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Advection Within Shallow Pore Waters of a Coastal Lagoon, Florida
Abstract
Ground water sources can be a significant portion of a local water budget in estuarine environments, particularly in areas with high recharge rates, transmissive aquifers, and permeable marine sediments. However, field measurements of ground water discharge are often incongruent with ground water flow modeling results, leaving many scientists unsure which estimates are accurate. In this study, we find that both measurements and model results are reasonable. The difference between estimates apparently results from the sources of water being measured and not the techniques themselves. In two locations in the Indian River Lagoon estuarine system, we found seepage meter rates similar to rates calculated from the geochemical tracers 222Rn and 226Ra. Ground water discharge rates ranged from 4 to 9 cm/d using seepage meters and 3 to 20 cm/d using 222Rn and 226Ra. In contrast, in comparisons to other studies where finite element ground water flow modeling was used, much lower ground water discharge rates of ∼0.05 to 0.15 cm/d were estimated. These low rates probably represent discharge of meteoric ground water from land-recharged aquifers, while the much higher rates measured with seepage meters, 222Rn, and 226Ra likely include an additional source of surface waters that regularly flush shallow (< 1 m depth) sediments. This resultant total flow of mixed land-recharged water and recirculated surface waters contributes to the total biogeochemical loading in this shallow estuarine environment.
... The average groundwater seepage rate over the study period was 0.025 gal/ft 2 /hr (1.25 L/m 2 /hr), indicating significant groundwater seepage to the lagoon. Cable et al. (2004) completed a study to determine the importance of submarine groundwater seepage to the northern IRL during the dry (May) and wet (August) seasons in 1999 and in the central Banana River Lagoon area in May, August, and December of 2000. Fifty-two field stations were established, with 28 located in the northern study area and 24 in the central Banana River Lagoon study area. ...
... The majority of data imply that groundwater discharge to Mosquito Lagoon is very important, and a review of the available hydrogeological data suggests that measurable seepage rates are possible. Although studies on flow and wave effects (water motion) on seepage meter results indicate negligible effects (Cable et al., 1997;Semmler, 2003), others believe these effects may be significant (Libelo and McIntyre, 1999;Shinn et al., 2002), or that shallow recirculating pore water derived from the overlying surface water column may represent a significant fraction of the measured groundwater input in the seepage meters (Cable et al., 2004). Possible mechanisms driving pore water advection include tides, waves, and bioturbation; bioirrigating organisms are the leading candidates (Martin et al., 2006). ...
... Unknown-few monitoring wells Data from Toth 1987;Belanger et al., 1997;Cable et al. 2004;Pandit et al., 2010. ...
Executive Summary
The conditions of terrestrial and estuarine resources within Canaveral National Seashore (CANA) were assessed and the threats and stresses to individual resources were identified and evaluated. With the exception of freshwater vegetation mapping, new data were not collected within CANA. Data and information derived from an intensive survey of the existing peer- reviewed and gray literature search were analyzed and synthesized to reveal the current state of each resource and to identify trends in resource quality and health.
... Seepage meters were developed originally by engineers studying leakage of canal bank linings (Israelson and Reeve 1944) but were largely ignored by the scientific research community until the 1970s when Lee (1977) demonstrated their effectiveness in evaluating groundwater seepage along shorelines. Since then, different forms of the seepage meter have been used with varying degrees of success in many freshwater and marine environments (e.g., Bokuniewicz 1980Bokuniewicz , 1992Prepas 1989, 1990;Belanger and Montgomery 1992;Reay et al. 1992;Libelo and MacIntyre 1994;Cable et al. 1996;Gallagher et al. 1996;Taniguchi and Fukuo 1996;Cable et al. 1997a,b;Corbett et al. 2000;Chanton et al. 2003;Cable et al. 2004). The simplicity of seepage meter design and deployment makes the device simultaneously attractive for field work and the bane of the researcher. ...
... Spatial and temporal seepage distribution-In the Indian River Lagoon system, seepage rates measured with seepage meters ranged from less than 1 to 12 cm d -1 throughout the project period (Table 1). A comparison of all four stations shows that the highest seepage rates were more common at the northern than central stations, similar to earlier findings (Martin et al. 2002;Cable et al. 2004). The elevated rate in the north may represent contributions to groundwater discharge from the Floridan Aquifer, which is more productive than the Surficial Aquifer. ...
... Studies using multiple techniques to measure SGD, such as tracer studies, seepage meters, and groundwater flow models, show that the magnitude of SGD strongly depends on the measurement techniques (e.g., Burnett et al. 2002;Cable et al. 2004). Our results suggest that seepage meters are a reliable technique for measuring pore water advection from sediments if the environment is calm. ...
Seepage meters, like most benthic flux chamber techniques, come with inherent concerns about how their presence may alter the environment and flow regimen of the benthic boundary layer and underlying sediments. Flow due to wave and current movement across topographic features induces a downward and upward flow field within the sediments surrounding the feature. We found this Bernoulli-induced flow is a real, but maybe minor, component of measured advection using seepage meters. This study was conducted in a Florida coastal lagoon to test the physical forcing mechanisms that may influence seepage measurements from sediments. Calculated Bernoulli seepage was within the measured background (∼1 to 2 cm day-1) expected from seepage meters when a plastic barrier beneath the device is used to inhibit natural seepage contributions. Nearby seepage measurements made with Lee-type seepage meters placed directly in the sediments ranged from 1 to 12 cm day -1. Thus, when seepage flow is very slow from sediments, Bernoulli-induced seepage may obscure the measurement. However, this study demonstrates that seepage in the Indian River Lagoon must be driven by forces other than Bernoulli-induced (pumped) flow. Suggestions for these forcing mechanisms highlight the uncertainty of the water source(s) in seepage measurements. In these Florida lagoon sediments, bioirrigation and terrestrial groundwater inputs are the most likely drivers, depending on distance from shore, benthic community composition, and continental recharge. Seepage measurements can be an excellent measure of advection in shallow-water marine sediments if Bernoulli-induced seepage is taken into account either experimentally or calculated based on local hydrographic and meteorological data. © 2006, by the American Society of Limnology and Oceanography, Inc.
... Their study further showed that the short-term influx problem can be alleviated by pre-filling the plastic bag with one liter of water before attaching it to the manual seepage meter. In addition to all the confounding factors mentioned above in using the conventional bag method, it is also very labor intensive (Taniguchi and Iwakawa 2001;Shinn et al. 2002;Cable et al. 2004). ...
... These automated approaches are able to detect low levels of flow rates, in addition to providing higher sampling frequency and resolving artifacts associated with the plastic bags, but not other artifacts. Despite the potential errors and detection limits inherent in the manual devices field evaluations of the "Lee-type" manual meters showed that consistent results can be obtained if one takes these potential problems into account (Cable et al. 2004;Taniguchi and Iwakawa 2001). ...
... The results of a seepmeter SGD rates in the table were averaged from the five different seepmeters that were deployed during the study. Burnett et al. (2002); c Cable et al. (2004); d Giblin and Gaines (1990); e Taniguchi et al. (2003b) Elsewhere, using manual seepage meters (Lee 1977), seepage rates ranging from 1 to 12 cm/day have been reported in the Indian River Lagoon system . While using both automated and manual seepage meters, Taniguchi et al. (2003a), measured SGD rates that ranged from 13.7 to 16.3 cm/day at Cockburn Sound, Western Australia. ...
Submarine groundwater discharge (SGD) assessments conducted both in the laboratory and at a field site in the northeastern Gulf of Mexico, using a continuous-heat type automated seepage meter (seepmeter) have shown that the device has the potential of providing long-term, high-resolution measurements of SGD. The improvements on the device using a simple inexpensive laboratory set up, have shown that: (1) connecting an extension cable to the seepmeter has a negligible effect on its measuring capability and, (2) influence of very low temperature (≤ 3 ºC) on seepmeter measurements can be accounted for by conducting calibrations at such temperatures prior to field deployments and, (3) salinity had no significant effect on the performance of the seepmeter. Calibration results from fresh water and sea water showed close agreement at a 95% confidence level significance between the data sets from the two media (R2 = 0.98).
The observed artifacts on seepmeter measurements associated with Bernoulli-induced flow, the vertically directed flow arising due to water movement across topographic features can significantly be reduced by burying (or submerging) the seepmeter to nearly the same level as the sediment topography. While the study revealed that in general wind speeds > 6 m/s were associated with enhanced SGD measurements in seepmeters with buried and unburied benthic chambers, the influence was greater in the unburied meters, and more pronounced for SGD rates < 2 cm/day. Comparatively, the seepmeter SGD measurements provided data that are comparable to manually-operated seepage meters, the radon geochemical tracer approach, and an electromagnetic (EM) seepage meter.
Study of the Sarasota Bay (SB) system revealed SGD advection rates ranging from 0.7 to 24.0 cm/day, except for rare isolated hot spot occurrences where higher rates were observed. In general, SGD estimates were relatively higher in the middle and south regions (5.9 – 24.0 cm/day) compared to the north region (0.7 – 5.9 cm/day). Although no obvious seawater nutrient concentration trend was revealed, the average N/P ratio was higher in the north compared to the middle and south regions of the SB system. The importance of SGD was evident in that about 40% of the regional nutrient fluxes were observed in the north while ~ 60% occurred in the middle and south regions combined. The latter two regions also had the highest overall nutrient flux per water volume ratio, compared to the north region, thus making them potentially more vulnerable to eutrophic conditions. On average, we estimate about 27% of total dissolved N in the SB system was derived via SGD.
... 222 Rn, radium isotopes, chloride, methane), seepage meters, piezometers, temperature, conductivity, water budgets, and modeling . It is becoming common, in fact, to use multiple methods to measure groundwater discharge because of its complex nature and the discrepancies found among methods used at the same location (Cable et al., 2004;. The two methods selected for this research, 222 Rn porewater distributions and seepage meters, have been used to quantify groundwater discharge in many previous studies (e.g. ...
... The two methods selected for this research, 222 Rn porewater distributions and seepage meters, have been used to quantify groundwater discharge in many previous studies (e.g. Cable et al., 1996Cable et al., , 2004Corbett et al., 1999Corbett et al., , 2000Michael et al., 2003;Burnett and Dulaiova, 2006;Martin et al., 2007;Fear et al., 2007;McCoy et al., 2007;Cable and Martin, 2008;Smith et al., 2008;Spruill and Bratton, 2008). ...
... Sediment slurry experiments (Martens et al., 1980;Corbett et al., 1998;Cable et al., 2004) were conducted to estimate the sedimentsupported levels of porewater 222 Rn at equilibrium with solidphase sediment. Sediment-supported 222 Rn ( 222 Rn produced from sediments) was measured by collecting vibracores down to 240 cm depth from nearshore sites close to the multi-level piezometers. ...
Radon-222 (222Rn) and ammonium (NH4+) were measured in interstitial water of the Neuse River Estuary (NRE), North Carolina, USA to determine the advective flux of NH4+ from sediments to the overlying water column. Porewater samples were collected over an annual cycle from multi-level piezometers installed in nearshore sites. NH4+ concentrations in sandy environments of the NRE were 10-fold higher than concentrations in the overlying water column. Shallow porewaters exhibited seasonal variations in NH4+ concentrations, which resulted in temporal changes in NH4+ flux from the sediment. Submarine groundwater discharge (SGD) was measured indirectly by using 222Rn as a tracer and directly via seepage meters. Discharge rates were variable depending upon the sampling location and season. The mean SGD was 9.1±1.5cmd−1 with a maximum SGD during spring at a rate of 13.6cmd−1 based on 222Rn porewater distribution. High porewater NH4+ concentrations in sandy nearshore sediments contributed NH4+ to the overlying water via groundwater discharge as an advective process. The overall mean NH4+ flux was 11.2±2.0mmolNH4+m−2d−1. Seasonal trends in groundwater seepage rates and NH4+ concentration suggest that groundwater is an important mechanism advecting nutrients from porewaters to surface waters, which is comparable to riverine NH4+ discharge. SGD N:P ratios (NH4+ as N) were >16:1, indicating that SGD is an important contributor of inorganic N for phytoplankton growth and may influence the NRE toward a less N-limited system. The data from this study will advance current understanding about the role of NH4+ in the progressive eutrophication of shallow estuarine ecosystems.
... Even though 222 Rn activities in groundwater are highly variable over small spatial scales (Table 1), they fall in the same range of values previously reported for the nearby Indian River Lagoon (Cable et al., 2004;Smith et al., 2008). The main factor controlling radon concentrations in these groundwaters appears to be sediment compaction. ...
... The groundwater discharge rates estimated here are on the same order of magnitude (in units of cm/day) as those observed at other coastal sites in Florida (Cable et al., , 2004Corbett et al., 2000;Li et al., 2009;Martin et al., 2006;Santos et al., 2009a;Smith et al., 2008;Swarzenski et al., 2007). We can also compare our values to those obtained from time series radon measurements in the Sebastian River. ...
... In areas where seepage through sandy sediments is the main mode of discharge, such as the Sebastian Estuary site, equilibrating water with sediments should be an excellent way to estimate the radon activity of advecting fluids. With flow rates on the order of centimeters per day (Cable et al., 2004;Martin et al., 2007), there should be ample time for the fluids to equilibrate with 226 Ra in the solid phases of the sedimentfull equilibrium is attained after ∼ 21 days. ...
River discharges are usually gauged at sites farther upstream than estuarine tidal reaches. As a result, global estimates of river water and nutrient fluxes to the ocean are likely underestimated as they often neglect groundwater discharge occurring in estuaries downstream of river gauging stations. We used radon and radium isotopes as tracers of groundwater discharge into the Sebastian River Estuary, a gaining stream in Florida, USA. We developed a spatially-distributed mass balance model that accounts for radon sources and sinks in waters above and below the estuarine pycnocline.Radium isotopes (224Ra, 223Ra, and 226Ra) were not enriched in groundwater relative to surface water and thus had limited usefulness as tracers at this specific site. The detection of fresh groundwater just beneath the sediment:water interface overlain by brackish bottom water implies that fresh groundwater dominates over saline groundwater in this salt wedge estuary. Lateral groundwater inputs from sandy banks into waters above the estuarine pycnocline were about 6-fold higher than inputs into waters below the pycnocline. Groundwater discharge rates into the surface layer of the estuary estimated from a radon mass balance ranged from 5 to 18 m3/s (or 18 to 62 cm/day if uniformly distributed throughout the entire estuary area). The fluxes into the bottom layer ranged from 0.8 to 1.1 m3/s (or 2.8 to 3.9 cm/day). These groundwater inputs augmented river discharges gauged upstream of the estuary tidal reaches by about 260% during the dry period and 135% during the wet period. As nutrient and other dissolved species are often highly enriched in groundwaters, groundwater probably controls surface water quality in this and other Florida estuaries.
... When multiple methods are applied at the same site, discharge estimates may vary by an order of magnitude or more, because the most commonly applied methods measure different components of SGD (e.g. Taniguchi et al., 2002; Cable et al., 2004 and references therein). This problem has been studied systematically at several coasts over the past five years ( Mulligan and Charette, 2006; Martin et al., 2007). ...
... This paper examines temporal and spatial distributions of pore water 222 Rn within the subterranean estuary of Indian River Lagoon, Florida (USA), to assess complexities of 222 Rn as a SGD tracer and to quantify fresh and marine advective fluxes from the sediments. Indian River Lagoon is a coastal water body where previous research provides a framework for the distribution and magnitudes of marine and terrestrial SGD (Cable et al., 2004; Martin et al., 2004; Cable et al., 2006; Martin et al., 2006 Martin et al., , 2007 Smith et al., 2008). A one-dimensional transport model, which includes advection, diffusion, non-local exchange, and production/decay, is employed to quantify how groundwater advection and shallow sediment irrigation contribute to observed pore water 222 Rn distributions. ...
... Submarine groundwater discharge to the Indian River Lagoon has been quantified using seepage meters (Belanger and Walker, 1990; Martin et al., 2004; Cable et al., 2006; Martin et al., 2006 Martin et al., , 2007), temperature (Martin et al., 2006), Cl − (Martin et al., 2004Martin et al., , 2006Martin et al., , 2007), Rn mass balance (Cable et al., 2004; Martin et al., 2007), and hydrologic mass balance or numerical models (Pandit and El-Khazen, 1990). Technique intercomparisons are often difficult because each method tends to measure different components of submarine groundwater discharge (recirculated seawater or fresh groundwater; e.g., Taniguchi et al., 2002; Cable et al., 2004). ...
