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Field Observations of Tidal and Seasonal Variations in Ground Water Discharge to Tidal Estuarine Surface Water
Abstract
A field study of the tidal and seasonal variations in upland, shoreline, and nearshore hydrological processes associated with ground water discharge from the unconfined Columbia Aquifer was conducted. The study, performed along a tidal subestuary of the Chesapeake Bay, involved measurement of water table elevation, ground water discharge, potentiometric had differentials across the sediment‐water interface, and ground water salinity. Fresh ground water discharge rates calculated from the measured water table gradient using the Dupuit assumptions varied from 6.1 X 10 ‐3 to 3.8 X 10 ‐2 m ³ /day per m of shoreline during the study period, May 1994 to September 1995. Variation in discharge rates were associated with seasonal recharge patterns.
Integrated total discharge rates based on seepage meter measurements decreased with distance offshore from a maximum of 3.3 L/m ² hr at the shoreline to 0.5 L/m ² hr at 30 m offshore Maximum instantaneous discharge rates calculated from potentiometric head differentials were much higher than integrated discharge rates and ranged from 19.2 L/m ² hr at 4.8 m offshore to 0.8 L/m ² hr at 19.8 m offshore. Instantaneous discharge rates were inversely correlated to tidal elevation and fluctuated rapidly decreasing from 19.2 L/m ² hr to 2.5 L/m ² hr during a several‐hour period.
Seasonal variations in salinity patterns within the transition zone of the Columbia Aquifer were observed and indicated a dependence on fresh ground water discharge and surface water salinity/density gradient was observed within the transition zone. The gradient was created, in part, by the infiltration of surface water into tidally exposed sediments. Seasonal periods of low fresh ground water discharge and high surface water salinity were associate with intrusion of the surficial mixing zone landward while seasonal periods of high fresh ground water discharge and low surface water salinity were associated with a seaward movement of the mixing zone.
... Fresh coastal groundwater is driven seaward by an inland hydraulic gradient, while density gradients along the lower Ghyben-Herzberg interface create convective seawater circulation that contributes a portion of saline SGD (Figure 1) [Cooper, 1959;Kohout, 1960]. In some beaches, a smaller, but chemically significant, intertidal cell of rapidly circulating saline water occurs landward of the zone of fresh SGD [Robinson et al., 1998;Michael et al., 2005;Robinson et al., 2007;Seidel et al., 2015;Beck et al., 2017]. Through tide and wave action, seawater moves up the beachface and infiltrates into the sand, where it circulates and mixes with the seaward-moving fresh groundwater. ...
... The intertidal circulation cell is highly dynamic and has been shown to respond to hydrologic, geologic, and topographic changes over various timescales (waves, tides, seasons) [Robinson et al., 1998;Robinson et al., 2006;Robinson et al., 2007;Bakhtyar et al., 2013;. Therefore, the flowpaths that control the delivery and availability of reactive particles and solutes into the circulation cell are also both spatially and temporally variable. ...
... As the driving forces and physical characteristics of the cell change, the extent and patterns of chemical reactions within and around the cell are expected to respond in concert. While the dynamic nature of the physical properties of the cell has been described in detail [Robinson et al., 1998;Michael et al., 2005;Robinson et al., 2007;, there are fewer observations concerning the location and relative rates of chemical reactivity within the cell and their relations to the patterns of mixing. ...
Beach aquifers host a dynamic and reactive mixing zone between fresh and saline groundwater of contrasting origin and composition. Seawater, driven up the beachface by waves and tides, infiltrates into the aquifer and meets the seaward-discharging fresh groundwater, creating and maintaining a reactive intertidal circulation cell. Within the cell, land-derived nutrients delivered by fresh groundwater are transformed or attenuated. We investigated this process by collecting pore water samples from multilevel wells along a shore-perpendicular transect on a beach near Cape Henlopen, Delaware, and analyzing solute and particulate concentrations. Pore water incubation experiments were conducted to determine rates of oxygen consumption and nitrogen gas production. A numerical model was employed to support field and laboratory interpretations. Results showed that chemically sensitive parameters such as pH and ORP diverged from salinity distribution patterns, indicating biogeochemical reactivity within the circulation cell. The highest respiration rates were found in the landward freshwater-saltwater mixing zone, supported by high dissolved inorganic carbon. Chlorophyll a, a proxy for phytoplankton, and particulate carbon did not co-occur with the highest respiration rates but were heterogeneously distributed in deeper and hypoxic areas of the cell. The highest rates of N2 production were also found in the mixing zone coinciding with elevated O2 consumption rates but closer to the lower discharge point. Model results were consistent with these observations, showing heightened denitrification in the mixing zone. The results of this work emphasize the relationship between the physical flow processes of the circulation cell and its biogeochemical reactivity and highlight the environmental significance of sandy beaches.
... The impact of tidal activity has long been recognized (Kohout 1964;Cooper 1964). For coastal and small island aquifers, the effect of tides (Harrison et al., 1971;Inouchi et al., 1990;Erskine, 1991;Aseervatham,1994;Robinson et al., 1998) and ...
... Only a few field and numerical studies (Stuyfzand, 1993;Robinson et al., 1998;Ataie-Ashtiani et al, 1999) have been done on the effects of geological heterogeneity, beach geometry and transient boundary conditions (tides and waves) on the salinity pattern in the near-shore groundwater. ...
... Field studies in unconfined coastal aquifers have shown that the patterns of groundwater discharge have been significantly influenced by the tide (Lewis 1987;Reay et al., 1992;Robinson et al., 1998;Uchiyama et al., 2000). ...
A field study of a 350m sand dune aquifer system has been carried out on the northern part of New South Wales, Australia. Bundled piezometers and monitoring wells were installed to measure fluid electrical conductivity and hydraulic heads. A FEFLOW model was developed to incorporate the varying head and salinity conditions on the model boundary.
Model results show the dynamic flow patterns of the groundwater caused by the tide and enhanced by the wave activity near the coast. Vertical head measurements and field observations were used to constrain the model parameters. Model simulations established the interaction of the sand dune aquifer with rainfall recharge, regional influx from nearby marsh and tidal creek. A geologic formation of low permeability significantly modified the groundwater flow patterns. A conceptual model, which features flow and salinity structure in the coastal sand dune was developed.
... The aquifer that the study will be based around is located on the North West coast of England and is shown on the map below. Groundwater discharge from unconfined aquifers to coastal waters can be a complex system, characterised by a fluctuating water table due to tidal activity, density-dependant flow along the saline/freshwater transition zone, and a dynamic ground water discharge region within the inter tidal zone (Robinson et al, 1998). An idealised sketch of the saline/freshwater interface can be seen in figure 1.2. ...
... The variability in the results from these data sets meant that the data collected showed very little. However, one of the things that it did show was that although it is often said that the tidal cycle follows a sine wave, "In coastal aquifers in contact with the ocean, sinusoidal fluctuations of groundwater levels occur in response to tides" (Todd, 1980), results from this experiment and others done by Robinson et al, (1998) suggest otherwise. The tidal measurements made at Rossall beach show an asymmetric pattern in the cycle (displayed in Chapter 3, figure 3.3.3.). ...
... This was calculated using the data provided by Her Majesties Coastguard Liverpool, shown in Appendix 3, figure A3.1. This pattern is due to the asymmetry of the tidal infiltration/draining process: it is easier for the water to flow into the unsaturated sediments at high tide than to drain away at low tide (Nielsen, 1990) (Robinson et al, 1998). This long recession as the tide recedes from high to low tide can be seen in data that was produced by Robinson et al, (1998) that is shown below in figure 5.1. ...