Pore water radon (222Rn) distributions from Indian River Lagoon, Florida, are characterized by three zones: a lower zone where pore water 222Rn and sediment-bound radium (226Ra) are in equilibrium and concentration gradients are vertical; a middle zone where 222Rn is in excess of sediment-bound 226Ra and concentration gradients are concave-downward; and an upper zone where 222Rn concentration gradients are nearly vertical. These 222Rn data are simulated in a one-dimensional numerical model including advection, diffusion, and non-local exchange to estimate magnitudes of submarine groundwater discharge components (fresh or marine). The numerical model estimates three parameters, fresh groundwater seepage velocity, irrigation intensity, and irrigation attenuation, using two Monte Carlo (MC) simulations that (1) ensure the minimization algorithm converges on a global minimum of the merit function and the parameter estimates are consistent within this global minimum, and (2) provide 90% confidence intervals on the parameter estimates using the measured 222Rn activity variance. Model estimates of seepage velocities and discharge agree with previous estimates obtained from numerical groundwater flow models and seepage meter measurements and show the fresh water component decreases offshore and varies seasonally by a factor of nine or less. Comparison between the discharge estimates and precipitation patterns suggests a mean residence time in unsaturated and saturated zones on the order of 5 to 7 months. Irrigation rates generally decrease offshore for all sampling periods. The mean irrigation rate is approximately three times greater than the mean seepage velocity although the ranges of irrigation rates and seepage velocities are the same. Possible mechanisms for irrigation include density-driven convection, wave pumping, and bio-irrigation. Simulation of both advection and irrigation allows the separation of submarine groundwater discharge into fresh groundwater and (re)circulated lagoon water.
... Freshwater is delivered to the studied portion of the Indian River Lagoon via direct precipitation, urban runoff, discharge of the Eau Gallie River and Crane Creek, and terrestrial SGD (Martin et al., 2007 ). Fluxes of SGD were previously determined at the study site using a combination of methods that included seepage meters, chemical and thermal tracers, and models of chemical profiles (excess 222 Rn, Ra isotope decay, Cl concentrations; Cable et al., 2004 Cable et al., , 2006 Martin et al., 2004 Martin et al., , 2006 Martin et al., , 2007 Smith et al., 2008a,b; Roy et al., 2010 ). The flux of terrestrial SGD decreases with distance offshore becoming negligible near the freshwater–saltwater seepage face within the subterranean estuary (Fig. 2). ...
... Further offshore beyond the freshwater–saltwater seepage face (e.g., in the vicinity of CIRL-39, 250 m offshore), bioirrigation results in rapid, daily exchange of lagoon waters with porewaters within the upper 70 cm of the sediments, leading to flow rates as large as 150 cm day À1 (Martin et al., 2004Martin et al., , 2006). Consequently, marine SGD is chiefly Indian River Lagoon water that is cycled through the lagoon bottom sediments by bioirrigation, whereas the total SGD consists of both a relatively small component of terrestrial SGD and this substantially larger component ($2 orders of magnitude larger) of marine SGD (Cable et al., 2004; Martin et al., 2004 Martin et al., , 2006 Martin et al., , 2007 Fig. 2 ). Exchange of Indian River Lagoon waters with the ocean occur via three inlets located within the southern part of the lagoon . ...
... Methods employed to determine volumetric fluxes of SGD that are used here in the mass balance model were previously described (Cable et al., 2004; Martin et al., 2004 Martin et al., , 2006 Martin et al., , 2007). Porewater (i.e., groundwater) samples for REE analysis were collected in April 2007 from three multilevel piezometers (i.e., multisamplers; Martin et al., 2003) along a near-shore transect that extends 30 m fromFig. ...
Porewater (i.e., groundwater) samples were collected from multi-level piezometers across the freshwater–saltwater seepage face within the Indian River Lagoon subterranean estuary along Florida’s (USA) Atlantic coast for analysis of the rare earth elements (REE). Surface water samples for REE analysis were also collected from the water column of the Indian River Lagoon as well as two local rivers (Eau Gallie River, Crane Creek) that flow into the lagoon within the study area. Concentrations of REEs in porewaters from the subterranean estuary are 10–100 times higher than typical seawater values (e.g., Nd ranges from 217 to 2409 pmol kg−1), with submarine groundwater discharge (SGD) at the freshwater–saltwater seepage face exhibiting the highest REE concentrations. The elevated REE concentrations for SGD at the seepage face are too high to be the result of simple, binary mixing between a seawater end-member and local terrestrial SGD. Instead, the high REE concentrations indicate that geochemical reactions occurring within the subterranean estuary contribute substantially to the REE cycle. A simple mass balance model is used to investigate the cycling of REEs in the Indian River Lagoon and its underlying subterranean estuary. Mass balance modeling reveals that the Indian River Lagoon is approximately at steady-state with respect to the REE fluxes into and out of the lagoon. However, the subterranean estuary is not at steady-state with respect to the REE fluxes. Specifically, the model suggests that the SGD Nd flux, for example, exported from the subterranean estuary to the overlying lagoon waters exceeds the combined input to the subterranean estuary from terrestrial SGD and recirculating marine SGD by, on average, ∼100 mmol day−1. The mass balance model also reveals that the subterranean estuary is a net source of light REEs (LREE) and middle REEs (MREE) to the overlying lagoon waters, but acts as a sink for the heavy REEs (HREE). Geochemical modeling and statistical analysis further suggests that this fractionation occurs, in part, due to the coupling between REE cycling and iron redox cycling within the Indian River Lagoon subterranean estuary. The net SGD flux of Nd to the Indian River Lagoon is ∼7-fold larger than the local effective river flux to these coastal waters. This previously unrecognized source of Nd to the coastal ocean could conceivably be important to the global oceanic Nd budget, and help to resolve the oceanic “Nd paradox” by accounting for a substantial fraction of the hypothesized missing Nd flux to the ocean.
... There have been few studies that have demonstrated the use of practical and effective methods to obtain broad-scale data on groundwater seepage across large (>100 km 2 ) coastal lagoons (e.g., Baudron et al., 2015;Santos et al., 2008). Many studies instead have focused on using small-scale methods (e.g., discrete water samples, vertical temperature profiles, seepage meters) or conducting in-depth studies of subset areas of large lagoons (e.g., Cable, Martin, Swarzenski, Lindenberg, & Steward, 2004;Haider, Engesgaard, Sonnenborg, & Kirkegaard, 2015;Stumpp et al., 2014). It may be difficult to extrapolate small-scale studies across entire lagoons where groundwater discharge is often temporally variable and spatially diffuse (Burnett, Bokuniewicz, Huettel, Moore, & Taniguchi, 2003). ...
... It may be difficult to extrapolate small-scale studies across entire lagoons where groundwater discharge is often temporally variable and spatially diffuse (Burnett, Bokuniewicz, Huettel, Moore, & Taniguchi, 2003). Small-scale methods for directly measuring seepage, such as Lee-type seepage meters (Lee, 1977), are a well-demonstrated method for measuring seepage (e.g., Cable et al., 2004;Duque et al., 2018;Leote et al., 2008;. In a previous study of the present study site, Te Waihora (Lake Ellesmere), seepage meters were installed mainly on the western and northwestern margins on the lagoon, and a smaller number on the eastern side and along the barrier, to spatially map and quantify groundwater discharge to the lagoon (Ettema & Moore, 1995). ...
Coastal lagoons are significant wetland environments found on coastlines throughout the world. Groundwater seepage may be a key component of lagoon water balances, though only a few studies have investigated large (>100 km2) coastal lagoons. In this study, we combined airborne thermal infrared imagery with continuous measurements of radon (222Rn—a natural groundwater tracer), conductivity, water temperature and dissolved oxygen to map groundwater seepage to a large coastal lagoon in New Zealand. We found evidence of seepage along the margins of the lagoon but not away from the margins. Our findings confirmed previously known seepage zones and identified new potential locations of groundwater inflow. Both point‐source and diffuse seepage occurred on the western and north‐western margins of the lagoon and parallel to the barrier between the lagoon and sea. These observations imply geologic controls on seepage. The combination of remote sensing and in‐situ radon measurements allowed us to effectively map groundwater discharge areas across the entire lagoon. Combined, broad‐scale qualitative methods built confidence in our interpretation of groundwater discharge locations in a large, dynamic coastal lagoon. This article is protected by copyright. All rights reserved.
... Commonly, analyses of groundwater discharge to surface water have been conducted using elemental and isotopic geochemistry (Moore, 1996;Grossman, 2002;Cable et al., 2004;Ni et al., 2011;Dimova et al., 2013) as well as density-dependent flow and transport simulation codes (Guo and Langevin, 2002;Murgulet and Tick, in press). Statistical methods such as analysis of variance (ANOVA), multivariate linear regression (MLR) and factor analysis on environmental data have also produced valuable models that aid in identifying variations in water quality and contamination sources in various hydrologic systems (Morehead et al., 2008;Thareja et al., 2011;Hae-Cheol and Montagna, 2012). ...
... Factor 2, explaining 24.7% of the model, is influenced by fluctuations in DO and pH which are likely responses to biogeochemical reactions and physical processes that cause changes in δ 13 C ratios (i.e., bacterial degradation) and/or Rn 222 concentrations (i.e., diffusion through sediment fluxes). Given that Rn 222 is well-established as a groundwater indicator (Moore, 1996;Cable et al., 2004), it is likely that Factor 2 represents the groundwater discharge component indicating that the source of depleted δ 13 C may be originating from bottom sediments (i.e., biogeochemical reactions) and groundwater (note the depleted δ 13 C signature of groundwater for the 2012 sampling events). Finally, the latent factor explaining 18.2% of the model is Factor 3 which is the salinization of the system through sources other than Gulf seawater including the impact of evapoconcentration, potential discharge of trapped oil field brines, and discharges from adjacent cooling ponds, or a combination of some or all of the above. ...
... The shallow (,2.5 m), nearshore sediments that make up the upper portion of the Surficial aquifer have horizontal and vertical hydraulic conductivities that average 8.3 m d 21 and 0.20 m d 21 , respectively (Martin et al. 2007). Previous studies have shown that SGD may be an important component of the overall hydrologic budget of Indian River Lagoon (Cable et al. 2004; Martin et al. 2004 Martin et al. , 2006); however, the significance is unclear because of large variations in discharge estimates and in sources of SGD (Martin et al. 2007). We consider here the concept of ''fair weather'' versus ''storm'' conditions in our examination of SGD and the subterranean estuary. ...
... 'fair weather'' as those conditions either averaged over an extended period of time (months to years) or based on data of limited temporal resolution. Pandit and El-Khazen (1990) used a finite element groundwater flow model to estimate specific discharge of terrestrially-sourced groundwater that averaged 0.23 cm d 21 along the length of the lagoon. Martin et al. (2004 Martin et al. ( , 2006) measured time-series of pore-water temperature and Cl 2 concentrations in the north-central lagoon and reported offshore SGD rates as high as 150 cm d 21 ; however, the fluid was determined to be recirculated lagoon water with no terrestrially-sourced groundwater. Nevertheless, both terrestrial groundwater and re ...
Hydrostatic balances between fresh and saline groundwater and saline surface water control the physical and chemical framework of subterranean estuaries, but they are responsive to high frequency (waves and tides), low frequency (seasonal recharge patterns), and episodic (storm) events. In this study, we document a salinity and pressure perturbation to the subterranean estuary in east-central Florida during and after the passage of Tropical Storm Tammy on 04 Oct 2005-05 Oct 2005 and Hurricane Wilma on 24 Oct 2005. These storms reversed hydraulic gradients, forced lagoon water into the aquifer, and shifted the outflow face landward. Salinity at 1.5 m and 2.5 m below a common datum converged on similar values intermediate between fresh and lagoon water salinities. The outflow face reestablished pre-storm conditions after 80 days at 15 m offshore, but more than 160 days at 30 m offshore, confirming that both the flow field and fluid sources control the position of the subterranean estuary. Episodic, high intensity events could influence the biogeochemical setting of the subterranean estuary and the overlying water body by altering redox conditions in the subterranean estuary during the landward migration of the dispersive mixing zone, increasing short-term discharge of potentially contaminated groundwater, and/or changing pore fluid residence time within the seepage face and along the mixing zone-seepage face front.
... In areas where seepage through sandy sediments is the main mode of discharge, as suspected here, equilibrating water with sediments from the study site should be an excellent manner to estimate the radon activity of advecting fluids (Burnett et al., 2007). With linear flow rates typically on the order of a few centimeters per day as documented for the nearby Indian River Lagoon (Cable et al., 2004;Smith et al., 2008), there should be ample time for the radon in subsurface fluids to equilibrate with the radium in the solid phases of the sediment ( 222 Rn will reach $97% equilibrium with 226 Ra in 20 days). ...
... The differences between the sediment equilibration and groundwater samples may be a result of sediment compaction and thus higher sediment-water ratios in the deeper layers sam-pled by the piezometer. As groundwater slowly moves upwards, it is likely that radon equilibrates and discharges to surface waters at a lower value, as observed elsewhere (Cable et al., 2004;Cable and Martin, 2008). Based on the curve fitting shown in Fig. 2a, together with the assumption that the radon (and thus groundwater) entered the canal further upstream than we sampled, we can extrapolate upstream to an assumed end-member concentration. ...
Naturally occurring 222Rn (radon; t1/2 = 3.8 days) is a good natural tracer of groundwater discharge because it is conservative and typically 2–3 orders of magnitude higher in groundwater than surface waters. In addition, new technology has allowed rapid and inexpensive field measurements of radon-in-water. Results from the C-25 Canal, a man-made canal in east-central Florida thought to be dominated by groundwater inflows, display how one can quickly assess a water body for locations of groundwater inputs. Although only the eastern portion of the canal was surveyed, use of a few assumptions together with some continuous radon measurements allowed reasonable estimates of the groundwater inflows to be made. Groundwater discharge estimates of 327,000 m3/day and 331,000 m3/day were measured for two stations based on determining the groundwater fraction of the total stream flow. This fraction in each case was calculated by correcting radon concentrations for decay over transit times determined from concentration differences between the apparent focal point of groundwater discharge (with a concentration of 520 ± 80 dpm/L) estimated to be ∼17.7 km upstream from the downstream sample locations. During the same period, an average flow of 312,000 ± 70,000 m3/day was determined from time-series measurements of radon at a fixed downstream location. Coincident current meter readings and a measured cross-section area allowed an independent assessment of the total stream discharge of 336,000 m3/day. The radon-derived estimates thus indicate that >90% of the total flow is groundwater derived, consistent with the known characteristics of this waterway.
... Their study further showed that the short-term influx problem can be alleviated by pre-filling the plastic bag with one liter of water before attaching it to the manual seepage meter. In addition to all the confounding factors mentioned above in using the conventional bag method, it is also very labor intensive (Taniguchi and Iwakawa, 2001;Shinn et al., 2002;Cable et al., 2004). ...
... These automated approaches are able to detect low levels of flow rates, in addition to providing higher sampling frequency and resolving artifacts associated with the plastic bags, but not other artifacts. Despite the potential errors and detection limits inherent in the manual devices field evaluations of the ''Lee-type'' manual meters showed that consistent results can be obtained if one takes these potential problems into account (Cable et al., 2004;Taniguchi and Iwakawa, 2001). ...
Submarine groundwater discharge (SGD) assessments were conducted both in the laboratory and at a field site in the northeastern Gulf of Mexico, using a continuous heat-type automated seepage meter (seepmeter). The functioning of the seepmeter is based on measurements of a temperature gradient in the water between downstream and upstream positions in its flow pipe. The device has the potential of providing long-term, high-resolution measurements of SGD. Using a simple inexpensive laboratory set-up, we have shown that connecting an extension cable to the seepmeter has a negligible effect on its measuring capability. Similarly, the observed influence of very low temperature (≤3 °C) on seepmeter measurements can be accounted for by conducting calibrations at such temperatures prior to field deployments. Compared to manual volumetric measurements, calibration experiments showed that at higher water flow rates (>28 cm day−1 or cm3 cm−2 day−1) an analog flowmeter overestimated flow rates by ≥7%. This was apparently due to flow resistance, turbulence and formation of air bubbles in the seepmeter water flow tubes. Salinity had no significant effect on the performance of the seepmeter. Calibration results from fresh water and sea water showed close agreement at a 95% confidence level significance between the data sets from the two media (R2 = 0.98). Comparatively, the seepmeter SGD measurements provided data that are comparable to manually-operated seepage meters, the radon geochemical tracer approach, and an electromagnetic (EM) seepage meter.