... Field and numerical studies have quantified the tidal effect on SGD and showed that tide-induced recirculation can represent a major portion of total SGD (e.g., Li et al., 1999;Taniguchi, 2002;Robinson et al., 2007c;Li et al., 2009). Seawater infiltration into an aquifer occurs on the rising tide and exfiltration (saline SGD) on the ebbing tide (Robinson et al., 1998;Sholkovitz et al., 2003;Robinson et al., 2007c). Infiltration is typically greatest from the mid-tide to high tide mark and exfiltration is greatest towards the low tide mark (Robinson et al., 2007b). ...
... When the instantaneous tide-induced water exchange and groundwater flows are averaged over a tidal cycle, seawater recirculation flow paths extend across the intertidal zone. This time-averaged recirculating flow can lead to the formation of an upper saline plume (USP, Fig. 1) (Robinson et al., 1998;Boufadel, 2000;Mango et al., 2004;Vandenbohede and Lebbe, 2005;Robinson et al., 2007b). Where the USP is present, FSGD travels below the USP and discharges near the low tide mark in a zone bounded by the USP and the saltwater wedge. ...
Sustainable coastal resource management requires sound understanding of interactions between coastal unconfined aquifers and the ocean as these interactions influence the flux of chemicals to the coastal ocean and the availability of fresh groundwater resources. The importance of submarine groundwater discharge in delivering chemical fluxes to the coastal ocean and the critical role of the subterranean estuary (STE) in regulating these fluxes is well recognized. STEs are complex and dynamic systems exposed to various physical, hydrological, geological, and chemical conditions that act on disparate spatial and temporal scales. This paper provides a review of the effect of factors that influence flow and salt transport in STEs, evaluates current understanding on the interactions between these influences, and synthesizes understanding of drivers of nutrient, carbon, greenhouse gas, metal and organic contaminant fluxes to the ocean. Based on this review, key research needs are identified. While the effects of density and tides are well understood, episodic and longer-period forces as well as the interactions between multiple influences remain poorly understood. Many studies continue to focus on idealized nearshore aquifer systems and future work needs to consider real world complexities such as geological heterogeneities, and non-uniform and evolving alongshore and cross-shore morphology. There is also a significant need for multidisciplinary research to unravel the interactions between physical and biogeochemical processes in the STE, as most existing studies treat these processes in isolation. Better understanding of this complex and dynamic system can improve sustainable management of coastal water resources under the influence of anthropogenic pressures and climate change.
... Because of the complex nature of coastal aquifer systems, flow and transport in such aquifer systems are driven primarily by transient fresh/seawater-level variations, salinity distributions, shoreline morphologies, heterogeneity of hydraulic properties, and various types of pollutants (Mao et al. 2006;Pool et al. 2015). Accurate measurements and simulations are essential for predicting contaminant migrations in near-shore groundwater systems (Robinson et al. 1998). ...
... Their results provided evidence that the USP was mainly driven by tidal oscillations, which resulted from water infiltrates into seawater-covered beach when the high tide and exfiltrates when the low tide. The USP and SI wedge confine a freshwater discharge zone in which freshwater discharges near the low-tide mark, so that USP is considerably as a part of submarine groundwater discharge in near-shore aquifer systems (Robinson et al. 1998;Taniguchi et al. 2002;Vandenbohede and Lebbe 2006;Robinson et al. 2006;Robinson et al. 2007bRobinson et al. , 2009. Improved models incorporating ocean forcing have been recently proposed to quantify the influence that ocean waves exert on the behaviors of USPs (e.g., Xin et al. 2010;Robinson et al. 2014). ...
This paper presents numerical investigations on quantifying the hydrodynamic effects of coastal environment factors, including tidal fluctuations, beach slopes, hydraulic conductivity, and hydraulic gradients on sea-derived benzene transport in unconfined coastal aquifers. A hydrologic transport and mixed geochemical kinetic/equilibrium reactions in saturated-unsaturated media model was used to simulate the spatial and temporal behaviors of the density flow and benzene transport for various hydrogeological conditions. Simulation results indicated that the tidal fluctuations lead to upper saline plumes (USPs) near the groundwater and seawater interfaces. Such local circulation zones trapped the seaward benzene plumes and carried them down in aquifers to the depth depending on the tide amplitudes and beach slopes across the coastal lines. Comparisons based on different tidal fluctuations, beach slopes, hydraulic conductivity, and hydraulic gradient were systematically conducted and quantified. The results indicated that areas with USPs increased with the tidal amplitude and decreased with the increasing beach slope. However, the variation of hydraulic conductivity and hydraulic gradient has relatively small influence on the patterns of flow fields in the study. The increase of the USP depths was linearly correlated with the increase of the tidal amplitudes. The benzene reactive transport simulations revealed that the plume migrations are mainly controlled by the local flow dynamics and constrained in the USP circulation zones. The self-cleaning process of a coastal aquifer is time-consuming, typically requiring double the time of the contamination process that the benzene plume reach the bottom of a USP circulation zone. The presented systematic analysis can provide useful information for rapidly evaluating seaward contaminants along a coastal line with available hydrogeological properties.
... Wrenn et al. [1997] reported salinity measurements in two beaches in Maine, and were the first to note an inverted salinity distribution in the intertidal zone, whereby the saltwater overlays freshwater. Robinson et al. [1998] presented field observation of seasonal salinity variations in a coastal unconfined aquifer that depended on fresh groundwater discharge and surface water salinity/density gradient. Boufadel [2000] used experiments and numerical modeling using the MARUN model to analyze the salinity distribution in a laboratory beach. ...
... This implies that the freshwater flow reaching the interface at 40 m is considerably reduced and is not capable of clearing of path "the tube" before the tide rises again. Thus, the salinity distribution in this gravel beach is unlike studies in sandy beaches [Boufadel, 2000;Robinson et al., 1998]. A similar salinity distribution was observed in a two-layered beach in Prince William Sound . ...
[1] This paper investigated the interaction of groundwater and seawater in a tidally influenced gravel beach. Field observations of water table, pore water salinity were performed. The two-dimensional finite element model MARUN was used to simulate observed water table and salinity. Based on field observations and model calibrations, a two-layered beach structure was identified which is characterized by a high-permeability surface layer underlain by a low-permeability lower layer. The salt wedge seaward of the low tide line was almost invariant in comparison with the strong fluctuations of the salinity plume in the surface layer of the intertidal zone. The presence of the two layers prevented the presence of a freshwater discharge “tube” between the upper saline plume and salt wedge. This is in contrast with the previous works where freshwater discharge tube was observed. The tide-induced submarine groundwater discharge (SGD) was estimated at 9 m3 d−1 m−1, a large value that is probably due to the large tidal range of ∼4.8 m and the very permeable surface layer. The freshwater-seawater dynamics revealed here may provide new insights into the complexity, intensity, and time scales of mixing between fresh groundwater and seawater in tidal beaches. The simulated water table of the beach was higher than the interface between the surface layer and the lower layer, which prevented Exxon Valdez oil from penetrating into the lower layer in 1989.