... Alternatively to the radon approach based on Eq. 1, SGD quantification is possible by physically capturing the discharging groundwater at the sea bottom by means of seepage meters (Lee, 1977;Taniguchi et al., 2003). However, in contrast to the radon approach, which results in data that allow integrating over time and space, seepage meter data result in SGD information restricted to point locations only (Cable et al., 2004;Povinec et al., 2012). ...
Mapping radon ( ²²² Rn) distribution patterns in the coastal sea is a widely applied method for localizing and quantifying submarine groundwater discharge (SGD). While the literature reports a wide range of successful case studies, methodical problems that might occur in shallow wind-exposed coastal settings are generally neglected. This paper evaluates causes and effects that resulted in a failure of the radon approach at a distinct shallow wind-exposed location in the Baltic Sea. Based on a simple radon mass balance model, we discuss the effect of both wind speed and wind direction as causal for this failure. We show that at coastal settings, which are dominated by gentle submarine slopes and shallow waters, both parameters have severe impact on coastal radon distribution patterns, thus impeding their use for SGD investigation. In such cases, the radon approach needs necessarily to allow for the impact of wind speed and wind direction not only during but also prior to the field campaign.
... Inputs of freshwater to the Indian River Lagoon within the study site include two small rivers (i.e., Eau Gallie River and Crane Creek; Figure 1), urban storm runoff, direct rainfall, and SGD . Previous studies evaluated SGD fluxes at the study site using a combination of techniques including Lee-type seepage meters (Lee, 1977), chemical and thermal tracers, and models of chemical profiles within the subterranean estuary (excess 222 Rn, Ra isotopes, Cl − measurements; Cable et al., 2004Cable et al., , 2006Martin et al., 2004Martin et al., , 2006Martin et al., , 2007Smith et al., 2008a,b). Lee-type seepage meters were installed at each of the nearshore EGN-x sites (Eau Gallie North, where x is distance offshore in m), except at the EGN-12.5 location (Figure 1C), as well as the offshore, CIRL sites (central Indian River Lagoon; Figure 1B) to estimate SGD fluxes within the Indian River Lagoon . ...
Rare earth elements (REE) and Nd isotope compositions of surface and groundwaters from the Indian River Lagoon in Florida were measured to investigate the influence of submarine groundwater discharge (SGD) on these parameters in coastal waters. The Nd flux of the terrestrial component of SGD is around 0.7±0.03 μmol Nd/day per m of shoreline across the nearshore seepage face of the subterranean estuary. This translates to a terrestrial SGD Nd flux of 4±0.2 mmol/day for the entire 5,880 m long shoreline of the studied portion of the lagoon. The Nd flux from bioirrigation across the nearshore seepage face is 1±0.05 μmol Nd/day per m of shoreline, or 6±0.3 mmol/day for the entire shoreline. The combination of these two SGD fluxes is the same as the local, effective river water flux of Nd to the lagoon of 12.7±5.3 mmol/day. Using a similar approach, the marine-sourced SGD flux of Nd is 31.4±1.6 μmol Nd/day per m of shoreline, or 184±9.2 mmol/day for the investigated portion of the lagoon, which is 45 times higher than the terrestrial SGD Nd flux. Terrestrial-sourced SGD has an εNd(0) value of −5±0.42, which is similar to carbonate rocks (i.e., Ocala Limestone) from the Upper Floridan Aquifer (−5.6), but more radiogenic than the recirculated marine SGD, for which εNd(0) is −7±0.24. Marine SGD has a Nd isotope composition that is identical to the εNd(0) of Fe(III) oxide/oxyhydroxide coated sands of the surficial aquifer (−7.15±0.24 and −6.98±0.36). These secondary Fe(III) oxides/oxyhydroxides formed during subaerial weathering when sea level was substantially lower during the last glacial maximum. Subsequent flooding of these surficial sands by rising sea level followed by reductive dissolution of the Fe(III) oxide/oxyhydroxide coatings can explain the Nd isotope composition of the marine SGD component. Surficial waters of the Indian River Lagoon have an εNd(0) of −6.47±0.32, and are a mixture of terrestrial and marine SGD components, as well as the local rivers (−8.63 and −8.14). Nonetheless, the chief Nd source is marine SGD that has reacted with Fe(III) oxide/oxyhydroxide coatings on the surficial aquifer sands of the subterranean estuary.
... Diffusive flux of tracer from sediments F dif Diffusion of radon and radium isotopes from bottom sediments into the overlying water column may represent a major source of tracers in some systems, particularly in areas covered by fine-grained sediments or sediments that contain high concentrations of 226 Ra and 228 Th (for 222 Rn and 224 Ra, respectively) and/or when inputs of tracers from pore water and other sources are comparatively low (Lambert and Burnett 2003;Rodellas et al. 2015b). Diffusive fluxes of radon and radium are routinely assessed via different methods and approaches, including laboratory incubations of sediment cores (Beck et al. 2007;Tamborski et al. 2017), sediment equilibration experiments (Cable et al. 2004;Wang et al. 2017), tracer concentrations measured in overlying waters (Gilfedder et al. 2015), or the 226 Ra concentration in sediments Baudron et al. 2015). Other studies have directly assumed that the tracer flux from diffusion is negligible (Schmidt et al. 2010;Trezzi et al. 2016). ...
Radium isotopes and radon are routinely used as tracers to quantify groundwater and porewater fluxes into coastal and freshwater systems. However, uncertainties associated with the determination of the tracer flux are often poorly addressed and often neglect all the potential errors associated with the conceptualization of the system (i.e., conceptual uncertainties). In this study, we assess the magnitude of some of the key uncertainties related to the determination of the radium and radon inputs supplied by groundwater and porewater fluxes into a waterbody (La Palme Lagoon, France). This uncertainty assessment is addressed through a single model ensemble approach, where a tracer mass balance is run multiple times with variable sets of assumptions and approaches for the key parameters determined through a sensitivity test. In particular, conceptual uncertainties linked to tracer concentration, diffusive fluxes, radon evasion to the atmosphere, and change of tracer inventory over time were considered. The magnitude of porewater fluxes is further constrained using a comparison of independent methods: (1) 224Ra and (2) 222Rn mass balances in overlying waters, (3) a model of 222Rn deficit in sediments, and (4) a fluid‐salt numerical transport model. We demonstrate that conceptual uncertainties are commonly a major source of uncertainty on the estimation of groundwater or porewater fluxes and they need to be taken into account when using tracer mass balances. In the absence of a general framework for assessing these uncertainties, this study provides a practical approach to evaluate key uncertainties associated to radon and radium mass balances.
... Martin et al. (2007); Martin et al. (2004); determined the SGD into the IRL using seepage meter measurements". Swarzenski (2001) estimated the SGD using the Radium Isotope technique", while Cable et al. (2004) also estimated the SGD using the Excess 222 Radium Flux Model technique". Mamoua et al. (2019); Pandit et al. (2016) determine SGD into IRL using numerical modeling". ...
The 250 km long shallow (maximum depth of four meters) Indian River Lagoon (IRL), located on the east coast of Florida, is an estuary of varying width (0.8 km to 8 km). As a result of land use changes in the IRL watershed the IRL is becoming more eutrophic, particularly in the northern reaches where major blooms of long duration (months) are becoming more frequent. A study was conducted over a 15 month period to determine the total nitrogen (TN) and total phosphorus (TP) loads into the IRL at Eau Gallie, Florida, due to groundwater seepage from the IRL watershed. The nutrient loads were determined by multiplying the measured TN and TP concentrations by groundwater seepage determined by the finite difference model SEAWAT. The numerical model was calibrated by comparing model predicted hydraulic heads with field measured hydraulic heads. The groundwater seepages, per unit length of shoreline, ranged between 0.62 m³/day to 0.84 m³/day. The concentration ranges for TN and TP were from 0.0003 mg/l to 5.9 mg/l, and 0.044 mg/l to 0.378 mg/l, respectively. The corresponding TN and TP loads, per meter of shoreline, ranged from 554 mg/day to 2,356 mg/day, and from 60 mg/day to 99 mg/day. The assessed TN and TP loads due to groundwater showed a high level of temporal and spatial variation, and can be a significant contributor to the eutrophy within the IRL.
... Different techniques were used by many researchers to estimate the SGD into the Indian River Lagoon (IRL), which is an estuary located on the east coast of Florida as shown in Fig. 1. Direct measurement of SGD into IRL by utilizing seepage meters was one of those techniques used by Zimmermann et al., (1985); Belanger and Walker (1990); Cable et al., (2004); and Martin et al., (2007). Elements like radon and radium have also been utilized as natural tracers to quantify the SGD into the IRL by some studies (Swarzenski et al., 2001;Martin et al 2004). ...
... Martin et al. (2007); Martin et al. (2004); determined the SGD into the IRL using seepage meter measurements. Swarzenski (2001) estimated the SGD using the Radium Isotope technique, while Cable et al. (2004) also estimated the SGD using the Excess 222 Radium Flux Model technique. Pandit et al. (2016) determine SGD into IRL using numerical modeling. ...
... Next, the equilibrated seawater in the RAD-H 2 O 250 mL bot- tles was immediately analyzed using a RAD7 detector. The equilibrium activity (C eq ) was calculated using the porosity (u) and wet bulk density (q wet ) of the sediments (Cable et al., 1996(Cable et al., , 2004). Then, the diffusive flux of 222 Rn was calculated using the following equation (Martens et al., 1980;Ullman & Aller, 1982): ...
Submarine groundwater discharge (SGD) has been recognized as an important pathway for nutrients into estuaries, coasts, and the adjacent seas. In this study, 222Rn was used to estimate the SGD‐associated nutrient fluxes into an aquaculture area in a typical tropical bay (Maowei Sea, China). The SGD into the Maowei Sea during June 2016 was estimated to be 0.36 ± 0.33 m d−1 and was associated with SGD‐derived dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicon (DSi) fluxes (mol d−1) of (4.5 ± 5.5) × 106, (5.3 ± 9.1) × 104, and (9.4 ± 9.3) × 106, respectively. The SGD‐derived nutrients (i.e., DIN, DIP, and DSi) were more than 1.9, 0.9, and 3.6 times the amounts in the local river input and served as dominant sources in the nutrient budgets in the Maowei Sea. Moreover, the N/P ratios in the SGD around the Maowei Sea were high (mean: 64), and these ratios likely exceeded the environmental self‐purification capacity, thereby enhancing the biomass and changing the phytoplankton community structure. Therefore, SGD processes with derived nutrients may affect the biogeochemical cycles and marine ecological environment in the Maowei Sea. Furthermore, the N/P ratios (∼67) in oysters are very close to those in the SGD in the Maowei Sea; this coincidence suggests that the high N/P ratios in the SGD are likely to be one of the most important sources that support oyster aquaculture, which might weaken the burden of water eutrophication in the Maowei Sea. SGD is the dominant source of nutrients to the Maowei Sea The SGD‐derived nutrients are higher in the Maowei Sea than those estimated in other similar studies SGD‐derived nutrients could support high‐quantity aquaculture (oyster) in the Maowei Sea
... The fact that the beach region, with intense hydrodynamic processes, presents a high 222 Rn activity indicates that the SGD may be active in maintaining such activity. Similar 222 Rn activities were used in other studies as evidence of intense SGD (Cable et al. 2004;Burnett et al. 1996;Dulaiova 2003, 2006). ...
Submarine groundwater discharge (SGD) is herein recognized as a significant pathway of material transport from land to the coastal SW Atlantic Ocean and thus, it can be a relevant factor affecting the marine biogeochemical cycles in the region. This paper focuses on the initial measurements of ²²⁶Ra, ²²⁸Ra and ²²²Rn made in Patagonia’s coastal zone of Chubut and Santa Cruz provinces (42°S–48°S, Argentina). ²²⁶Ra activity ranged from 2.9 to 73.5 dpm 100 L⁻¹, and ²²⁸Ra activity ranged from 11.9 to 311.0 dpm 100 L⁻¹ in groundwater wells. The radium activities found in Patagonia’s marine coastal regions and adjacent shelf indicate significant enrichment throughout the coastal waters. Groundwater samples presented the largest ²²²Rn activity and ranged from 2.66 to 1083 dpm L⁻¹. Conversely, in the coastal marine environment, the ²²²Rn activity ranged from 1.03 to 6.23 dpm L⁻¹. The Patagonian coastal aquifer showed a larger enrichment in ²²⁸Ra than in ²²⁶Ra, which is a typical feature for sites where SGD is dominant, probably playing a significant role in the biogeochemistry of these coastal waters.
... Although recent studies recognize the role of groundwater on many aspects of river processes (Cook, 2013;Sear et al., 1999), few studies have attempted to analyze the groundwater-surface water signal in the hydrology and water chemistry of impaired rivers and the degree of interaction, suggesting that this area merits further research. Multiple methods have been utilized to investigate the contribution of groundwater to surface water in rivers such as mathematical models (Frei et al., 2009), geochemical tracers, statistical methods (Cable et al., 2004;Su et al., 2012), and seepage meters (Lee, 1977). ...
Hydrologic alterations in coastal rivers and semi-arid climates affect water chemistry. • Stable isotopes of δ 18 O and δD constrain well the magnitude of groundwater contribution. • Variable degrees of groundwater discharge occur downstream and upstream of the dam. • Electrical resistivity methods show sources and quality of groundwater discharge. • Evaporation effects significantly alter surface water and groundwater quality. There is a lack of understanding and methods for assessing the effects of anthropogenic disruptions, (i.e. river fragmentation due to dam construction) on the extent and degree of groundwater-surface water interaction and geochemical processes affecting the quality of water in semi-arid, coastal catchments. This study applied a novel combination of electrical resistivity tomography (ERT) and elemental and isotope geochemistry in a coastal river disturbed by extended drought and periodic flooding due to the operation of multiple dams. Geochemical analyses show that the saltwater barrier causes an increase in salinity in surface water in the downstream river as a result of limited freshwater inflows, strong evaporation effects on shallow groundwater and mostly stagnant river water, and is not due to saltwater intrusion by tidal flooding. Discharge from bank storage is dominant (~ 84%) in the downstream fragment and its contribution could increase salinity levels within the hyporheic zone and surface water. When surface water levels go up due to upstream freshwater releases the river temporarily displaces high salinity water trapped in the hyporheic zone to the underlying aquifer. Geochemical modeling shows a higher contribution of distant and deeper groundwater (~40%) in the upstream river and lower discharge from bank storage (~13%) through the hyporheic zone. Recharge from bank storage is a source of high salt to both upstream and downstream portions of the river but its contribution is higher below the dam. Continuous ERT imaging of the river bed complements geochemistry findings and indicate that while lithologically similar, downstream of the dam, the shallow aquifer is affected by salinization while fresher water saturates the aquifer in the upstream fragment. The relative contribution of flows (i.e. surface water releases or groundwater discharge) as related to the river fragmentation control changes of streamwater chemistry and likely impact the interpretation of seasonal trends.
... However, while radon budgets produce an estimate of " total " SGD, i.e., freshwater inputs + re-circulated seawater (Mulligan and Charette, 2006 ), radium budgets primarily assess the salty component of SGD given that radium is normally absent in fresh groundwater but might be mobilized from sediment particles in case of saline water influence (Webster et al., 1995). Even so, the variety of ubiquitous temporally and spatially variable sediment–water exchange mechanisms that also act as sources of radon (Cable et al., 2004; Martin et al., 2004; Colbert et al., 2008a, b) and short-lived radium isotopes to surface waters (Webster et al., 1994; Hancock and Murray, 1996; Hancock et al., 2000; Hammond, 2007, 2008; Gonneea et al., 2008) cannot be ignored. Correctly identifying both the fluid source and composition is thus an important task (Mulligan and Charette, 2006; Burnett et al., 2006 ). ...
Natural radioactive tracer-based assessments of basin-scale submarine
groundwater discharge (SGD) are well developed. However, SGD takes place in
different modes and the flow and discharge mechanisms involved occur over a
wide range of spatial and temporal scales. Quantifying SGD while
discriminating its source functions therefore remains a major challenge.