... This can be achieved by the installation of piezometers and seepage meters to measure hydraulic conductivity [10][11][12][13][14]. Piezometers can often puncture the interface, bypassing thin surficial layers. Therefore, sediment cores can be used to measure the hydraulic conductivity across the SWI through a laboratory test and/or grain size analysis; however, protocols differ greatly by site location [6,15,16]. During SWI grain size analysis, the organic matter must be considered because the grain size properties are not conservative and can change during analysis (e.g., loss of organic material). ...
Stormwater ponds are intended to be used for mitigating floods, improving water quality, and recharging groundwater. The sediment-water interface (SWI) of stormwater ponds exhibits properties that influence surface water–groundwater exchanges similar to naturally occurring surface water bodies. However, these ponds are rarely monitored over time to account for their functionality. As organic and inorganic sediments accumulate on the pond bed, the ability of the SWI to conduct water is influenced by sediment deposition, accumulation, and compaction, as well as organic matter content and other biological processes. Two augmented methods, a sediment core permeability cell and an in situ aluminum tube and manometer, were evaluated for measuring the hydraulic conductivity of the SWI. The grain size, hydraulic conductivity, and percentage of organic matter were compared between two ponds constructed 22 years apart. Both methods were effective at measuring the hydraulic conductivities, especially in challenging encountered field situations, albeit with some shortcomings. The in situ method yielded data from sediments with low hydraulic conductivities due to thermal heating, expansion of the water, and the release of biogenic-derived gas from the sediments within the aluminum tube. The converted sediment core permeability cells generated the most consistent measurements. Grain size and hydraulic conductivities were correlated to pond age. The mean and effective grain sizes, as well as hydraulic conductivities of the older pond, were statistically lower than the younger pond in both shallow and deeper depths. Measurement of the changes in the SWI of stormwater ponds is important to protect urbanized areas from flood damage, control the quality and quantity of runoff, and maintain their groundwater recharge function.
... The presence of a freshwater-saltwater interface (FSI) in coastal aquifers affects the dynamic response of the freshwater balance to cyclical climate variability at timescales corresponding to daily tidal cycles [6][7][8], seasonal recharge oscillations [9][10][11][12], and Atlantic multidecadal oscillation (AMO) [13]. Seasonal oscillations in recharge (R) are reflected in measured or modeled submarine groundwater discharge at the coastline, but with time lags (∆t) of up to five months attributed to the delayed response of the FSI to the R signal [10,14]. ...
The dynamic behavior of the freshwater-saltwater interface (FSI) in coastal aquifers can introduce unexpected lags between recharge and stream discharge, especially when recharge is forced by long-term cyclical precipitation patterns. This work seeks to assess these FSI impacts at the watershed scale. Recharge-discharge time lags were evaluated in 68 watersheds overlying the Floridan Aquifer System in the coastal region of the southeastern United States (Florida, Georgia, and South Carolina). Utilizing the strength of the Atlantic multidecadal oscillation (AMO) signal in this region, 10–20 year averaged recharge and discharge time series were used for the selected watersheds. Lags of 10–25 years between recharge and discharge were found in 16% of the basins considered, possibly induced by a dynamic FSI which responded slowly to the AMO-scale recharge signal. Freshwater storage coefficients (S) were estimated from time series of change-in-storage and groundwater level, with 11 basins showing S>1.5 indicating water storage well above that expected for unconfined aquifers. These 11 basins with both multidecadal recharge-discharge time lags and high S values showed a positive linear relationship between time lag and FSI depth with slope 0.016 yr/m (R-squared = 0.30). These large time lags may be directly impacting the management of these basins as they obscure water and solute mass balances in the southeastern US.
... The effect of tidal oscillations on the freshwater-saltwater interface (FSI) has been the subject of far fewer studies. Sea tides impact the width of the FSI and groundwater discharge [11][12][13]. Ataie-Ashtiani et al. [14] showed that tidal oscillations favor seawater intruding further inland and result in a wider FSI than what would occur from the tidal effect alone. La Licata et al. [15] compared the results of solute transport in the transition zone with and without tidal effects, concluding that contaminant and salinity concentration are more mixed under the influence of tidal variations. ...
The temperature distribution of shallow sectors of coastal aquifers are highly influenced by the atmospheric temperature and recharge. However, geothermal heat or vertical fluxes due to the presence of the saline wedge have more influence at deeper locations. In this study, using numerical models that account for variable density, periodic oscillations of temperature have been detected, and their origin has been attributed to the influence exerted by recharge and tides. The combined analysis of field data and numerical models showed that the alternation of dry and wet periods modifies heat distribution in deep zones (>100 m) of the aquifer. Oscillations with diurnal and semidiurnal frequencies have been detected for groundwater temperature, but they show differences in terms of amplitudes and delay with electrical conductivity (EC). The main driver of the temperature oscillations is the forward and backward displacement of the freshwater–saltwater interface, and the associated thermal plume generated by the upward flow from the aquifer basement. These oscillations are amplified at the interfaces between layers with different hydraulic conductivity, where thermal contours are affected by refraction.
... Groundwater discharging across the seabed (submarine groundwater discharge; SGD) consists of fresh, brackish, and saline groundwater (Robinson et al., 1998;Taniguchi et al., 2019;Santos et al., 2021). Nutrient concentrations in SGD are often higher than concentrations in riverine surface water, thus SGD fluxes can negatively affect coastal water quality, contribute to eutrophication, and compromise coastal marine ecosystems (Johannes, 1980;Moore, 1999;Slomp and Van Cappellen, 2004;Guo et al., 2020). ...
Dissolved inorganic carbon (DIC) and total alkalinity (AlkT) fluxes to the nearshore ocean can directly impact the rates of primary production, coral reef formation, coastal ocean acidification, and continental shelf ecology. Current understanding of the transformations that DIC and AlkT undergo as they move from land to sea are limited, leading to difficulties in estimating future DIC and AlkT export that may be altered under a changing climate. While much research has focused on carbon fluxes in carbon-rich mangroves and coastal wetlands, DIC and AlkT transformations and distributions in sandy beach aquifers, which are comparatively carbon-poor, have not been studied as extensively. We monitored DIC and AlkT concentrations in a sandy beach system over six sampling events spanning two years. Substantial changes to DIC and AlkT occurred along subsurface flowpaths due to aerobic respiration and anoxic reactions, resulting in an additional mean flux to the ocean of 191 and 134 mmol/d per meter length of shoreline, respectively. The chemical alterations occurred within the saltwater-freshwater mixing zone beneath the beach surface. Both aerobic and anaerobic reactions actively contributed to DIC and AlkT production within the system, as indicated by DIC: AlkT and dDIC:dAlkT ratios relative to the theoretical dilution line. The work indicates that beach aquifers support active transformation of inorganic carbon and highlights a potentially important and overlooked source of DIC and AlkT to coastal systems.
... Intrinsically linked groundwater and surface water systems in coastal ecosystems represent interaction and transformation zones and these are widely reported (Glover 1959, Simpson et al 2003. Groundwater discharge from aquifers as well as infiltration of saline water into sediment is induced by tidal activity in other estuarine ecosystems such as in the Chesapeake Bay and the Swan-Canning estuary (Robinson et al 1998, Smith and Turner 2001, Acworth and Dasey 2003. Groundwater discharge is also recognized as a significant source of nutrients such as for nitrogen and phosphorus into coastal systems (Simmons, 1992, Krest et al 2000, Moore et al 2002, Hwang et al 2005. ...
The Bay of Bengal receives nitrogen inputs from multiple sources and the potential role of nitrogen-metabolizing microbial communities in the surface water is not well understood. The nitrogen budget estimate shows a deficit of 4.7 ± 2.4 Tg N yr-1, suggesting a significant role of dissolved organic nitrogen remineralization in fuelling ecosystem processes. Unravelling the process of remineralization leading to increasing concentrations of dissolved inorganic nitrogen (DIN) in coastal ecosystems such as mangroves require a better understanding of the composition of functional resident bacterioplankton communities. Bacterioplankton communities were elucidated from eight stations along different estuaries spanning west to east of northeast coastal Bay of Bengal to understand the influence of DIN on shaping these communities. The eight stations were differentiated into ‘low’ and ‘high’ DIN stations based on a DIN concentration, with five stations with High DIN concentration (>45 µM) and three stations with Low DIN concentration (<40µM). The V3-V4 region of 16S rRNA was amplified and sequenced to elucidate resident bacterioplankton community structure from environmental DNA. Proteobacteria, Bacteroidetes, and Firmicutes were the dominant bacterioplankton phyla across all stations. Nitrogen-fixing groups such as Nitrospirae, Lentisphaerae, Chloroflexi, and Planctomycetes make up about 1% of the bacterioplankton communities. Abundances of Spirochaetes and Tenericutes showed a positive correlation with DIN. Pseudomonadales, Alteromonadales, and Desulfovibrionales were found to distinctly vary in abundance between low and High DIN stations. Predicted metagenomic profiles from taxonomically derived community structures indicated bacterial nitrate-nitrite reductase to be negatively correlated with prevalent DIN concentration in High DIN stations but positively correlated in Low DIN stations. This trend was also consistent for genes encoding for nitrate/nitrite response regulators and transporter proteins. This indicates the need to delineate functional bacterioplankton community structures to better understand their role in influencing rates and fluxes of nitrogen within mangroves.
... Tidal activity can often induce a fluctuating water table as well as infiltration of surface water into sediments, forming a surficial mixing zone with groundwater discharging from the adjacent aquifer (Robinson et al. 1998;Ataie-Ashtiani et al. 1999;Zhou et al. 2006). The groundwater behavior in an unconfined aquifer with a mild sloping face in response to a tide is affected by two factors that do not exist in a confined aquifer. ...
Groundwater in the coastal area is relatively vulnerable to the contamination by seawater intrusion, which makes it unsuitable for drinking or irrigation. This study was carried out along the coastal aquifer of southern Tamil Nadu, India. To access the impact of mining, the coast was divided into three sites, namely (i) non-placer sand area, (ii) active mining area and (iii) inland dune area. The non-placer sand area is assumed to be undisturbed and is considered as reference site. The inland dunes sand area is the area with no active mining but within the impact of the mining activity. Hydrogeochemical and groundwater table characteristics of shallow coastal aquifer system in the mining and non-mining area were investigated to identify the salinization process. The Na/Cl ratio, correlation matrix and ionic relationship between major ions showed a marked increase in salinization in the active mining area and nearby wells. The reverse ion exchange and seawater intrusion control the groundwater chemistry along the active mining aquifers. The spatial visualization of electrical conductivity, salinity, chloride and groundwater quality index map (GWQI) that reflects active mining areas are exhibiting poor water quality and are comparatively low in non-placer mining areas. Gibb’s diagram representing evaporation is the dominant process more than the rock water interaction and precipitation. The groundwater level fluctuation in both inland dune and non-placer sand area aquifers blocks a little variation due to lack of rainfall, irrespective of volumes of water recharge and over pumping of groundwater for irrigational purposes. In the active mining region, the groundwater level shows high fluctuation of ±3 to ±5 m below the ground level. Depth profile study indicates the highly depleted groundwater level in the active mining region that has induced higher EC value and salinity. The process might, therefore, be related to the saltwater encroachments. Tidal induces changes in water level in the Karamaniyar river estuary and near the active mining wells around 1.03 m and the Vembar river estuary water level increased in the wells around 0.68 m.
... These gradients were estimated at mid-to-low tide, and would be expected to vary tidally as observed elsewhere in this region (Beck et al. 2016). Groundwater discharge was calculated according to the Darcy equation (v = K h [dh/dl]), assuming a regional hydraulic conductivity (K h ) of 0.001 cm/s (Gallagher et al. 1996;McFarland and Bruce 2006;Robinson et al. 1998). A seepage face of 5 m width was estimated. ...
... Fresh groundwater in coastal aquifers mixes with saline groundwater prior to its discharge to the ocean as submarine groundwater discharge (SGD; Li et al., 2008;Michael et al., 2005;Robinson et al., 1998Robinson et al., , 2007. SGD includes the discharge of fresh groundwater (blue, Figure 1), convectively recirculated seawater at the base of the beach (lower interface, Figure 1), and circulating brackish water due to tide and wave activity on the beach (intertidal circulation cell, Figure 1). ...
Biogeochemical reactions within intertidal zones of coastal aquifers have been shown to alter the concentrations of terrestrial solutes prior to their discharge to surface waters. In organic‐poor sandy aquifers, the input of marine organic matter from infiltrating seawater supports active biogeochemical reactions within the sediments. However, while the seasonality of surface water organic carbon concentrations (primary production) and groundwater mixing have been documented, there is limited understanding of the transience of various organic carbon pools (pore water particulate, dissolved, sedimentary) within the aquifer and how these relate to the location and magnitudes of biogeochemical reactions over time. To understand the relationship between changes in groundwater flow and the seasonal migration of geochemical patterns, beach pore water and sediment samples were collected and analyzed from six field sampling events spanning 2 years. While the seasonally dynamic patterns of aerobic respiration closely followed those of salinity, redox conditions and nutrient characteristics (distributions of N and P, denitrification rates) were unrelated to contemporaneous salinity patterns. This divergence was attributed to the spatial variations of reactive particulate organic carbon distributions, unrelated to salinity patterns, likely due to filtration, retardation, and immobilization dynamics during transport within the sediments. Results support a “carbon memory” effect within the beach, with the evolution and migration of reaction patterns relating to the distribution of these scattered carbon pools as more mobile solutes move over less mobile pools during changes in hydrologic conditions. This holds important implications for the prediction and quantification of biogeochemical reactions within beach systems.
... The temporal fluctuations of flow caused by tidal oscillations also significantly affect the dynamic behaviors of salt water transport in coastal areas. The tidal action across a sloping beach surface drives sea water into the beach aquifer at high tidal level, forming a salt water circulation above the fresh water discharge zone and an upper saline plume in the intertidal region (Robinson et al. 1998(Robinson et al. , 2006Westbrook et al. 2005;Kuan et al. 2012). Furthermore, this upper saline plume represents a potentially more active zone for mixing and reaction than the dispersion zone of the lower salt water wedge, due to its faster flow rates and lower transit times (Robinson et al. 2007). ...