However, correctly identifying both the fluid source and composition is
critical. When multiple sources of the tracer of interest are present,
failure to adequately discriminate between them leads to inaccurate
attribution and the resulting uncertainties will affect the reliability of
SGD solute loading estimates. This lack of reliability then extends to the
closure of local biogeochemical budgets, confusing measures aiming to
mitigate pollution.Here, we report a multi-tracer study to identify the sources of SGD,
distinguish its component parts and elucidate the mechanisms of their
dispersion throughout the Ria Formosa – a seasonally hypersaline lagoon in
Portugal. We combine radon budgets that determine the total SGD (meteoric +
recirculated seawater) in the system with stable isotopes in water
(δ2H, δ18O), to specifically identify SGD source
functions and characterize active hydrological pathways in the catchment.
Using this approach, SGD in the Ria Formosa could be separated into two
modes, a net meteoric water input and another involving no net water
transfer, i.e., originating in lagoon water re-circulated through permeable
sediments. The former SGD mode is present occasionally on a multi-annual
timescale, while the latter is a dominant feature of the system. In the
absence of meteoric SGD inputs, seawater recirculation through beach
sediments occurs at a rate of
∼ 1.4 × 106 m3 day−1. This implies that the
entire tidal-averaged volume of the lagoon is filtered through local sandy
sediments within 100 days ( ∼ 3.5 times a year), driving an estimated
nitrogen (N) load of ∼ 350 Ton N yr−1 into the system as
NO3−. Land-borne SGD could add a further
∼ 61 Ton N yr−1 to the lagoon. The former source is
autochthonous, continuous and responsible for a large fraction (59 %) of
the estimated total N inputs into the system via non-point sources, while the
latter is an occasional allochthonous source capable of driving new
production in the system.
... We assume salinity represents a proxy of freshwater discharge (i.e., direct rainfall, upstream river inputs, urban runoff, and groundwater discharge), dissolved oxygen represents a proxy of photosynthesis and respiration, and radon is a proxy of fresh and saline groundwater discharge. Radon sources other than groundwater discharge (molecular diffusion from bottom sediments and decay of dissolved 226 Ra) were not quantified in Gold Coast canals but were demonstrated to be minor sources of radon to other shallow coastal systems located within 100 km from the Gold Coast Makings et al., 2014) and overseas (Berelson et al., 1982;Cable et al., 2004). Therefore, we treat radon as an unambiguous qualitative tracer for groundwater exchange which likely includes a combination of fresh groundwater discharge and seawater recirculation in canal beach sediments (i.e., tidal pumping). ...
... Methods that have proven useful in the identification of SGD sites and subsequent quantification have developed along four, often coincident, themes: (i) geochemical tracers, (ii) geophysical techniques, (iii) numerical modeling, and (iv) actual physical measurements of fluid exchange across the sediment/water interface . The suite of geochemical tracers that are employed to identify and quantify SGD include: Cl − (Martin et al., 2004), temperature (Taniguchi, 2000; Taniguchi et al., 2003), four naturally occurring Ra isotopes (Moore et al., 2002; Crotwell and Moore, 2003; Charette et al., 2001; Swarzenski et al., 2001), 222 Rn (Cable et al., 1996Cable et al., , 1997Cable et al., , 2004 Hussain et al., 1999; Burnett and Dulaiova, 2003; Lambert and Burnett, 2003), and CH 4 (Corbett et al., 1999Corbett et al., , 2000). Recent developments in the acquisition of continuous or timeseries resistivity data (Manheim et al., 2004; Swarzenski et al., 2004a) in coastal sediments can yield powerful information on the dynamic position and movement of the fresh water/saltwater interface down to depths in excess of 10's m. ...
A suite of naturally occurring radionuclides in the U/Th decay series (222Rn, 223,224,226,228Ra) were studied during wet and dry conditions in Tampa Bay, Florida, to evaluate their utility as groundwater discharge tracers, both within the bay proper and within the Alafia River/estuary — a prominent free-flowing river that empties into the bay. In Tampa Bay, almost 30% of the combined riverine inputs still remain ungauged. Consequently, groundwater/surface water (hyporheic) exchange in the discharging coastal rivers, as well as submarine groundwater discharge (SGD) within the bay, are still unresolved components of this system's water and material budgets.
... The study site is a shore-normal transect that extends across the seepage face of Indian River Lagoon, Florida (Fig. 1). Details of hydrostratigraphy, hydrogeology, and non-local exchange are discussed elsewhere (Cable et al., 2004; Martin et al., 2004 Martin et al., , 2006 Martin et al., , 2007 Pandit and El-Khazen, 1990; Toth, 1988). The hydraulic conductivity of the upper several meters of sediment ranges from 10 − 2 to 10 − 8 cm/s but is more homogenous in the upper 70 cm sediments ranging from 10 − 2 to 10 − 4 cm/s (data based on 28 cores, Hartl, 2006). ...
Iron oxides are important terminal electron acceptors for organic carbon (OC) remineralization in subterranean estuaries, particularly where oxygen and nitrate concentrations are low. In Indian River Lagoon, Florida, USA, terrestrial Fe-oxides dissolve at the seaward edge of the seepage face and flow upward into overlying marine sediments where they precipitate as Fe-sulfides. The dissolved Fe concentrations vary by over three orders of magnitude, but Fe-oxide dissolution rates are similar across the 25-m wide seepage face, averaging around 0.21 mg/cm2/yr. The constant dissolution rate, but differing concentrations, indicate Fe dissolution is controlled by a combination of increasing lability of dissolved organic carbon (DOC) and slower porewater flow velocities with distance offshore. In contrast, the average rate constants of Fe-sulfide precipitation decrease from 21.9 × 10- 8 s- 1 to 0.64 × 10- 8 s- 1 from the shoreline to the seaward edge of the seepage face as more oxygenated surface water circulates through the sediment. The amount of OC remineralized by Fe-oxides varies little across the seepage face, averaging 5.34 × 10- 2 mg/cm2/yr. These rates suggest about 3.4 kg of marine DOC was remineralized in a 1-m wide, shore-perpendicular strip of the seepage face as the terrestrial sediments were transgressed over the past 280 years. During this time, about 10 times more marine solid organic carbon (SOC) accumulated in marine sediments than were removed from the underlying terrestrial sediments. Indian River Lagoon thus appears to be a net sink for marine OC.
... Although groundwater discharge and its potential ecological importance have been previously investigated (e.g. Portnoy et al., 1998;Cable et al., 2004;Urish and McKenna, 2004), the importance of groundwater as a mode of delivery of nutrients to near-shore coastal areas is still an area requiring more detailed study (e.g. Burnett et al., 2002;Slomp and van Cappellen, 2004;Bowen et al., 2007). ...
Nutrient fluxes from developed catchments are often a significant factor in the declining water quality and ecological functioning in estuaries. Determining the relative contributions of surface water and groundwater discharge to nutrient-sensitive estuaries is required because these two pathways may be characterized by different nutrient concentrations and temporal variability, and may thus require different remedial actions. Quantifying the volumetric discharge of groundwater, which may occur via diffuse seepage or springs, remains a significant challenge. In this contribution, the total discharge of freshwater, including groundwater, to two small nutrient-sensitive estuaries in Prince Edward Island (Canada) is assessed using a unique combination of airborne thermal infrared imaging, direct discharge measurements in streams and shoreline springs, and numerical simulation of groundwater flow. The results of the thermal infrared surveys indicate that groundwater discharge occurs at discrete locations (springs) along the shoreline of both estuaries, which can be attributed to the fractured sandstone bedrock aquifer. The discharge measured at a sub-set of the springs correlates well with the area of the thermal signal attributed to each discharge location and this information was used to determine the total spring discharge to each estuary. Stream discharge is shown to be the largest volumetric contribution of freshwater to both estuaries (83% for Trout River estuary and 78% for McIntyre Creek estuary); however, groundwater discharge is significant at between 13% and 18% of the total discharge. Comparison of the results from catchment-scale groundwater flow models and the analysis of spring discharge suggest that diffuse seepage to both estuaries comprises only about 25% of the total groundwater discharge. The methods employed in this research provide a useful framework for determining the relative volumetric contributions of surface water and groundwater to small estuaries and the findings are expected to be relevant to other fractured sandstone coastal catchments in Atlantic Canada. Copyright © 2009 John Wiley & Sons, Ltd.
... Based on discrepancies between water balances and coastal measurements, recent studies have recognized that the measured volumes of SGD include both terrestrial and marine components (LI et al., 1999;BURNETT et al., 2003;CABLE et al., 2004;MARTIN et al., 2004). For example, MOORE (1996) used 226Ra to estimate that ground water discharge to the South Atlantic Bight was as great as 40% of river inputs. ...
Dissolved constituent fluxes from the sediment to the water column are important in estuarine environments. Benthic nutrient source depends on the mechanisms driving advection, the advective transport rate, and the concentration of nutrients in the discharged water, all of which depend on the source of discharged water and water-solid reactions along its flow. Pore water advection has been measured at rates of 3 to 6 cm/d using seepage meters in the Banana River Lagoon, Florida. Diffusion, advection, and reaction modeling of SiO2 profiles in pore water indicate that advection and reactions are more important than diffusion in the upper I in of the sediments. Advection results from the recirculation of the overlying water column through the bottom sediments, oxygenating the lagoon water, while pore water lacks oxygen. As lagoon water recirculates through sediments, the subsequent loss of oxygen enhances regeneration of buried organic matter. Using measured seepage rates and average pore water concentrations of nutrients, N and P fluxes from the sediment are estimated to be 33 to 38 mu g/cm(2)/y and 3 to 5 mu g/cm(2)/y, respectively. On the basis of sedimentation rates and the average concentrations of N and P in the sediment, the fluxes of N and P to the sediment are estimated to be 9 to 38 mu g/cm(2)/y and 2 to 6 mu g/cm(2)/y, respectively. These values suggest that 100% more N and 30% more P may discharge with recirculating lagoon water than is deposited in the sediment. Because the source of most pore water is surface water, the excess nutrients appear to originate from organic matter regeneration at or near the sediment-water interface, thereby elevating their concentrations in pore waters and depleting their concentration in the buried sediment. This regeneration of nutrients appears to limit their burial rate in the lagoon.
... Marine and terrestrial SGD could be separated through observations of variations in chemical compositions of pore water at the outflow face. Pore water chemistry has been used recently to estimate mixing rates across the sediment-water interface [Schluter et al., 2000; Cable et al., 2004; Martin et al., 2004; Martin et al., 2006], but none of these studies directly sampled the outflow face. Our objectives here are to separate marine and terrestrial SGD and to compare their magnitudes in a microtidal lagoon in Florida using pore water chemistry and seepage meters. ...
1] Magnitudes of terrestrial (fresh) and marine (saline) sources of submarine groundwater discharge (SGD) are estimated for a transect across Indian River Lagoon, Florida. Two independent techniques (seepage meters and pore water Cl À concentrations) show terrestrial SGD decreases linearly to around 22 m offshore, and these techniques, together with a model based on the width of the outflow face, indicate a cumulative discharge of between 0.02 and 0.9 m 3 /d per meter of shoreline. Seepage meters and models of the deficiencies in 222 Rn activity in shallow sediments indicate marine SGD discharges of roughly 117 m 3 /d per meter of shoreline across the entire 1800-m-wide transect. Two surface streams nearest the transect have an average discharge of about 28 m 3 /d per meter of shoreline. Marine SGD is thus 4 times greater then surface water discharge and more than 2 orders of magnitude greater than terrestrial SGD. The magnitude of the terrestrial SGD is limited by the amount of regional precipitation, evaporation, recharge, and groundwater usage, while marine SGD is limited only by processes circulating marine water into and out of the sediments. The large magnitude of marine SGD means that it could be important for estuarine cycling of reactive components such as nutrients and metals with only slight modification from estuarine water compositions. The small magnitude of terrestrial SGD means that large differences from estuarine water composition would be required to affect chemical cycling.
... The objective of our experiments was to label the benthic microalgae at the sediment surface with stable isotope tracers and follow the fate of that label. Thus, we used moderate-to-high vertical flow rates, relative to natural systems (e.g., Burnett et al. 2003;Cable et al. 2004;Sholkovitz et al. 2003). However, we designed the perfusionator to allow for fine-scale control of flow rates to suit the needs of a particular study, so the perfusionator should be adaptable to numerous applications. ...
The water–sediment interface is a dynamic zone where the benthic and pelagic environments are linked through exchange and
recycling of organic matter and nutrients. However, it is often difficult to measure rate processes in this zone. To that
end, we designed an experimental apparatus for continuous and homogeneous perfusion of sediment porewater with dissolved conservative
(SF6, Rhodamine WT dye) and isotopic (H13CO3− and 15NH4+) tracers to study nitrogen and carbon cycling by the sediment microbial community of shallow illuminated sediments. The perfusionator
consists of a 60-cm ID × 60-cm high cylinder that includes a reservoir for porewater at the base of the sediment column. Porewater
amended with conservative and stable isotopic tracers was pumped through a mixing reservoir and upward through the overlying
sediments. We tested the perfusionator in a laboratory setting, as part of an outdoor mesocosm array, and buried in coastal
sediments. Conservative and isotopic tracers demonstrated that the porewater tracers were distributed homogeneously through
the sediment column in all settings. The perfusionator was designed to introduce dissolved stable isotope tracers but is capable
of delivering any dissolved ionic, organic, or gaseous constituent. We see a potentially wide application of this technique
in the aquatic and marine sciences in laboratory and field settings.
KeywordsStable isotopes–Mesocosms–Porewater advection–Water–sediment interface–Sediment column
... Some recent studies have attempted to reveal the impact of SGD on the distributions of salinity, trace elements, and natural radionuclides (Rn and Ra isotopes) in the pore water of permeable sediments. However, these studies are mostly for the geochemistry of deep pore water for the upper 10 m layer of coastal sediments (Cable et al. 2004; Martin et al. , 2006 Charette et al. 2005; Charette and Sholkovitz 2006). Thus, in this study, we focused on the distribution of salinity and nutrients in shallow pore water in the upper 30 cm layer of surface sediments, with high depth resolution, in order to look closely at the ventilation processes of nutrients in the intertidal zone where active wave set-up and recirculation of seawater are taking place. ...
In order to determine the temporal and spatial variations of nutrient profiles in the shallow pore water columns (upper 30
cm depth) of intertidal sandflats, we measured the salinity and nutrient concentrations in pore water and seawater at various
coastal environments along the southern coast of Korea. In the intertidal zone, salinity and nutrient concentrations in pore
water showed marked vertical changes with depth, owing to the active exchange between the pore water and overlying seawater,
while they are temporally more stable and vertically constant in the sublittoral zone. In some cases, the advective flow of
fresh groundwater caused strong vertical gradients of salinity and nutrients in the upper 10 cm depth of surface sediments,
indicating the active mixing of the fresher groundwater with overlying seawater. Such upper pore water column profiles clearly
signified the temporal fluctuation of lower-salinity and higher-Si seawater intrusion into pore water in an intertidal sandflat
near the mouth of an estuary. We also observed a semimonthly fluctuation of pore water nutrients due to spring-neap tide associated
recirculation of seawater through the upper sediments. Our study shows that the exchange of water and nutrients between shallow
pore water and overlying seawater is most active in the upper 20 cm layer of intertidal sandflats, due to physical forces
such as tides, wave set-up, and density-thermal gradient.
... Estimates of groundwater seepage to the lagoon vary widely (Martin et al. 2002), ranging from 3.4 3 10 7 m 3 yr 21 (Pandit and El-Khazen [1990] using groundwater modeling) to 1.6 3 10 13 m 3 yr 21 (Belanger and Walker [1990] using seepage meters). Cable et al. (2004) attributed the disparity in seepage rate estimates to a distinction between land recharged-derived seepage estimated with groundwater models and the sum of land-recharged-and recirculated surface water-derived seepage measured with seepage meters. In this study, the base temperature (Eq. ...
The distilling effect of evaporation and the diluting effect of precipitation on salinity at two estuarine sites in the humid
subtropical setting of the Indian River Lagoon, Florida, were evaluated based on daily evaporation computed with an energy-budget
method and measured precipitation. Despite the larger magnitude of evaporation (about 1,58 mm yr−1) compared to precipitation (about 1,180 mm yr−1) between February 2002 and January 2004, the variability of monthly precipitation induced salinity changes was more than
twice the variability of evaporation induced changes. Use of a constant, mean value of evaporation, along with measured values
of daily precipitation, were sufficient to produce simulated salinity changes that contained little monthly (root-mean-square
error = 0.33‰ mo−1 and 0.52‰ mo−1 at the two sites) or cumulative error (<1‰ yr−1) compared to simulations that used computed daily values of evaporation. This result indicates that measuring the temporal
variability in evaporation may not be critical to simulation of salinity within the lagoon. Comparison of evaporation and
precipitation induced salinity changes with measured salinity changes indicates that evaporation and precipitation explained
only 4% of the changes in salinity within a flow-through area of the lagoon; surface water and ocean inflows probably accounted
for most of the variability in salinity at this site. Evaporation and precipitation induced salinity changes explained 61%
of the variability in salinity at a flow-restricted part of the lagoon.