This study employed a coupled water‐air two‐phase flow and saltwater transport model to analyze the behaviors of generated airflow in unsaturated zones and the fluctuations of salinity at the salt‐freshwater interface in a two‐layered unconfined aquifer with a sloping beach surface subjected to tidal oscillations. The simulation results show that as the new dynamic steady‐state including effects of tidal fluctuations is reached through multiple tidal cycles, the dispersion zone in the lower saltwater wedge is broadened because freshwater/saltwater therein flows continuously landward or seaward during tidal cycles. The upper salt‐freshwater interface exhibits more vulnerable to the tidal fluctuations, and the variation of salinity therein is periodic, which is irrelevant to the hydraulic head but is influenced by the direction and velocity of surrounding water‐flow. With the tidal level fluctuating, airflow is mainly concentrated in the lower permeable layer due to the restraint of the upper semi‐permeable layer, and the time‐lag between the pore‐air pressure and the tidal level increases with distance from the coastline. The effect of airflow in unsaturated zones can be transmitted downward, causing both the magnitude of salinity and its amplitude in the upper salt‐freshwater interface to be smaller for the case with airflow than without airflow due to the resistance of airflow to water‐flow. Sensitivity analysis reveal that distributions of airflow in unsaturated zones are affected by the permeability of the upper/lower layer and the van Genuchten parameter of the lower layer, not by the van Genuchten parameter of the upper layer, whereas the salinity fluctuations in the salt‐freshwater interface are affected only by soil parameters of the lower layer. This article is protected by copyright. All rights reserved.
... Harvey et al., 1987;Gardner, 2005;Wilson and Gardner, 2006;Gibbes et al., 2008b;Xia and Li, 2012) and field measurements (e.g. Robinson et al., 1998;Hughes et al., 1998;Gibbes et al., 2008a;Røy et al., 2008;Xia and Li, 2012). These studies considered the groundwater flow in sandy tidal flats and salt marsh soils at different scales that vary from cm to m. ...
Frequent and unexplained iron enrichments have been localized in buried, Holocene, marine sediments in coastal areas. These buried sediments are nowadays often part of groundwater systems in densely populated deltas. Here, the Krabbenkreek intertidal/supratidal area in the Netherlands is considered as a present analogue for diagenetic processes in these near-shore sediments. We evaluate the role of tidally driven groundwater flow alternations on the transport and speciation of Fe in deep tidal sediments. This was achieved by examining geochemical sediment and porewater data in the context of 3-D geohydrological modelling results of the groundwater system. The Krabbenkreek area presents clear hydrologic and geochemical zonations across the supratidal/intertidal area. Due to the low hydraulic conductivity, groundwater flow and infiltration of rain water is limited in the mud-rich upper sediments of the supratidal flat. Despite tidal fluctuations of the hydraulic heads up to 1.5 m, the corresponding alternating groundwater flow is relatively minor in comparison to the net groundwater flow. Tidal variations in the groundwater force field alone did not lead to pronounced local enrichments in reactive Fe. Differences in reactive Fe contents in deep sediments can be related to textural differences, implying that no indications for local, diagenetic Fe enrichments were found in the sediments down to 5 m depth. Contents of FeS and amorphous Fe(III) are remarkably low indicating little current dynamics in Fe redox cycling in these deep sediments, while the porewater chemistry indicates that pyrite oxidation prevails. Long-term oxidation of pyrite is reflected by the presence of Fe(III) oxides in the high marsh sediments while pyrite still dominates in the middle marsh and the intertidal sediments. Absence of active Fe redox cycling can be attributed to low respiratory activity in the sediments. Previous studies reported that sedimentary organic matter in the vicinity of the investigated area can be predominately refractory, which can probably explain the low respiratory activity in the Krabbenkreek sediments.
... These studies have all shown that contaminants may remain trapped below the beach surface for a long time and thus continue to harm the marine and coastal ecosystems (Peterson et al. 2003). To avoid nearshore pollution events, accurate prediction of transport mechanisms in coastal aquifers is crucial for managing coastal developments (Robinson et al. 1998;Zhang et al. 2001). ...
This study employed experimental and numerical methods to assess the behavior of conservative solute transport for a selected temporary solid waste site in a reclamation area in western Taiwan. Calibrating a site-specific numerical model, finite element model of water flow through saturated-unsaturated media (FEMWATER), relies on observations from field- and laboratory-scale hydraulic tests and spatial-temporal monitoring. The field-scale experiment used a modified hydraulic tomography survey (MHTS) to identify near surface aquifer stratifications and estimate the distribution of saturated hydraulic conductivity. The pressure plate experiments provided parameters for the van Genuchten soil characteristic model. Sensitivity analyses were then conducted based on varied recharge rates and dispersivities applied to the calibrated model. Observations of groundwater levels and salinity in the wells indicated that the regional groundwater flow was from southeast to northwest. In addition, a shallow freshwater layer was noted in the study area. The tidal-induced amplitudes for water level fluctuation in the wells ranged from 2 to 20 cm, depending on their distance from the seawater body. MHTS showed clear stratification, similar to that of well loggings at the storage site. The hydraulic conductivity at the test site ranged from 8 to 10 m/day, which is close to that obtained from the laboratory falling head test. The results of particle-tracking modeling showed that the critical recharge rate for the site needed to enhance plume traveling is 1000 mm/year. The increase in dispersivity values induced a decrease in plume travel time of up to 1000 days from the site to the coastal line. A special case for pulse releasing solute at the site shows that the key factor in controlling plume migration is the recharge rate. This is due to the low natural head gradient in the reclamation area. The results therefore suggest that a land drainage system near the site can play an important role in contaminant transport in the reclamation area.
... Seawater, or saltwater, intrusion is a complicated hydrogeological process, due to the impact of aquifer properties, anthropogenic activities (e.g., intensive groundwater pumping, irrigation practices), recharge rates, variable density flow, tidal activity and effects relating to global climate change, such as sea level rise (Ghassemi et al., 1993;Robinson et al., 1998;Smith and Turner, 2001;Simpson and Clement, 2004;Narayan et al., 2007;Werner and Simmons, 2009;Wang et al., 2015). Understanding the complex interactions between groundwater, surface water and seawater is thus essential for effective management of coastal water resources (Mondal et al., 2010). ...
Groundwater is an important water resource for agricultural irrigation and urban and industrial utilization in the coastal regions of northern China. In the past 5 decades, coastal groundwater salinization in the Yang–Dai river plain has become increasingly serious under the influence of anthropogenic activities and climatic change. It is pivotal for the scientific management of coastal water resources to accurately understand groundwater salinization processes and their causative factors. Hydrochemical (major ion and trace element) and stable isotopic (δ¹⁸O and δ²H) analysis of different water bodies (surface water, groundwater, geothermal water and seawater) were conducted to improve understanding of groundwater salinization processes in the plain's Quaternary aquifer. Saltwater intrusion due to intensive groundwater pumping is a major process, either by vertical infiltration along riverbeds which convey saline surface water inland, and/or direct subsurface lateral inflow. Trends in salinity with depth indicate that the former may be more important than previously assumed. The proportion of seawater in groundwater is estimated to have reached up to 13 % in shallow groundwater of a local well field. End-member mixing calculations also indicate that the geothermal water with high total dissolved solids (up to 10.6 g L⁻¹) with depleted stable isotope compositions and elevated strontium concentrations (> 10 mg L⁻¹) also mixes locally with water in the overlying Quaternary aquifers. This is particularly evident in samples with elevated Sr ∕ Cl ratios (> 0.005 mass ratio). Deterioration of groundwater quality by salinization is also clearly exacerbated by anthropogenic pollution. Nitrate contamination via intrusion of heavily polluted marine water is evident locally (e.g., in the Zaoyuan well field); however, more widespread nitrate contamination due to other local sources such as fertilizers and/or domestic wastewater is evident on the basis of NO3 ∕ Cl ratios. This study provides an example of how multiple geochemical indicators can delineate different salinization processes and guide future water management practices in a densely populated water-stressed coastal region.
... Saltwater-freshwater mixing in beach aquifers has long been thought to occur mainly in the salt wedge (see Fig. 1) dispersion zone ( Cooper et al., 1964;Robinson et al., 2007a;Pool et al., 2014). How- ever, integrated field salinity measurements (Robinson et al., 1998;Urish and McKenna, 2004;Michael et al., 2005;Robinson et al., 2007b;Lebbe, 2006, 2007;Abarca et al., 2013;Heiss and Michael, 2014) and electrical resistivity tomography profiles ( Turner and Acworth, 2004;Morrow et al., 2010;Befus et al., 2013;Buquet et al., 2016) important mixing zone, the upper saline plume (USP) (see Fig. 1), where saltwater-freshwater mixing is faster than that in the classical salt wedge. USP is an inverted structure with dense saltwater above light freshwater in shallow beach aquifers (Ataie-Ashtiani et al., 1999;Boufadel, 2000;Mango et al., 2004). ...
... For example, some investigators have seen agreement between measurements made by seepage meters and other methods in flowing streams (Rosenberry and Pitlick, 2009;Kennedy et al., 2010) while others have reported large uncertainties in measured flows, or were unable to operate seepage meters under the conditions they encountered (Cey et al., 1998;Zamora, 2006). The reasons for these uncertainties have been attributed to design limitations of the devices (Isiorho and Meyer, 1999;Murdoch and Kelly, 2003;Simpkins, 2006;Rosenberry, 2008), disturbance of flow paths due to instrument installation (Hutchinson and Webster, 1998), velocity heads imposed by waves and moving water interfering with seepage meter operation (Shinn et al., 2002), gas release in bed sediments (Kennedy et al., 2010), improper seals between the drums and the beds (Cey et al., 1998), small-scale spatial heterogeneities (Robinson et al., 1998;Bokuniewicz et al., 2004), and the combined effect of slow seepage rates with a moving streambed, causing scour or burial of the seepage meters (Zamora, 2006). Other methods, such as multiple tracer injections in streambeds (Zellweger, 1994) and the measurement of isotopic and temperature signatures to infer groundwater inflows into streams (Cook et al., 2003) may be similarly influenced by streambed heterogeneity, difficult installation, and the challenges of working in a continuously evolving system (due to the transport of sediment, temporal variability of flow, etc.). ...
The streambed point velocity probe (SBPVP) measures in situ groundwater velocities at the groundwater-surface water interface without reliance on hydraulic conductivity, porosity, or hydraulic gradient information. The tool operates on the basis of a mini-tracer test that occurs on the probe surface. The SBPVP was used in a meander of the Grindsted Å (stream), Denmark, to determine the distribution of flow through the streambed. These data were used to calculate the contaminant mass discharge of chlorinated ethenes into the stream. SBPVP data were compared with velocities estimated from hydraulic head and temperature gradient data collected at similar scales. Spatial relationships of water flow through the streambed were found to be similar by all three methods, and indicated a heterogeneous pattern of groundwater-surface water exchange. The magnitudes of estimated flow varied to a greater degree. It was found that pollutants enter the stream in localized regions of high flow which do not always correspond to the locations of highest pollutant concentration. The results show the combined influence of flow and concentration on contaminant discharge and illustrate the advantages of adopting a flux-based approach to risk assessment at the groundwater-surface water interface. Chlorinated ethene mass discharges, expressed in PCE equivalents, were determined to be up to 444 kg/yr (with SBPVP data) which compared well with independent estimates of mass discharge up to 438 kg/yr (with mini-piezometer data from the streambed) and up to 372 kg/yr crossing a control plane on the streambank (as determined in a previous, independent study).
... Field studies of the groundwater discharge process in unconfined coastal aquifers show that the tide can significantly influence the temporal and spatial patterns of groundwater discharge as well as the salt concentration in the near-shore groundwater (Robinson and Gallagher 1993;Robinson et al. 1998). ...
Tidal variation and water level in aquifer is an important function in the coastal environment, this study attempts to find the relationship between water table fluctuation and tides in the shallow coastal aquifers. The study was conducted by selecting three coastal sites and by monitoring the water level for every 2-h interval in 24 h of observation. The study was done during two periods of full moon and new moon along the Cuddalore coastal region of southern part of Tamil Nadu, India. The study shows the relationship between tidal variation, water table fluctuations, dissolved oxygen, and electrical conductivity. An attempt has also been made in this study to approximate the rate of flow of water. Anyhow, the differences are site specific and the angle of inclination of the water table shows a significant relation to the mean sea level, with respect to the distance of the point of observation from the sea and elevation above mean sea level.
... Additionally, the fast response of water level to precipitation supports a high hydraulic conductivity in this karst massif; thus, Litno Cave is the largest conduit among many others that are widespread in the karst of this coastal region. The high hydraulic conductivity explains why the tides in the estuary are in phase and do not have a reduced amplitude with distance to the estuary as in other systems (Robinson et al. 1998;Befus et al. 2013). ...
Although temperature is a nonconservative tracer, it often provides useful information to understand hydrological processes. This study explores the potential of temperature to characterize the hydrological dynamics of a submarine spring and its coastal karst aquifer in Krka Estuary (Croatia). The estuary is well stratified and its water column has a clear thermocline. A network of loggers was designed to monitor the temperature along vertical profiles in the estuary and the coastal aquifer, taking advantage of an anchialine cave that enabled access to the subterranean estuary. The location of the thermocline in the groundwater, which defines the upper boundary of the saline intrusion, depends on (1) the recharge of the aquifer via infiltration of precipitation, (2) the evolution of the thermocline in the estuary, and (3) the tidal oscillations. The sources of water flowing though the anchialine cave were identified: brackish water from the estuary above the thermocline, saline water from the estuary below the thermocline, and freshwater from infiltrated precipitation. A conceptual model is described that characterizes the hydrological dynamics of this coastal aquifer and its interactions with the estuary. Thus, at least for some hydrological settings, temperature is a valid tracer to characterize the main hydrological processes. The measurement of temperature is inexpensive compared to other (conservative) tracers. Therefore, for those hydrological settings that have water masses with distinct temperatures, the use of temperature as a tracer to establish conceptual models of the hydrological dynamics is encouraged.
... Seawater/saltwater intrusion is a complicated hydrogeological process due to the impact of aquifer 59 properties, anthropogenic activities (e.g., intensive groundwater pumping, irrigation practices), recharge 60 rate, variable density flow between the estuary and adjacent fresh groundwater system, tidal/surge activity 61 and global climate change ( Ghassemi et al., 1993;Robinson et al., 1998;Smith and Turner, 2001;Simpson 62 Hydrol. Earth Syst. ...
Groundwater is the important water resource for agricultural irrigation, urban and tourism development and industrial utilization in the coastal region of north China. In the past five decades, coastal groundwater salinization in the Yang-Dai River coastal plain has become more serious than ever before under natural climate change and anthropogenic activities. It is pivotal for the scientific management of coastal water resources to accurately understand groundwater salinization processes and its inducement. Hydrochemical and stable isotopic (δ¹⁸O and δ²H) analysis for the different water bodies (surface water, groundwater, geothermal water, and seawater) were applied to provide a better understanding of the processes of groundwater salinization in the Quaternary aquifers. Saltwater intrusion is the major aspect and can be caused by vertical infiltration along the riverbed at the downstream areas of rivers during the tide/surge period, and lateral inflow into fresh aquifer derived from intensively pumping groundwater. Seawater proportion can reach ~ 13 % in the well field. High mineralized geothermal water (TDS up to 10.6 g/L) with the indicator of paleoseawater relics (lower Cl / Br ratios relative to modern seawater) overflows into the cold Quaternary aquifers. Groundwater salinization can also be exacerbated by the anthropogenic activities (e.g., irrigation return-flow with solution of fertilizers, domestic wastewater discharge). Additionally, the interaction between surface water and groundwater can make the groundwater freshening or salinizing in different sections to locally modify the groundwater hydrochemistry. The cease of the well field and establishment of anti-tide dam in the Yang River estuary area have effective function to contain the development of saltwater intrusion. This study can guide the future water management practices, and provide research approaches and foundation for further investigation of seawater intrusion in this and similar region.