... Likewise the magnitude of pore water exchange (marine sources to SGD) that may be included in total SGD estimates will vary depending upon local wind stress and tides, resident benthic burrowing organisms, bottom topography, and sediment type. Elucidating the relative magnitude of these sources in total SGD is critical to our complete understanding of coastal hydrological and biogeochemical mass balances (Cable et al., 2004; Martin et al., 2004 Martin et al., , 2006 Martin et al., , 2007). Although recent research has aimed at quantifying magnitudes of the water and chemical fluxes associated with submarine groundwater discharge in order to address fate and transport of nutrients and contaminants from the terrestrial to the marine habitat (e.g. ...
Transport between pore waters and overlying surface waters of Flamengo Bay near Ubatuba, Brazil, was quantified using natural and artificial geochemical tracers, 222Rn, Cl−, and SF6, collected from multi-level piezometers installed along a transect perpendicular to the shore. Eight sampling ports positioned along the length of the piezometers allowed sampling of pore waters at discrete depth intervals from 10 to 230 cmbsf (centimeters below seafloor). Small volume samples were collected from the piezometers using a peristaltic pump to obtain pore water depth profiles. Pore water 222Rn is deficient in shallow sediments, allowing application of a diffusion-corrected 222Rn exchange rate. This model estimates the magnitude of pore water exchange rates to be about 130–419 cm/day. An SF6-saturated fluorescein dye tracer was gently pumped into deep pore waters and exchange rates estimated from this method range from 29 to 185 cm/day. While absolute rates are higher using 222Rn than SF6, rates are of similar magnitudes and the trends with distance from shore are the same – flow is greatest 6 m from shore and decreases by more than 50% further offshore. A Cl− mass balance indicates the greatest fraction of fresh SGD occurs along an apparent preferential flow path in sediments within 5–7 m of the shoreline (87%). Recirculating bay waters through sediments dominate pore water advection at 10 m offshore where only 4% of the flow can be attributed to a freshwater source. Both fresh and marine sources combine to make up submarine groundwater discharge to coastal water bodies. The magnitude of fresh aquifer discharge is often a spatially variable and minor component of the total discharge.
... The importance of bioirrigation in exchange of pore fluids in lagoon settings was highlighted by recent work in Florida (Martin et al., 2006). This is likely an important process in Chincoteague Bay as well, and may explain some of the rapid penetration of young saline water into groundwater beneath this estuary and similar settings (Bratton et al., 2004; Cable et al., 2004). The approach used in this investigation, particularly the greater depth and time components obtained by drilling and age dating, could be productively applied to experiments like the intercomparison studies of submarine groundwater discharge that have been done at geologically distinct sites in Florida, New York, Italy, Australia, Brazil, and Mauritius (Burnett et al., 2003Burnett et al., , 2006). ...
To better understand large-scale interactions between fresh and saline groundwater beneath an Atlantic coastal estuary, an offshore drilling and sampling study was performed in a large barrier-bounded lagoon, Chincoteague Bay, Maryland, USA. Groundwater that was significantly fresher than overlying bay water was found in shallow plumes up to 8 m thick extending more than 1700 m offshore. Groundwater saltier than bay surface water was found locally beneath the lagoon and the barrier island, indicating recharge by saline water concentrated by evaporation prior to infiltration. Steep salinity and nutrient gradients occur within a few meters of the sediment surface in most locations studied, with buried peats and estuarine muds acting as confining units. Groundwater ages were generally more than 50 years in both fresh and brackish waters as deep as 23 m below the bay bottom. Water chemistry and isotopic data indicate that freshened plumes beneath the estuary are mixtures of water originally recharged on land and varying amounts of estuarine surface water that circulated through the bay floor, possibly at some distance from the sampling location. Ammonium is the dominant fixed nitrogen species in saline groundwater beneath the estuary at the locations sampled. Isotopic and dissolved-gas data from one location indicate that denitrification within the subsurface flow system removed terrestrial nitrate from fresh groundwater prior to discharge along the western side of the estuary. Similar situations, with one or more shallow semi-confined flow systems where groundwater geochemistry is strongly influenced by circulation of surface estuary water through organic-rich sediments, may be common on the Atlantic margin and elsewhere.
... A substantial number of studies have been performed on the nature of groundwater discharge to the IRL, most of which indicate a range in groundwater advection rates between 3 and 25 cm d À1 in the upper 70 cm of sediments. These studies employed seepage meters (Zimmermann et al., 1985;Cable et al., 2004Cable et al., , 2006Martin et al., 2004), geochemical tracers Martin et al., 2004), heat flux , and modeling (Smith et al., 2008a) to estimate fluxes. Terrestrial, meteoric discharge is isolated to within about 25 m of the western shoreline of the IRL (Martin et al., 2007;Smith et al., 2008a), whereas recirculated seawater dominates the fluxes elsewhere, often driven by bioirrigation and tropical storm events (Smith et al., 2008b). ...
The natural flux of groundwater into coastal water bodies has recently been shown to contribute significant quantities of nutrients and trace metals to the coastal ocean. Groundwater discharge and hyporheic exchange to estuaries and rivers, however, is frequently overlooked though it often carries a distinctly different chemical signature than surface waters. Most studies that attempt to quantify this input to rivers use multiple geochemical tracers. However, these studies are often limited in their spatial and temporal extents because of the labor-intensive nature of integrating multiple measurement techniques. We describe here a method of using a single tracer, 222Rn, to rapidly characterize groundwater discharge into tidally-influenced rivers and streams. In less than one week of fieldwork, we determined that of six streams that empty into the Indian River Lagoon (IRL), Florida, three (Eau Gallie River, Turkey Creek, and Main Canal) did not receive substantial groundwater inputs, one canal (C-25 Canal) was dominated by groundwater exchange, and the remaining two (Sebastian River system and Crane Creek) fell somewhere in between. For more detailed discharge assessments, we focused on the Sebastian River system, a stratified tidal river estuary, during a relatively dry period (June) and a wet period (July) in 2008. Using time-series 222Rn and current velocity measurements we found that groundwater discharge into all three branches of the Sebastian River increased by 1–2 orders of magnitude during the wetter period. The estimated groundwater flow rates were higher than those reported into the adjacent IRL, suggesting that discharge into these rivers can be more important than direct discharge into the IRL. The techniques employed here should work equally well in other river/stream systems that experience significant groundwater discharge. Such assessments would allow area managers to quickly assess the distribution and magnitude of groundwater discharge nature into rivers over large spatial ranges.
... A simple mixing line drawn through the respective endmember concentrations suggests non-conservative mixing for all four Ra isotopes. Excluding the thermal spring-derived discharge off west-central Florida (Fanning et al., 1981), and the phosphate-enhanced Ra isotopes observed in Tampa Bay, such Ra activities are typical for Florida coastal waters and estuaries (Burnett et al., 1990;Swarzenski et al., 2001;Cable et al., 2004). ...
The distributions of dissolved organic carbon (DOC), Ba, U, and a suite of naturally occurring radionuclides in the U/Th decay series (²²²Rn, 223,224,226,228Ra) were studied during high- and low-discharge conditions in the Loxahatchee River estuary, Florida to examine the role of submarine groundwater discharge in estuarine transport. The fresh water endmember of this still relatively pristine estuary may reflect not only river-borne constituents, but also those advected during active groundwater/surface water (hyporheic) exchange. During both discharge conditions, Ba concentrations indicated slight non-conservative mixing. Such Ba excesses could be attributed either to submarine groundwater discharge or particle desorption processes. Estuarine dissolved organic carbon concentrations were highest at salinities closest to zero. Uranium distributions were lowest in the fresh water sites and mixed mostly conservatively with an increase in salinity. Suspended particulate matter (SPM) concentrations were generally lowest (< 5 mg L− 1) close to zero salinity and increased several-fold (∼ 18 mg L− 1; low discharge) toward the seaward endmember, which may be attributed to dynamic resuspension of bottom sediments within Jupiter Inlet.
Meteoric groundwater discharge (MGD) to coastal regions transports terrestrial freshwater and nutrients that may alter coastal ecosystems by supporting harmful algal blooms. Estimation of MGD-driven nutrients is crucial to assess potential effects on coastal zones. These estimates require a reliable assessment of MGD rates and pore water nutrient concentrations below subterranean estuaries. To estimate nutrient delivery into a subterranean estuary in the Indian River Lagoon, FL., pore water and surface water samples were collected from nested piezometers along a selected transect on five sampling episodes. Groundwater hydraulic head and salinity were measured in thirteen onshore and offshore piezometers. Numerical models were developed, calibrated, and validated using SEAWAT to simulate MGD flow rates. Lagoon surface water salinity exhibits no spatial but mild temporal variation between 21 and 31. Pore water salinity shows tremendous variation in time and space throughout the transect except in the middle region of the lagoon which exhibits uniform but elevated salinities up to 40. Pore water salinity as low as that of freshwater happens to occur in the shoreline regions during most of the sampling episodes. Both pore water and surface water show remarkably higher total nitrogen TN than total phosphorus TP concentrations and most TN is exported as NH4, reflecting the effect of mangroves on the geochemical reactions that reduce NO3 into NH4. Nutrient contributions of pore water and lagoon water exceed the Redfield TN/TP molar ratio in all sampling trips by up to a factor of 48 and 4, respectively. Estimated TP and TN fluxes receives by the lagoon via MGD are 41-106 and 113-1478 mg/d/m of shoreline. The molar TN/TP ratio of nutrient fluxes exceeds the Redfield ratio by a factor of up to 3.5 which indicates the potential of MGD-driven nutrients to alter the lagoon water quality and support harmful algal blooms.
The influx of fresh groundwater and re-circulated sea water into coastal ecosystem occurs through the submarine groundwater discharge (SGD). Measurement of salinity, radium tracers (²²⁴Ra, and ²²⁶Ra isotopes) and nutrients in estuarine water, coastal surface water and groundwater during December 2019 estimated the SGD and associated nutrient fluxes near the Karameniyar estuary (Gulf of Mannar) and surroundings of the Manapad region at southern part of Tamil Nadu state in India. The presence of excessive radium tracers revealed that the SGD was contributing to Ra desorption from the sediments and enrichment in the coastal waters. We estimated SGD of approximately 0.03–0.59 m³ m⁻² d⁻¹ for the Manapad region and relatively more homogeneous but comparatively less values in the Karameniyar estuary (0.03–0.34 m³ m⁻² d⁻¹). Higher average values of dissolved inorganic nitrogen (DIN; 43.62 μmol L⁻¹) and soluble reactive phosphate (SRP; 1.848 μmol L⁻¹) suggested greater influence of SGD on the overall coastal water nutrient budget. This study also indicated simultaneous occurrence of fresh and saline SGD in this region.
This review of studies that quantified fluxes with seepage meters in marine settings in the last decades shows the historical evolution of this device and the knowledge acquired during this period. Coastal environments are differentiated from freshwater settings due to water salinity and the effects of tides and waves that have important implications for the measurement approach and generated results. The framework in which seepage meters have been used in marine settings has evolved in parallel to the understanding of submarine groundwater discharge. This review of seepage meter research shows: an uneven distribution of studies in the world with some densely-studied regions and an absolute lack of data in other regions; a dominance of studies where only seepage meters were used compared to studies that combined seepage meter measurements with values determined with radioactive tracers or hydraulic calculations; and a variety of publication outlets with different focuses (hydrology, oceanography or multidisciplinary). The historical overview of the research conducted with seepage meters shows the wide range of seepage meter applications – from simply measuring fluxes at local scales to larger studies that extrapolate local results to estimate fluxes of water, nutrients, and other solutes at regional and global scales. A variety of automated seepage meters have been developed and used to better characterize short-term groundwater-seawater exchange, including the effects of waves and tides. We present recommendations and considerations to guide seepage meter deployment in marine settings, as seepage meters are still the only method that quantifies directly the interaction between groundwater and surface water.
Assessing submarine groundwater discharge (SGD) into lagoons and bays can be helpful to understand biogeochemical processes, especially nutrient dynamics. In the present paper, radium (Ra) isotopes were used to quantify SGD in two typical tropical lagoons (Laoye Lagoon (LY Lagoon) and Xiaohai Lagoon (XH Lagoon)) of Eastern Hainan Island, China. The Ra mass balance model provided evidence that SGD plays an important role in the hydrology of the LY Lagoon and the XH Lagoon, delivering average SGD fluxes of 1.7 × 106 (94 L m−2 d−1) and 1.8 × 106 (41 L m−2 d−1) m3 d−1, respectively. Tidal pumping was one of the important driving forces for SGD fluxes in the LY and the XH Lagoons. Tidal-driven SGD into the tidal channels of both lagoons can account for approximately 10% of the total SGD flux into the lagoons. In addition, the dissolved inorganic nutrient budgets were reassessed in the LY Lagoon and the XH Lagoon, which showed that SGD was the major source of nutrients entering the LY Lagoon and that the LY Lagoon behaved as a source for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) and as a sink for dissolved silicate (DSi). Nutrient loads in the XH Lagoon were mainly derived from riverine inputs and SGD, and the XH Lagoon behaved as a source for DIP, but a sink for DIN and DSi. This article is protected by copyright. All rights reserved.
Natural radioactive tracer-based assessments of basin-scale Submarine Groundwater Discharge (SGD) are well developed, but because of the different modes in which SGD takes place and the wide range of spatial and temporal scales under which the flow and discharge mechanisms involved occur, quantifying SGD while discriminating its source functions remains a major challenge. Yet, correctly identifying both the fluid source and composition is critical: when multiple sources of the tracer of interest are present, failure to adequately discriminate between them will lead to inaccurate attribution and the resulting uncertainties will affect the reliability of SGD solute loading estimates. This lack of reliability then extends to the closure of local biogeochemical budgets, confusing measures aiming to mitigate pollution.
Here, we report a multi-tracer study to identify the sources of SGD, distinguish its component parts and elucidate the mechanisms of their dispersion throughout the Ria Formosa – a seasonally hypersaline lagoon in Portugal. We combine radon budgets that determine the total SGD (meteoric + recirculated seawater) in the system with stable isotopes in water (2H, 18O), to specifically identify SGD source functions and characterize active hydrological pathways in the catchment. Using this approach, SGD in the Ria Formosa could be separated into a net water input and another involving no net water transfer, i.e. originating in seawater recirculation through permeable sediments. The former SGD mode is present occasionally on a multiannual timescale, while the latter is a permanent feature of the system. In the absence of meteoric SGD inputs, seawater recirculation through beach sediments occurs at a rate of ~ 1.4 × 106 m3 day−1, implying the entire tidal-averaged volume of the lagoon is filtered through local sandy sediments within 100 days, or about 3.5 times a year, driving an estimated nitrogen (N) load of ~ 350 t N yr−1 into the system as NO3−. Land-borne SGD could add a further ~ 61 t N yr−1 to the lagoon. The former source is autochthonous, continuous and responsible for a large fraction (59 %) of the estimated total N inputs into the system via non-point sources, while the latter is an occasional allochthonous source, so more difficult to predict, but capable of driving new production in the system.
A study was conducted from July 2002 through June 2006 in order to assess the significance of submarine groundwater discharge (SGD) to Sarasota Bay (SB), Florida. The assessment approaches used in this study included manual seepage meters, geochemical tracers (radon, 222Rn and methane, CH4), and subseafloor resistivity measurements. The estimated SGD advection rates in the SB system were found to range from 0.7 to 24.0 cm/day, except for some isolated hot spot occurrences where higher rates were observed. In general, SGD estimates were relatively higher (5.9–24.0 cm/day) in the middle and south regions of the bay compared to the north region (0.7–5.9 cm/day). Average dissolved inorganic nutrient concentrations within the SB water column ranged: 0.1–11 μM (NO2+NO3), 0.1–9.1 μM (NH4) and 0.2–1.4 μM (PO4). The average N/P ratio was higher in the north compared to the middle and south regions of the bay. On average, we conservatively estimate that about 27% of the total N in the SB system was derived via SGD. The prevalence of shallow embayed areas in the SB system and the presence of numerous septic tanks in the surrounding settlements enhanced the potential effects of nutrient rich seepages. Statistical comparison of the quantitative approaches revealed a good agreement between SGD estimates from manual seepage meters and those derived from the 222Rn model (p=0.67; α=0.05; n=18). CH4 was found to be useful for qualitative SGD assessments. CH4 and 222Rn were correlated (r2=0.31; α=0.05; n=54). Large scale resistivity surveys showed spatial variability that correlates more clearly with lithology than with SGD patterns.