... Coastal beaches subjected to oceanic forcing (e.g., tides and waves) involve very complex driving mechanisms for subsurface pore water flow and solute transport processes [Abdollahi-Nasab et al., 2010;Geng et al., 2015;Heiss, 2011;Michael et al., 2005;Moore, 1999;Robinson et al., 2006;Xin et al., 2010]. Tidal action induces significant saltwater-freshwater mixing and recirculation and forms two distinct saline plumes in beaches: the classical saltwater wedge and an upper saline plume that overlays a ''tube,'' whereby fresh groundwater discharges near the low tide mark [Boufadel, 2000;Brovelli et al., 2007;Robinson et al., 1998]. Waves superimposing on tides enhance the mixing and recirculation between seawater and groundwater and induce a convoluted driving force for groundwater flow and solute transport due to the nonlinear interactions between tides and waves [Baldock and Hughes, 2006;Geng and Boufadel, 2015c;Sous et al., 2013;Xin et al., 2015]. ...
Evaporation has been recognized as a major driving force affecting coastal aquifer systems. However, its effects on subsurface flow and salinity structure have not been investigated in sufficient detail. This paper presents field measurements and numerical simulations of pore-water flow and subsurface salinity structure in the supratidal zone of a sandy beach subjected to evaporation as well as rainfall. It was found that evaporation significantly increased pore-water salinity, up to 85 g/L, within a shallow layer, approximately 10 cm below the beach surface. The induced density gradient generated salt fingers near the beach surface, which caused local groundwater circulation (i.e., fingering flow). However, unlike inland aquifers, the salt fingering was significantly diminished by tidal action that prompted the horizontal mixing of salt in the beach. The subsequent precipitation (e.g., rainfall) diluted the evaporation-induced high saline plume near the beach surface, and drove the plume to migrate further downward; the plume gradually dispersed and was diluted along the groundwater pathways. The simulation results indicated that evaporation as well as precipitation at the beach surface induced complex driving mechanisms for supratidal groundwater flow. Depending on the intensity at the beach surface, evaporation and rainfall significantly altered the pore water flow and associate solute transport processes in the supratidal zone of the beach.
... The modeling sensitivity analysis showed that doubling total recharge has a large effect on groundwater flow paths, transit times, and discharge locations (Figs. 4, S4, S5). Thus, it follows that rates and spatial distributions of groundwater discharge vary seasonally as has been observed previously (Michael et al., 2005;Robinson et al., 1998). Overland runoff and rainfall to riparian zones during storm events are also important components of aquifer recharge, which are at temporal scales inherently neglected in a steady-state model, resulting in reaches of streambed that are simulated as dry (Fig. 2B), despite flowing intermittently. ...
Submarine groundwater discharge (SGD) is a small portion of the global water budget, but a potentially large contributor to coastal nutrient budgets due to high concentrations relative to stream discharge. A numerical groundwater flow model of the Inland Bays Watershed, Delaware, USA, was developed to identify the primary hydrogeologic factors that affect groundwater discharge rates and transit times to streams and bays. The distribution of groundwater discharge between streams and bays is sensitive to the depth of the water table below land surface. Higher recharge and reduced hydraulic conductivity raised the water table and increased discharge to streams relative to bays compared to the Reference case (in which 66% of recharge is discharged to streams). Increases to either factor decreased transit times for discharge to both streams and bays compared to the Reference case (in which mean transit times are 56.5 and 94.3 years, respectively), though sensitivity to recharge is greater. Groundwater-borne nitrogen loads were calculated from nitrogen concentrations measured in discharging fresh groundwater and modeled SGD rates. These loads combined with long SGD transit times suggest groundwater-borne nitrogen reductions and estuarine water quality improvements will lag decades behind implementation of efforts to manage nutrient sources. This work enhances understanding of the hydrogeologic controls on and uncertainties in absolute and relative rates and transit times of groundwater discharge to streams and bays in coastal watersheds.
... The infiltration of seawater by waves and tides alters the salinity distribution in the beach aquifer, the directions and rates of groundwater flow, and the rates of submarine groundwater discharge (SGD) (Michael et al., 2005;Vandenbohede and Lebbe, 2005;Robinson et al., 2007;Li et al., 2008). As a result of this infiltration, seawater circulating through beach aquifers can comprise a significant portion of SGD (Gallagher et al., 1996;Robinson et al., 1998;Boehm et al., 2006;Sieyes et al., 2008). Furthermore, mixing between through-flowing freshwater and circulating saltwater in the beach aquifer has been shown to impact biogeochemical processes that affect the transport of land-derived solutes to the sea (Charette and Sholkovitz, 2002;Kroeger and Charette, 2008;Santos et al., 2008;Spiteri et al., 2008;Loveless and Oldham, 2009;Santoro, 2009). ...
... Semidiurnal and diurnal variations in SGD result from changes in sea-surface height between high and low tides. When the sea-surface height is lower, at low tide, the hydraulic gradient between land and sea is steeper, increasing seaward groundwater flow (Taniguchi, 2002), and most studies where SGD has been measured at both high and low tides have recorded this effect (e.g., Robinson et al., 1998;Garrison et al., 2003;Lambert and Burnett, 2003;Shellenbarger et al., 2006). For example, at Shelter Island, New York, SGD flux was approximately 30 times greater at low tide than at high tide (Sholkovitz et al., 2003). ...
This chapter provides a review of the current state of knowledge on submarine groundwater discharge (SGD) and the associated fluxes of nutrients, trace metals, microbes, pharmaceuticals, and other terrestrial constituents to coastal waters. We review methods of estimating SGD, present flux estimates from different locations worldwide, and discuss how various hydrogeologic features such as topography, aquifer substrate, climate, waves, and tides affect SGD. We discuss the range of nutrient and metal concentrations observed in groundwater and their relationship to land use, and explore the chemical changes that nutrients and metals undergo during their seaward journey through the aquifer. Climate change is likely to affect both the quantity and the quality of SGD, and we investigate these effects, which are only beginning to be studied. The chapter concludes with a discussion of active areas of SGD research, including expanding the geographic scope of SGD studies; characterizing and reducing the uncertainty associated with SGD measurements; understanding the behavior of nutrients, metals, and other pollutants in the subterranean estuary; and modeling SGD on a global scale.
... Meanwhile, this study also examined temporal variation in SGD rates. Several studies have shown that the magnitude of SGD is mainly controlled by semidiurnal, diurnal, and semimonthly (spring-neap) tidal fluctuations (Kim and Hwang 2002; Taniguchi 2002; Taniguchi et al. 2008b), while monthly variation in SGD has been shown where the hydraulic gradient has a well-defined seasonal cycle (Robinson et al. 1998; Kelly and Moran 2002; Michael et al. 2005; Charette 2007). Coastal zones in the Sea of Japan are generally characterized by a tideless regime (Nishida 1980), wherein sea surface height shows clear seasonal trends with respective highs and lows inFig. 1 summer and winter (Kasai et al. 2010). ...