We trace pathways of Fe reactions in the Indian River Lagoon (Florida, USA) subterranean estuary using Fe isotopes to provide new constraints on Fe-isotopic fractionation in a sulfide-bearing subterranean estuary. Porewater δ56Fe values increase from − 1.16‰ at 115 cm depth to + 0.2‰ at 7 cm depth due to isotope fractionation in three distinct lithostratigraphic zones. The deepest zone contains orange sands with elevated Fe-oxide contents (0.2 wt.%) that dissolve through diagenetic Fe-oxide reduction and elevate Fe concentrations in porewaters (100 to 300 μM/l). This reaction causes porewater δ56Fe values to be ~ 1‰ lighter than the sediment δ56Fe values. An intermediate zone contains white Fe-poor sands, with Fe-oxide contents < 0.1 wt.% and dissolved Fe concentrations < 20 μM/l. This zone is a sink for dissolved Fe through adsorption of isotopically heavy dissolved Fe(II) onto mineral surfaces. This adsorption results in porewater δ56Fe values that are as much as 1.8‰ lighter than sediment δ56Fe values. The uppermost zone contains organic carbon and Fe-sulfide rich black sediments with low dissolved Fe (< 1 μM/l) and elevated porewater sulfide (up to 600 μM/l) concentrations. Precipitation of isotopically light Fe-sulfides increases the porewater δ56Fe values as much as 0.68‰ more than corresponding sediment δ56Fe values. The near-surface Fe-sulfide precipitation delivers to the lagoon dissolved Fe with slightly positive δ56Fe values, averaging about + 0.24‰, via submarine groundwater discharge (SGD). Iron-sulfide precipitation in sulfide-containing subterranean estuaries thus may result in a previously unidentified source of isotopically heavy Fe to the coastal oceans.
Pollution of rivers with excess nutrients due to groundwater discharge, storm water runoff, surface loading, and atmospheric deposition is an increasing environmental concern worldwide. While the storm water runoff and surface loading of nutrients into many rivers have been explored in great detailed, the groundwater discharge of nutrients into the rivers has not yet been thoroughly quantified. This study ascertained the shallow groundwater discharges and nutrient loads into the Lower St. Johns River (LSJR), FL USA. The groundwater discharges were obtained using Darcy's law along with field measured hydrological parameters, whereas the groundwater nutrient loads were calculated based on the groundwater discharges and the field measured nutrient concentrations. The average rate of groundwater discharge per unit cross-section area over the four selected sites along the LSJR was about 1.2×10−2 m3m−2 d−1. The average loads of groundwater nutrients into the adjacent LSJR were 10.6 and 5.6mgm−2 d−1, respectively, for nitrate- and nitrite-nitrogen (NOx-N) and total phosphorus (TP). In general, seasonal variations of the groundwater levels were larger than the river stages, whereas site variations of groundwater nutrient concentrations were larger than seasonal variations of groundwater nutrient concentrations. Results from this study are useful for estimation of groundwater contamination and river eutrophication.
Subterranean estuary occupies the transition zone between hypoxic fresh groundwater and oxic seawater, and between terrestrial and marine sediment deposits. Consequently, we hypothesize, in a subterranean estuary, biogeochemical reactions of Fe respond to submarine groundwater discharge (SGD) and sea level rise. Porewater and sediment samples were collected across a 30-m wide freshwater discharge zone of the Indian River Lagoon (Florida, USA) subterranean estuary, and at a site 250 m offshore. Porewater Fe concentrations range from 0.5 μM at the shoreline and 250 m offshore to about 286 μM at the freshwater–saltwater boundary. Sediment sulfur and porewater sulfide maxima occur in near-surface OC-rich black sediments of marine origin, and dissolved Fe maxima occur in underlying OC-poor orange sediments of terrestrial origin. Freshwater SGD flow rates decrease offshore from around 1 to 0.1 cm/day, while bioirrigation exchange deepens with distance from about 10 cm at the shoreline to about 40 cm at the freshwater–saltwater boundary. DOC concentrations increase from around 75 μM at the shoreline to as much as 700 μM at the freshwater–saltwater boundary as a result of labile marine carbon inputs from marine SGD. This labile DOC reduces Fe-oxides, which in conjunction with slow discharge of SGD at the boundary, allows dissolved Fe to accumulate. Upward advection of fresh SGD carries dissolved Fe from the Fe-oxide reduction zone to the sulfate reduction zone, where dissolved Fe precipitates as Fe-sulfides. Saturation models of Fe-sulfides indicate some fractions of these Fe-sulfides get dissolved near the sediment–water interface, where bioirrigation exchanges oxic surface water. The estimated dissolved Fe flux is approximately 0.84 μM Fe/day per meter of shoreline to lagoon surface waters. Accelerated sea level rise predictions are thus likely to increase the Fe flux to surface waters and local primary productivity, particularly along coastlines where groundwater discharges through sediments.
Time-series measurements of chloride (Cl-) concentrations in lagoon and pore waters of an estuary on the east coast of Florida (Indian River Lagoon) demonstrate exchange of lagoon surface water to depths of ∼40 cm in the sediment in less than 46 h. The exchange rate may be as fast as 150 cm d-1 based on models of the decay in the amplitude of diurnal temperature variations and the time lag of maxima and minima of the temperature variations at depths of 15 and 30 cm below the sediment-water interface. These flow rates indicate a minimum residence time of 0.33 d for the pore water. Considering the small tides and waves, rate of the exchange, and large number of bioturbating organisms in the Indian River Lagoon, the exchange of water is driven largely by bioirrigation. The exchange provides a greater flux of excess radon-222 from the sediment to the lagoon than would occur from diffusion alone. The exchange also pumps oxygenated water into the sediments, thereby enhancing organic carbon remineralization and the flux of nitrogen from sediments to the lagoon water. High rates of exchange across the sediment-water interface indicate that marine sources are volumetrically more important than terrestrial sources to submarine groundwater discharge in the permeable sediments of this estuary. © 2006, by the American Society of Limnology and Oceanography, Inc.
Submarine ground water discharge is suggested to be an important pathway for contaminants from continents to coastal zones, but its significance depends on the volume of water and concentrations of contaminants that originate in continental aquifers. Ground water discharge to the Banana River Lagoon, Florida, was estimated by analyzing the temporal and spatial variations of Cl− concentration profiles in the upper 230 cm of pore waters and was measured directly by seepage meters. Total submarine ground water discharge consists of slow discharge at depths > ∼70 cm below seafloor (cmbsf) of largely marine water combined with rapid discharge of mixed pore water and estuarine water above ∼70 cmbsf. Cl− profiles indicate average linear velocities of ∼0.014 cm/d at depths > ∼70 cmbsf. In contrast, seepage meters indicate water discharges across the sediment-water interface at rates between 3.6 and 6.9 cm/d. The discrepancy appears to be caused by mixing in the shallow sediment, which may result from a combination of bioirrigation, wave and tidal pumping, and convection. Wave and tidal pumping and convection would be minor because the tidal range is small, the short fetch of the lagoon limits wave heights, and large density contacts are lacking between lagoon and pore water. Mixing occurs to ∼70 cmbsf, which represents depths greater than previously reported. Mixing of oxygenated water to these depths could be important for remineralization of organic matter.
A novel technique for monitoring of submarine groundwater discharge (SGD) in coastal zones based on an in situ underwater gamma-ray spectrometry of radon-decay products is described. Several sites were visited during the IAEA’2002 expedition offshore Donnalucata in the south-eastern Sicily. Continuous monitoring of 222Rn in the beach spring at Donnalucata has shown variable 222Rn activity concentrations in groundwater (from 12 to ), depending inversely on water levels during tide. Spatial variations of SGD have been observed in the Donnalucata boat basin. Average 222Rn activity concentrations in seawater varied from ∼0.1 to , showing an inverse relationship with salinity. A continuous monitoring carried out at the site closest to the coast has also revealed an inverse relationship of 222Rn activity concentration on tide and salinity. The 222Rn concentrations in seawater varied from during high tides to during low tides, confirming the tidal influence on SGD.
The natural geochemical tracer 222Rn was used to quantify submarine groundwater discharge (SGD) onto the continental shelf of Onslow Bay, North Carolina. Water column samples were collected aboard the R/V Cape Hatteras on July 21–26, 2002, and an additional nearshore water column transect and groundwater samples were collected in 2005/2006. Assessment of SGD was accomplished using a mass balance approach that quantified sources and sinks of radon, including benthic flux, exchange across the pycnocline or air–sea interfaces, horizontal transport into and out of the study area, and a water column inventory. Four independent geochemical box models were developed to quantify SGD regionally and with distance from shore.Overall, water column inventories and diffusion rates decreased with distance from shore. Average water column inventories were 8520 ± 1310, 7230 ± 1190, and 760 ± 510 dpm m−2 for three shore-parallel boxes from nearshore to offshore, and resulted in a regional average of 5800 ± 1050 dpm m−2 for the Regional box model. Diffusion rates of radon through the sediment–water interface were 0.9 ± 0.2, 0.6 ± 0.1, and 0.4 ± 0.1 dpm m−2 min−1 for the near to offshore models, and averaged 0.6 ± 0.1 dpm m−2 min−1 for the Regional box model. SGD estimates were calculated using two end-member 222Rn activities for the advecting fluids which allowed a distinction between terrestrially-driven SGD and total SGD. Terrestrially-driven and total SGD estimates averaged 0.2 and 0.7 cm d−1, respectively. The calculated terrestrially-driven SGD is as important in the delivery of fresh water as riverine sources to Onslow Bay and a significant contributor to the South Atlantic Bight.
Radon-222 is a good natural tracer of groundwater flow into the coastal ocean. Unfortunately, its usefulness is limited by the time consuming nature of collecting individual samples and traditional analysis schemes. We demonstrate here an automated system which can determine, on a “continuous”basis, the radon activity in coastal ocean waters. The system analyses ^222Rn from a constant stream of water passing through an air-water exchanger that distributes radon from the running flow of water to a closed air loop. The air stream is feed to a commercial radon-in-air monitor which determines the concentration of ^222Rn by collection and measurement of the emitting daughters, ^214Po and ^218Po, via a charged semiconductor detector. Since the distribution of radon at equilibrium between the air and water phases is governed by a well-known temperature dependence, the radon concentration in the water is easily calculated.
The construction and operation of a multi-level piezometer (multisampler) designed to collect pore water from permeable sediments up to 230 cm below the sediment-water interface is described. Multisamplers are constructed from 11/2 inch schedule 80 PVC pipe. One-quarter-inch flexible PVC tubing leads from eight ports at variable depths to a 11/2 inch tee fitting at the top of the PVC pipe. Multisamplers are driven into the sediment using standard fence-post drivers. Water is pumped from the PVC tubing with a peristaltic pump. Field tests in Banana River Lagoon, Florida, demonstrate the utility of multisamplers. These tests include collection of multiple samples from the permeable sediments and reveal mixing between shallow pore water and overlying lagoon water.
We use 228Ra and 226Ra to determine the mass
balance of dissolved inorganic nitrogen (DIN) and dissolved reactive
phosphorus (DRP) in the North Inlet salt marsh-estuarine system. While
this system has only minor freshwater inputs of nutrients or radium, it
is an extremely productive ecosystem. In addition, there are significant
exports of these dissolved species to the coastal ocean. Saline
groundwater in this estuarine system contains nutrient and radium
concentrations more than an order of magnitude greater than surface
waters. Using a radium mass balance, we estimate the groundwater
discharge necessary to support the export of radium to the coastal ocean
and the corresponding flux of nutrients from the groundwater. From these
calculations, we show that the underlying aquifer supplies nutrients
sufficient to support the net primary productivity of the salt marsh
ecosystem and to account for the known export of nutrients from the
marsh. We conclude that the major nutrient source to the North Inlet,
South Carolina, salt marsh is the saline aquifer lying just beneath the
surface of the marsh. Furthermore, extrapolation of the nutrient export
to include other South Carolina marshes suggests that nutrient fluxes
from salt marshes to the coastal ocean rival riverine nutrient fluxes
for the region.
We investigated the influence of flood-induced channel changes on the hyporheic zone of 4th-and 5th-order reaches of a mountain stream network. Preflood versus postflood comparisons were made in three study reaches from well networks established before and reestablished after a major flood. Flood effects were scale dependent and varied with channel constraint and the dominant channel forming process. Large changes were observed in unconstrained stream reaches where channel incision drove large changes in subsurface flow paths and the extent of the hyporheic zone. However, subreach scale differences were apparent. In the lower portion of the studied reach, channel incision lowered the water table, leading to abandonment of secondary channels, and decreased the extent of the hyporheic zone that previously extended more than 30 m into the floodplain. In contrast, the extent of the hyporheic zone increased at the head of the studied reach where channel incision steepened head gradients through a meander bend. In another unconstrained reach, lateral channel jumps dramatically altered exchange flow paths. However, the extensive hyporheic zone was maintained throughout the reach. Less change was observed in the constrained stream reach where both the depth and area of sediment available to be reworked by the flood were limited by bedrock constraining the width of the valley floor. This flood dramatically changed the hyporheic zone at the three study sites and these physical changes are expected to be biologically important, considering the role of the hyporheic zone in stream ecosystem processes.
Shinn et al. (2002) seem convinced that seepage meters are not a practical instrument to use in coastal environments for questions related to advection across the sediment-water interface. While their premise is certainly worth considering since so many scientific studies are tackling the issue of ground water discharge in recent decades, we disagree that they have sufficient evidence to support their conclusions. Their field experiments produced more technical questions for us about experimental design than answered questions concerning the magnitude of seepage rates in Florida Bay. The practicality of seepage meters as a measurement tool for ground water discharge is a persistent question. Seepage meters are easy to make and easy to use. Their simplicity alone worries some scientists. In many studies where multiple techniques have been applied or where control experiments have been performed in conjunction with field measurements, seepage meters provide consistent reliable results. We suggest some other questions to ask. What is the effect of altering the collection bag to seepage meter volume ratio on the anomalous short-term influx? Does that volume ratio control the magnitude of the hydraulic gradient between the bag and the seepage meter? How significant is the short-term influx when a 7-1 collection bag is placed in strong currents? How might this short-term influx be alleviated under different circumstances? How would the seepage results appear if tidal experiments were not performed as a net tidal effect, but instead evaluated seepage measurements on shorter time scales, such as less than a tidal cycle? The conclusions drawn by Shinn et al. (2002) are generalizations that are not very well supported by their experimental design or by their results. While their argument that Bernoulli's Principle affects seepage in this environment is not completely convincing, we agree that it may still be a contributing factor to some fraction of flow measurements. It is apparent (Fig. 1; Cable et al. 1997a; Chanton et al. 2003) that these devices respond to forces beyond the venturi effect. More quantitative work needs to be performed to assess this possible effect, as well as work to investigate its temporal and spatial variability and forces driving this flux. The measurements made by seepage meters, if made carefully and with some control experiments applied, are not artifacts. These measurements likely represent multiple water sources, such as infiltrating seawater and meteoric ground water fluxes. Mechanisms that drive pore water fluxes across the sediment-water interface may include wave-pumping (Bernoulli's principle applied), but experiments to test this mechanism and other possible driving forces need to be performed. Shinn et al. (2002) have given us all something to think about, but we suggest that some moderation of their conclusions is needed. We eagerly anticipate and actively work to elucidate the sources of water measured in coastal benthic advective studies.
Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m–2 yr–1 P and 694 mg m–2 yr–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m–2 yr–1 P), while sediment burial of N (1890–4027 mg m–2 yr–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.
The Floridan aquifer system of the SE USA is comprised of a thick sequence of carbonate rocks that are mostly of Paleocene to early Miocene age and that are hydraulically connected in varying degrees. The aquifer system consists of a single vertically continous permeable unit updip and of 2 major permeable zones (the Upper and Lower Floridan aquifers) separated by one of 7 middle confining units downdip. Neither the boundaries of the aquifer system or of its component high- and low-permeability zones necessarily conform to either formation boundaries or time-stratigraphic breaks. The rocks that make up the Floridan aquifer system, its upper and lower confining units, and a surficial aquifer have been separated into several chronostratigraphic units. The external and internal geometry of these stratigraphic units is presented on a series of structure contour and isopach maps and by a series of geohydrologic cross sections and a fence diagram. -from Author
Dissolved helium and radon anomalies are used to quantify groundwater input to Florida Bay waters. The method relies on the fact that groundwater dissolves large quantities of 4He and 222Rn isotopes, radioactive decay products of the uranium-chain elements, which accumulate over geological time periods. Seasonal surveys in Florida Bay show average helium concentration anomalies of 13.6% and 16.5% in the summer and winter, respectively. Excess 222Rn, in excess of that in equilibrium with 226Ra, was found to vary from 3 dpm.L-1 (disintegration per minute per liter) in the summer to 2 dpm.L-1 in the winter. The fact that such anomalies are present in the 1.5 m deep unstratified waters is a strong indication of groundwater input to the Bay. A simple box model based on helium data yields a groundwater flux of 2.5-4.0 cm.d-1 in the summer and 10-16 cm.d-1 in the winter while the same model results in a flux of 0.8-1.7 cm.d-1 using radon data. The difference between the flux figures obtained from helium and radon may be explained by the two-layer structure of the aquifer system underlying Florida Bay.
This chapter discusses the radon tracing of submarine groundwater discharge (SGD) in coastal environments. Direct discharge of groundwater into the coastal zone may be an important material flux pathway from land to sea in some areas. It has been largely ignored because of the difficulty in assessing its magnitude. While measurement problems persist, there is a growing recognition that groundwater flow into the sea is important. This chapter reviews an approach, using a simple one-dimensional model, to measure SGD via use of 222Rn as a natural tracer. Radon has certain advantages over other potential geochemical tracers of groundwater discharge. Typically, it is greatly enriched in groundwater compared to seawater; it can be measured at very low concentrations, and is completely conservative. On the other hand, as a gas it is subject to losses at the air–sea interface which may limit its use in shallow water environments. The radon tracing is an excellent qualitative tool for identifying areas of spring or seepage inputs in most coastal environments. It is a good quantitative tool in shallow marine environments characterized by large amounts of SGD.
Ahs tract Benthic fluxes of dissolved nutrients and manganese from biologically disturbed, relatively unpolluted sediment in Narragansett Bay, Rhode Island, have been measured. Analyses of the vertical gradients of chemical species dissolved in port waters and the uptake of 22Na from the overlying water permits evaluation of the contribution of biological advection and molecular diffusion to the transport of dissolved materials across the sediment-water interface. The activity of bottom-dwelling organisms appears to be about as important as molecular diffusion in most cases. The sum of the independently estimated contributions by both mechanisms is in good agreement with integrated benthic fluxes measured in situ. Sulfate and oxygen oxidize comparable amounts of organic matter in these sediments.
Vertical profiles of dissolved inorganic carbon in the Pacific show a well-defined concentration maximum at about 2.5-km depth due to oxidation and solution of particulate carbon from the surface. The ΣCO2 concentrations are inversely correlated with C14/C12 ratios (ΔC14 values), so that ΔC14 profiles have a mid-depth minimum but the absolute C14 concentration below 1.3 km is essentially constant. The ΣCO2 maximum is associated with a corresponding carbonate alkalinity maximum, and, in the South Pacific, with the deep oxygen minimum. In the North Pacific the O2 minimum lies above 1 km but the ΣCO2 and alkalinity maximum concentrations remain at about 2.5 km. The vertical profiles below the advective core of Intermediate Water are treated as stationary in the Eulerian sense, in a ‘z-diffusion water mass’ with vertical diffusion and advection, production from particulate flux, and radioactive decay. For a radioisotope, the mixing parameter K/w is obtained from salinity and/or potential temperature; the production or consumption ratio J/w is obtained from the stable isotope profile, and the upwelling velocity w from the profiles of absolute concentration of radioisotope. The ΣCO2 profiles in conjunction with geochemical estimates of the particulate flux also give a direct estimate of w ∼ 6 m/yr, consistent with estimates from various physical considerations. The CO2–O2 relationship approaches linearity only in the lowermost few hundred meters of the section because of the boundary-value effects. C14 profiles are consistent with w = 2 to 20 m/yr because of the scatter of individual measurements, but the observation that the concentration is essentially constant with depth gives w = 7 m/yr, K = 2 cm2/sec. Horizontal variations of C14 in bottom water are treated for a flowing open system affected by vertical diffusion from above, particulate flux or consumption, and radioactive decay. From 60°S to 40°N, at 3.5-km depth the absolute C14 concentration is constant, and the observed 50‰ decrease in ΔC14 is due to the increase in ΣCO2. The C14 concentration at all depths below 1.3 km and at all latitudes is constant because the production of C14 by particulate transport is approximately equal to the radioactive decay rate. Thus the vertical C14 gradient vanishes, and there is no vertical diffusive flux of radiocarbon. However, downward diffusion of stable carbon from the ΣCO2 maximum dilutes the specific activity of bottom water and constitutes a source of ‘dead carbon’ in the sea, which is continually added from above along any horizontal trajectory. The specific activity changes are therefore not the result of a ‘closed-system’ flow time as has previously been assumed in deriving bottom water flow velocities and do not give a direct record of elapsed flow time.
The fluxes of 226Ra (half-life = 1600 years) and 228Ra (half-life = 5.7 years) from the North Inlet salt marsh to the sea are much larger than can be supported by decay of their Th parents in the surface marsh sediments. These fluxes are sustained almost entirely by groundwater flow through the marsh. An average groundwater flow of approximately 10 cm3 cm−2 day−1 is indicated if the groundwater activities we have measured are representative. The fluxes of 223Ra (half-life = 11.4 day) and 224Ra (half-life = 3.6 day) are factors of 22, and ten more than those expected from the flux of 226Ra. Groundwater also sustains most of the flux of the short-lived isotopes.The measured Ra activity ratio pattern in the marsh creeks matches the groundwater signature but is distinct from the pattern of the parent thorium isotopes in the sediment. We present a model to explain the anomalous distribution pattern of these isotopes.Despite their large throughput, the inventories of desorbable 226Ra and 228Ra in the top 15 cm sediment layer are very low. Nevertheless, the activities of 226Ra and 228Ra in the porewaters are large, indicating a low distribution coefficient (∼10) for radium and a short retention time (∼10 days) in the surface sediment layer.We surmise that groundwater flow may be a significant source of radium isotopes in the waters of shallow estuaries and coastal margins. This source must be recognized while considering mass balance of any tracer, be it radium, nutrients, other metals, or δ18O.
The behavior of radon in the sea-floor region provides a useful test of theories which describe mass transport in sediments. We have made measurements of Rn-222 and Ra-226 in near-bottom waters and near-surface sediments at the same location. The distribution of radon in sediments can be described by a simplified diagenetic equation when advection, adsorption, and bioturbation are ignored. Sediment measurements show a radon deficit relative to radium emanation. A reasonable balance is found between integrated radon deficit in sediment and radon surplus in the overlying water.In most cores radium increased with depth in the top 10 cm of sediment. This implies that bioturbation and other mixing processes do not homogenize the radium concentration in the zone of diffusive radon loss, and that radium is diffusing out of the sediments.Radon leakage is less than that predicted by previous authors. Radon leakage depends upon the physical distribution of radium in marine sediments. We present a model that predicts leakage of 30–40% for normal deep-sea sediments, in agreement with measured values.Radon surplus in near-bottom waters depends upon the radium distribution, radon leakage, and effective diffusion coefficients. These in turn depend on the properties of the sediment, such as composition, accumulation rate, and porosity. As we learn how these factors interact we may be able to infer sedimentary features from measurements of radon in overlying waters.
Freshwater discharge to a shallow coastal embayment was measured with two upland hydrologic and three embayment physical methods for 2 yr. Parallel measurements from the five methods ranged from 3,900 (±630) to 9,400 (±3,400) m ³ d ⁻¹ , and four of the methods showed close agreement and averaged 4,800 (± 670) m ³ d ⁻¹ . The most precise estimate of discharge was from a chloride balance, while the best understanding of the rate and pattern of groundwater flow was from a Darcian streamtube approach.
Groundwater dominated the freshwater budget, accounting for >95% of the total annual input, and was partitioned almost equally between direct seepage to embayment waters and seepage to a stream with final discharge via surface flow. Freshwater inputs decreased rapidly toward the mouth of the estuary and >80% entered into the upper half. The lack of fixed watershed boundaries resulted in large errors in both the location and area of the topographically defined watershed when compared to a watershed defined by water‐table mapping. Seasonal variations were found in both the boundaries of the watershed (8%) and in groundwater discharge (6‐fold) in response to changing water‐table gradients due to recharge. Hydrologic alterations of the upland through the import of water and the increased recharge from impermeable surfaces led to an apparent increase in the total freshwater discharge to the embayment of nearly 50% over “natural” levels.
Because groundwater discharge along coastal shorelines is often concentrated in zones inhabited by fringing wetlands, accurately estimating discharge is essential for understanding its effect on the function and maintenance of these ecosystems. Most previous estimates of groundwater discharge to coastal wetlands have been temporally limited and have used only a single approach to estimate discharge. Furthermore, groundwater input has not been considered as a major mechanism controlling pore-water flushing. We estimated seasonally varying groundwater discharge into a fringing estuarine wetland using three independent methods (Darcy's Law, salt balance, and Br 2 tracer). Seasonal patterns of discharge predicted by both Darcy's Law and the salt balance yielded similar seasonal patterns with discharge maxima and minima in spring and early fall, respectively. They differed, however, in the estimated magnitude of discharge by two- to fourfold in spring and by 10-fold in fall. Darcy estimates of mean discharge ranged between 28.0 and 80 L m 22 d 21 , whereas the salt balance predicted groundwater discharge of 0.6 to 22 L m 22 d 21 . Results from the Br 2 tracer experiment estimated discharge at 16 L m 22 d 21 , or nearly equal to the salt balance estimate at that time. Based upon the tracer test, pore-water conductivity profiles, and error estimates for the Darcy and salt balance approaches, we concluded that the salt balance provided a more certain estimate of groundwater discharge at high flow (spring). In contrast, the Darcy method provided a more reliable estimate during low flow (fall). Groundwater flushing of pore water in the spring exported solutes to the estuary at rates similar to tidally driven surface exchange seen in previous studies. Based on pore-water turnover times, the groundwater- driven flux of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and NH to the estuary was 1 4 11.9, 1.6, and 1.3 g C or g N m 22 wetland for the 90 d encompassing peak spring discharge. Groundwater-induced flushing of the wetland subsurface therefore represents an important mechanism by which narrow fringing marshes may seasonally relieve salt stress and export material to adjacent water masses.
Laboratory and field tests revealed that there was an anomalous, short-term influx of water into plastic bags after they were attached to seepage meters. At Narrow Lake, Alberta, the anomalous, short-term (30 min) influx of water averaged 237 ml to bags that were initially empty, but the anomaly was effectively eliminated when bags were prefilled with 1000 ml of water before they were attached to seepage meters. The impact of the anomaly on calculated seepage rates was greatest when seepage rates were low, eg. 0.3 ml m-2 min-1. The anomaly may be due to mechanical properties of the bag, and it may be alleviated by partially filling bags before they are attached to seepage meters. -from Authors
Radon-222 is a good natural tracer of groundwater flow into the coastalocean.
Unfortunately, its usefulness is limited by the time consuming natureof collecting
individual samples and traditional analysis schemes. We demonstratehere an automated
system which can determine, on a “continuous”basis, the radon activity in coastal
ocean waters. The system analyses 222Rn from a constant
stream of water passing through an air-water exchangerthat distributes radon from
the running flow of water to a closed air loop.The air stream is feed to a
commercial radon-in-air monitor which determinesthe concentration of
222Rn by collection and measurement of theemitting
daughters, 214Po and 218Po,
via a charged semiconductordetector. Since the distribution of radon at equilibrium
between the air andwater phases is governed by a well-known temperature dependence,
the radonconcentration in the water is easily calculated.
The response of seepage meters was evaluated in a nearshore marine environment where water motion effects are more pronounced than in lake settings, where these meters have been used traditionally. Temporal and spatial variations of seepage, as well as potential artifacts, were evaluated using empty and 1000-ml pre-filled bag measurements. Time-series measurements confirmed earlier observations that anomalously high fluxes occur during the early stages (≤10min) of collection. As deployment times increased (30–60min), measured flow rates stabilized at a level thought to represent the actual seepage flux. Pre-filling the plastic measurement bags effectively alleviated this anomalous, short-term influx. Reliable seepage measurements required deployment times sufficient to allow a net volume of at least 150ml into the collection bag. Control experiments, designed by placing seepage meters inside sand-filled plastic swimming pools, served as indicators of external effects on these measurements, i.e. they served as seepage meter blanks. When winds were under 15 knots, little evidence was found that water motion caused artifacts in the seepage measurements. Tidal cycle influences on seepage rates were negligible in the present study area, but long-term temporal variations (weeks to months) proved substantial. Observed long-term changes in groundwater flux into the Gulf of Mexico correlated with water table elevation at a nearby monitoring well.
The use of seepage meters to identify nearshore seepage patterns and to quantify seepage in lakes was evaluated with a Monte Carlo simulation model. The model simulated seepage flux as would be derived from seepage meter measurements along a transect extending from the shore of a hypothetical lake to 40 or 100m offshore. Along the transect, simulated seepage velocities decreased exponentially with distance from shore according to patterns measured at Narrow Lake, Alta., and Lake Sallie, MN. To determine statistical parameters needed in the model, seepage flux was measured in situ with closely spaced seepage meters at four different sites in Narrow Lake. Seepage velocities within a small area of lakebed were log-normally distributed, and the variance was positively correlated with mean seepage flux. The modeling indicated that the most sensitive parameter affecting the accuracy of seepage meter estimates of seepage patterns and average seepage flux along the transect was the variability in the spatial distribution of seepage flux within a small area of lakebed. There was little improvement in the accuracy of estimates of seepage patterns or transect flux when more than ten seepage meters were simulated along the transect, when the transect was “sampled” more than twice, or when seepage meters along the transect were simulated to follow a stratified rather than a systematic design.
Measurements of the benthic flux of four naturally occurring radium isotopes in a shallow lagoon in the Bega River estuary has provided information on the types and rates of transport processes operating in the lagoon sediments. The measurement techniques included Ra mass budgets of the lagoon, Ra fluxes into benthic chambers, and modelling of the pore water and solid phase Ra profiles in a sediment core. The sediment profile of Pb-210, and the solid phase and pore-water profiles of the longer-lived Ra isotopes, Ra-228 (half-life 5.7 years) and Ra-226 (half-life 1600 years), indicate bioturbation to a depth of 10 cm. A diffusion-bioturbation model has been used to assess the relative importance of molecular diffusion and bioturbation as transport processes controlling the benthic Aux of Ra. The flux of the shortest-lived isotope, Ra-224 (half-lift: 3.7 days), is not significantly enhanced by bioturbation, and its Bur is consistent with diffusion-controlled release. However bioturbation enhances the Ra-228 Bur by a factor of more than two over the flux due to molecular diffusion alone. Modelled pore-water profiles and Bur calculations are consistent with a bioturbation time scale between 0.5 and 2 years. The measured benthic flux of Ra-226 is much greater than can be accounted for by the modelled profile, and may be due to slow Ra-226 desorption from the sediment, variable sediment accumulation rates, or groundwater flow. Based on Ra-226 pore-water and flux measurements at the time of this study, groundwater how has an upper limit of 0.3 cm d(-1). Copyright (C) 2000 Elsevier Science Ltd.
The activities and consequently the bioturbational effects of deposit-feeding organisms are largely restricted to a narrow surficial zone of marine sediments with a worldwide, environmentally invariant mean of 9.8 cm with a standard deviation of 4.5 cm. Currently available theories of infaunal behavior cannot predict quantitatively this observation. A new simple model that accounts for the feedback between resource (food) abundance, its reactivity, and the intensity of bioturbation leads to a quantitative estimate of 9.7 cm. This model constitutes a fundamental advance in our understanding bioturbation.
The direct discharge of groundwater into the coastal zone has received
increased attention in the last few years. We now know that this process
represents an important pathway for material transport between land and
sea. Groundwater discharge often contains higher concentrations of
dissolved nutrients and other components than does river water; thus, it
can play an important role in the health of coastal ecosystems.
Evaluation of seepage data from a network of 50 permanently deployed submarine seepage meters, specially constructed from fiberglass, indicates that the devices artificially advect (Bernoulli effect) shallow ground water. Reverse flow into the rock was not observed even when adjacent piezometers installed 2-m to 20-m below the rock-water interface indicated negative groundwater heads. Quantitative testing of five different designs, including conventional end-of-oil-drum designs, indicates that meters presenting positive relief on the sea floor are subject to the Bernoulli effect when placed in areas where there are waves and/or currents. Advection does not appear to be caused by flexing of the collection bags.
An automatic seepage meter using a heat pulse method was developed to obtain a continuous measurement of ground-water seepage rates. According to calibrations of the automatic seepage meter fitted with a 50 cm diameter collection funnel, seepage rates from 2 X 10-5 to 5 X 10-4 cm/sec can be obtained by measuring the time when the temperature as measured by a thermistor peaks after applying a heat pulse. The automatic seepage meter was used to measure continuous seepage rates into Lake Biwa, Japan. The ground-water seepage rate measured by the automatic seepage meter in Lake Biwa changed by six times within 12 hours. The automatic seepage meter is useful for surface-/ground-water studies, because a continuous seepage rate can be obtained without errors caused by the resistance of a collection bag to water flow.
Submarine ground water discharge to the ocean has the potential to create estuarine conditions near the point of discharge, thereby dramatically altering local benthic habitats and ecology. Aerial thermal infrared imaging along the southwestern margin of Delaware Bay indicated abundant discharge at Cape Henlopen, Delaware, adjacent to the Atlantic Ocean. On the sandflat there, we have documented low salinity in sedimentary pore waters within 20 m of the beachface that are associated with dense assemblages (in thousands per square meter) of a deep, burrow-dwelling polychaete worm, Marenzelleria viridis, otherwise regarded as a species characteristic of fresher, oligohaline conditions. Where present, M. viridis is a numerical and biomass dominant in a benthic community strikingly different from that in nearby nonseep locations. At Cape Henlopen, the ecological role of the ground water discharge appears to be a multifaceted one. Seeps are localized regions of significantly reduced salinity, stabilized temperature, increased nutrient flux, high microalgal abundance, and enhanced sediment stability. M. viridis feeds on sediment diatoms and may provide an important trophic linkage between microalgal growth fueled by nutrients associated with the discharging ground water and worm-feeding predators such as bottom fish or shorebirds common on the Cape Henlopen sandflat. Calculations based on our sampling suggest that nutrients supplied by the ground water substantially exceed what is needed to support benthic biomass and productivity estimated for this site.
Submarine ground water discharge is suggested to be an important pathway for contaminants from continents to coastal zones, but its significance depends on the volume of water and concentrations of contaminants that originate in continental aquifers. Ground water discharge to the Banana River Lagoon, Florida, was estimated by analyzing the temporal and spatial variations of Cl− concentration profiles in the upper 230 cm of pore waters and was measured directly by seepage meters. Total submarine ground water discharge consists of slow discharge at depths > ∼70 cm below seafloor (cmbsf) of largely marine water combined with rapid discharge of mixed pore water and estuarine water above ∼70 cmbsf. Cl− profiles indicate average linear velocities of ∼0.014 cm/d at depths > ∼70 cmbsf. In contrast, seepage meters indicate water discharges across the sediment-water interface at rates between 3.6 and 6.9 cm/d. The discrepancy appears to be caused by mixing in the shallow sediment, which may result from a combination of bioirrigation, wave and tidal pumping, and convection. Wave and tidal pumping and convection would be minor because the tidal range is small, the short fetch of the lagoon limits wave heights, and large density contacts are lacking between lagoon and pore water. Mixing occurs to ∼70 cmbsf, which represents depths greater than previously reported. Mixing of oxygenated water to these depths could be important for remineralization of organic matter.
We examined the importance of nitrogen inputs from groundwater and runoff in a small coastal marine cove on Cape Cod, MA, USA. We evaluated groundwater inputs by three different methods: a water budget, assuming discharge equals recharge; direct measurements of discharge using bell jars; and a budget of water and salt at the mouth of the Cove over several tidal cycles. The lowest estimates were obtained by using a water budget and the highest estimates were obtained using a budget of water and salt at the Cove mouth. Overall there was more than a five fold difference in the freshwater inputs calculated by using these methods.
Nitrogen in groundwater appears to be largely derived from on site septic systems. Average nitrate concentrations were highest in the region where building density was greatest. Nitrate in groundwater appeared to behave conservatively in sandy sediments where groundwater flow rates were high (> 11/m2/h), indicating that denitrification was not substantially reducing external nitrogen loading to the Cove.
Nitrogen inputs from groundwater were approximately 300 mmol-N/m3/y of Cove water. Road runoff contributed an additional 60 mmol/m3/y. Total nitrogen inputs from groundwater and road runoff to this cove were similar in magnitude to river dominated estuaries in urbanized areas in the United States.
Sandy subtidal sediments are part of the earth's largest filter system. Water flow through bottom sediments is driven by wave action on the sea surface. The mechanisms involved are described, including a theoretical deduction and field measurements. As an example, the total water exchange through part of the West Atlantic shelf is computed and the influence of the phenomenon is discussed from a biological point of view and with regard to its importance for the world's oceans.
We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth.
Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below.
The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.
Anthropogenic activities on coastal watersheds increase nutrient concentrations of groundwater. As groundwater travels downslope
it transports these nutrients toward the adjoining coastal water. The resulting nutrient loading rates can be significant
because nutrient concentrations in coastal groundwaters may be several orders of magnitude greater than those of receiving
coastal waters. Groundwater-borne nutrients are most subject to active biogeochemical transformations as they course through
the upper 1 m or so of bottom sediments. There conditions favor anaerobic processes such as denitrification, as well as other
mechanisms that either sequester or release nutrients. The relative importance of advective vs. regenerative pathways of nutrient
supply may result in widely different rates of release of nutrients from sediments. The relative activity of denitrifiers
also may alter the ratio of N to P released to overlying waters, and hence affect which nutrient limits growth of producers.
The consequences of nutrient (particularly nitrate) loading include somewhat elevated nutrient concentrations in the watercolumn,
increased growth of macroalgae and phytoplankton, reduction of seagrass beds, and reductions of the associated fauna. The
decline in animals occurs because of habitat changes and because of the increased frequency of anoxic events prompted by the
characteristically high respiration rates found in enriched waters.
Evaluation of seepage data from a network of 50 permanently deployed submarine seepage meters, specially construc ted from
fiberglass, indicates that the devices artificially advect (Bernoulli effect) shallow ground water. Reverse flow into the
rock was not observed even when adjacent piezometers installed 2-m to 20-m below the rock-water interface in dicated negative
groundwater heads. Quantitative testing of five different designs, including conventional end-of-oildrum designs, indicates
that meters presenting positive relief on the sea floor are subject to the Bernoulli effect when placed in areas where there
are waves and/or currents. Advection does not appear to be caused by flexing of the collection bags.
The flushing of Florida’s Indian River Lagoon is investigated as a response to tidal and low-frequency lagoon-shelf exchanges
in the presence of freshwater gains and losses. A one-dimensional computer model uses the continuity equation to convert water-level
variations into both advective transport within the lagoon and lagoon-shelf exchanges. The model also incorporates transport
by longitudinal diffusion. Flushing is quantified by calculating the 50% renewal time, R50, for each of 16 segments. R50 is calculated for tidal exchanges enhanced by 0–30 cm nontidal fluctuations in coastal sea level, then for a range of rainfall
rates. In both series of simulations, results suggest that in the northern sub-basin, R50 increases dramatically with distance from the inlet due to relatively weak tidal and nontidal exchanges. A 50% renewal occurs
in about one tidal cycle just inside Sebastian Inlet; at the northern end of the northern sub-basin, R50 is over 230 d, and only coastal sea-level variations on the order of 30 cm and/or dry season rainfall rates decrease R50 to less than 1 yr. R50 is 1 wk or less throughout the central and southern sub-basins, where lagoon-shelf exchanges occur through two inlets. Simulations
involving seasonal variations in precipitation and evaporation indicate that maximum and minimum rates of freshwater input
lead minimum and maximum salinities by time periods on the order of 2–3 wk for the lagoon as a whole and in the northern sub-basin.
The central and southern sub-basins respond in 1–2 wk.
Rainfall events cause episodic discharges of groundwaters contaminated with septic tank effluent into nearshore waters of
the Florida keys, enhancing eutrophication in sensitive coral reef communities. Our study characterized the effects of stormwater
discharges by continuously (30-min intervals) measuring salinity, temperature, tidal stage, and dissolved oxygen (DO) along
an offshore eutrophication gradient prior to and following heavy rainfall at the beginning of the 1992 rainy season. The gradient
included stations at a developed canal system (PP) on Big Pine Key, a seagrass meadow in a tidal channel (PC), a nearshore
patch reef (PR), a bank reef at Looe Key National Marine Sanctuary (LK), and a blue water station (BW) approximately 9 km
off of Big PIne Key. Water samples were collected at weekly intervals during this period to determine concentrations of total
nitrogen (TN), ammonium (NH4
+), nitrate plus nitrite NO3
− plus NO2
−), total phosphorus (TP), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP), and chlorophyll a (chl a). Decreased salinity immediately followed the first major rainfall at Big Pine Key, which was followed by anoxia (DO <0.1
mg I−1), high concentrations of NH4
+ (≈24 μM), TDP (≈1.5 μM), and chl a (≈20 μg I−1). Maximum concentration of TDP (≈0.30 μM) also followed the initial rainfall at the PC, PR, and LK stations. In contrast,
NH4
+ (≈4.0 μM) and chl a (0.45 μg I−1) lagged the rain event by 1–3 wk, depending on distance from shore. The highest and most variable concentrations of NH4
+, TDP, and chl a occurred at PP, and all nutrient parameters correlated positively with rainfall. DO at all stations was positively correlated
with tide and salinity and the lowest values occurred during low tide and low salinity (high rainfall) periods. Hypoxia (DO
<2.5 mg I−1) was observed at all stations follwing the stormwater discharges, including the offshore bank reef station LK. Our study
demonstrated that high frequency (daily) sampling is necessary to track the effects of episodic rainfall events on water quality
and that such effects can be detected at considerable distances (12 km) from shore. The low levels of DO and high levels of
nutrients and chl a in coastal waters of the Florida Keys demand that special precautions be exercised in the treatment and discharge of wastewaters
and land-based runoff in order to preserve sensitive coral reef communities.
Theoretical diffusive flux rates for dissolved reactive phosphate (DRP) were determined for sediments in a small area of the
Indian River, Florida for the period March–May 1982. Flux rates from the sediment varied from 29 to 70 × 10−6g per m2 per day in seagrass associated sediments to 3–25 × 10−6g per m2 per day for an area devoid of seagrass. Simultaneous measurements of groundwater seepage velocities indicated greater velocities
in seagrass associated sediments (1.03 × 10−6m per sec) than an area devoid of grass (0.77 × 10−6m per sec). Measured seepage flux accounted for more than 99% of the combined estimated diffusive and seepage flux of DRP
for nearshore seagrass sediments. Also noted was an apparent direct relationship between tidal height, DRP and seepage velocity
in nearshore sediments (25 m from shore) which further demonstrates the importance of hydrogeologic variables to these areas.
A hypothesis was tested to determine if a relationship exists between rates of submarine groundwater discharge and the distribution
of seagrass beds in the coastal, nearshore northeastern Gulf of Mexico. As determined by nonparametric statistics, four of
seven seagrass beds in the northeastern Gulf of Mexico had significantly greater submarine groundwater discharge compared
with adjacent sandy areas, but the remainder exhibited the opposite relationship. We were thus unable to verify if a relationship
exists between submarine groundwater discharge and the distribution of seagrass beds in the nearshore sites selected. A second
objective of this study was to determine the amount of nitrogen and phosphorus delivered to nearshore areas by submarine groundwater
discharge. We considered new nutrient inputs to be delivered to surface waters by the upward flux of fresh water. This upward
flux of water encounters saline porewaters in the surficial sediments and these porewaters contain recycled nutrients; actual
nutrient flux from the sediment to overlying waters includes both new and recycled nutrients. New inputs of nitrogen to overlying
surface waters for one 10-km section of coastline, calculated by multiplying groundwater nutrient concentrations from freshwater
wells by measured seepage rates, were on the order of 1,100±190 mol N d−1. New and recycled nitrogen fluxes, calculated by multiplying surficial porewater concentrations by measured seepage rates,
yielded fluxes of 3,600 ±1,000 mol N d−1. Soluble reactive phosphate values were 150±40 mol P d−1 using freshwater well concentrations and 130±3.0 mol P d−1 using porewater concentrations. These values are comparable to the average nutrient delivery of a small, local river.
Atlantic tidal fluctuations drive pressure head variations in shallow offshore wells drilled into the limestone subsurface on both the Florida Bay and Atlantic sides of Key Largo, Florida, USA. We tested the hypothesis that these pressure head variations influence groundwater flow and that flux rate variability is associated with tidal variability. We used an automated Rn monitor to make continuous measurements of 222Rn, a natural tracer of groundwater discharge, in Florida Bay waters. We also deployed three types of seepage meters, including an automated heat pulse meter to collect a continuous record of seepage from the sediments. Drum type seepage meters inserted into soft sediments and fiberglass meters cemented to the rocky bay floor were utilized with pre-filled 4-l bag collectors, and monitored on an hourly basis. Maximum Rn inventories in Florida Bay waters were associated with high tide on the Atlantic side of the island. Modeling of the Rn variation indicated variable groundwater discharge rates with maximum flux occurring at high Atlantic tide. Seepage meter results in Florida Bay were consistent with 222Rn modeling. Florida Bay seepage meter rates showed positive correlation with Atlantic tide, meter 1, r=0.63, n=12, p0.025 and meter 2, r=0.67, n=12, p0.025. A seepage meter offshore of the Atlantic side of Key Largo exhibited rates that were inversely correlated with Atlantic tide (r=0.87, n=9, p0.005) showing negative rates when the tide was high, and positive rates when the tide was low. Overall, our results are consistent with the hypothesis of Reich et al. (2002), that pressure head variations driven by Atlantic tide influence groundwater seepage rate variability in Florida Bay off Key Largo. Effectively, as proposed by Reich et al. (2002), Key Largo functions as a semi-permeable dam separating Florida Bay and the Atlantic Ocean.
An assessment of developing eutrophic conditions in small temperate lagoons along the coast of Rhode Island suggests that
in such shallow, macrophyte based systems the response to nutrient enrichment differs from that described for plankton based
systems. The nitrogen loadings per unit area of the salt ponds are 240–770 mmol N per m2 per year. Instead of the high nutrient concentrations, increased phytoplankton biomass and turbidity, leading to eventual
loss of benthic macrophytes described for such systems as the Chesapeake, Patuxent and Appalachicola Bay, nutrient enrichment
of the Rhode Island lagoons has led to increased growth of marine macroalgae. The increased macroalgal growth appears to alter
the benthic habitat and a shift from a grazing to detrital food chain appears to be impacting important shellfisheries. As
more extensive areas of organic sediments develop, geochemical cycling changes, resulting in higher rates of nitrogen remineralization
and accelerated eutrophication. The major sources of nitrogen inputs to the salt ponds have been identified and a series of
management initiatives have been designed to limit inputs from present and potential development within the watersheds of
the lagoons.
The Peel Inlet in Western Australia was used to study the cause of eutrophic conditions in an estuary. In addition to large quantities of nitrogen and phosphorus (at low concentrations) entering the inlet from rivers and drains from agricultural areas, the urban contribution via groundwater was identified. The average nitrogen concentration of urban groundwater under an area serviced with septic tank systems was 12 parts 10−6. Because of the nature of the soils, very few groundwater samples contained appreciable phosphorus concentrations. However, one area close to the inlet had semiconfined groundwaters with nitrogen and phosphorus concentrations as high as 100 and 0·4 parts 10−6 respectively. An obsolete dumping site for human excreta was also identified near the urban area, and this is suspected of having contributed to very high concentrations of nitrogen and phosphorus in groundwater which may have already reached the estuary and caused excessive algal growth. Groundwater contours showed the potential for groundwater movement from part of the urban area and also from the obsolete dumping site.