... Paired measurements of amplified and unamplified tube seepage meter and Darcian seepage rates (Figure 4) show relatively poor correlation (R 2 = 0.3 and 0.27, respectively), at least part of which is likely associated with inherent small-scale spatial variability in the streambed (Kennedy et al. 2007, and the fact that paired tube seepage meter and Darcian measurements were not exactly colocated. Previous field comparisons of traditional seepage meters to Darcian methods show variable results and general lack of correlation (Woessner and Sullivan 1984;Cey et al. 1998;Robinson et al. 1998;Bokuniewicz et al. 2004;Mulligan and Charette 2006). ...
We designed and evaluated a "tube seepage meter" for point measurements of vertical seepage rates (q), collecting groundwater samples, and estimating vertical hydraulic conductivity (K) in streambeds. Laboratory testing in artificial streambeds show that seepage rates from the tube seepage meter agreed well with expected values. Results of field testing of the tube seepage meter in a sandy-bottom stream with a mean seepage rate of about 0.5 m/day agreed well with Darcian estimates (vertical hydraulic conductivity times head gradient) when averaged over multiple measurements. The uncertainties in q and K were evaluated with a Monte Carlo method and are typically 20% and 60%, respectively, for field data, and depend on the magnitude of the hydraulic gradient and the uncertainty in head measurements. The primary advantages of the tube seepage meter are its small footprint, concurrent and colocated assessments of q and K, and that it can also be configured as a self-purging groundwater-sampling device.
... Estimates of SGD vary over several orders of magnitude (a few to 10 6 m 3 /yr per meter shoreline) due to the natural spatial variability in hydrological, geological and climatological parameters (Burnett et al., 2001;Taniguchi et al., 2002;Gallardo and Marui, 2006). In particular, the extent of seawater circulation could vary considerably de-pending on the studied scale, from wave setup and tidal-driven short-term circulation (Robinson et al., 1998;Li et al., 1999), through seasonal (Michael et al., 2005;Gonneea et al., 2013a), and up to the multi-annual dispersive circulation along the freshsaline water interface (Cooper, 1959). Determining the circulation type and time scale is highly important to the characterization of water-rock interaction and to the assessment of its impact on oceanic mass balances. ...
Radium isotopes in the Dead Sea Lake and the surrounding aquifer were studied in order to define the processes controlling their activities in the lake and in the groundwater. 226Ra activities in the groundwater show a significant removal of 226Ra from the lake water as they enter the aquifer. Short-lived radium isotopes show a nonlinear relation with salinity which is caused by the effect of salinity on the adsorption of radium. Simulations of radium distribution in the aquifer, using the code of SUTRA-MS, were done in order to estimate the adsorption distribution coefficient of radium in the aquifer. 228Ra activities in the Dead Sea water are much higher than the expected activities according to the radium sources to the Dead Sea. These observations together with the removal of 226Ra in the Dead Sea groundwater may indicate a large-scale saline water circulation in the aquifer.
... Semidiurnal and diurnal variations in SGD result from changes in sea-surface height between high and low tides. When the sea-surface height is lower, at low tide, the hydraulic gradient between land and sea is steeper, increasing seaward groundwater flow (Taniguchi, 2002), and most studies where SGD has been measured at both high and low tides have recorded this effect (e.g., Robinson et al., 1998;Garrison et al., 2003;Lambert and Burnett, 2003;Shellenbarger et al., 2006). For example, at Shelter Island, New York, SGD flux was approximately 30 times greater at low tide than at high tide (Sholkovitz et al., 2003). ...
... The circulation of saltwater in coastal aquifers is a well-known phenomenon (Bear, 1979;Cooper, 1959;Henry, 1964;Todd, 1980). Its flow patterns vary from short-time-scale, such as wave and tidal driven (Robinson et al., 2007a;Robinson et al., 2007bRobinson et al., , 2014Robinson et al., , 1998Xin et al., 2010Xin et al., , 2014 through seasonal scale (Michael et al., 2005), up to long term time-scale of density-driven dispersive circulation (Cooper, 1959). This study focuses on the density-driven dispersive circulation flow patterns. ...
... Depuis 20 ans, la communauté scientifique internationale n'a de cesse d'étudier le biseau salé (Burnett et al., 2006;Post and Abarca, 2010). Des études de cas (Alberti et al., 2009;Delinom, 2009;Escolero et al., 2007;Ghassemi and Jakeman, 1996;Kouzana et al., 2009;Prieto, 2005), et le développement d' outils d'analyse, de surveillance et de modélisation (Comte and Banton, 2007;Holliday et al., 2007;Koopmans and Berg, 2011;Michael et al., 2003;Moore et al., 2008;Scanlon et al., 2002;Swarzenski et al., 2001) ont montré l'importance de la prise en compte dans les modèles numériques, des facteurs naturels comme l'anisotropie des paramètres hydrauliques (Abarca, 2006;Voss and Souza, 1987), la zone non saturée (Zhang et al., 2001), la structure géologique de l'aquifère, la variation de la recharge (Anderson and Emanuel, 2010;Comte et al., 2010;Michael et al., 2005; et les variations du niveau océanique (Li et al., 2008;Robinson et al., 2011;Vandenbohede and Lebbe, 2007) . L'introduction de ces facteurs naturels sont indispensables lors de la réalisation de modèles numériques prédictifs d'aide à la gestion (Abd-Elhamid and Javadi, 2011;Bobba, 1993;Kopsiaftis et al., 2009;Voss, 1999). ...
Studies of coastal aquifers based on a constant mean sea level generally not consider the impact of oceanic oscillations. Our approach combines a long-term recording of “La Nappe des Sables” (adjacent to the fringing reef system of l'Ermitage, La Reunion) with numerical models in order to investigate the influence of these oceanic seasonal oscillations on groundwater hydrodynamics, seawater intrusions and submarine groundwater discharges. Analyzing both the behavior of the aquifer and the oceanic domain (reef), this study integrates a continuous recording of 600 days. It shows that the geometry and the hydrodynamic parameters chosen in the model, correlated to three types of oceanic oscillations, have variable consequences on the hydraulic head, the dispersive front and submarine groundwater discharges. Moreover, taking into account different complex oceanic boundaries in transient simulations over this 600 days period generates a better reproducibility of recorded signals (hydraulic head and salinity). These results underline that a long-term recording of oceanic oscillations is necessary for a good understanding of hydrodynamic mechanisms within coastal aquifers. The global signal must be decomposed with a harmonic analysis in order to identify the respective contributions of wave set up and tidal pumping to this signal. Our models reveal that the complexity of the natural system must be accounted for realistically in future numerical studies. The combination of hydrogeological and oceanographical data will allow the quantification of the impact of oceanic seasonal oscillations on such environments.
... Here the word "uplift" is referred to as the vertical motion and dislocation of the soil and rock formations below the base of the excavations caused by the hydraulic pressure in the formations, wherein tidal inundation is prevented. Field studies of the groundwater discharge process in unconfined coastal aquifers show that the tide can significantly influence the temporal and spatial patterns of groundwater discharge as well as the salt concentration in the near-shore groundwater (Robinson et al. 1998; Robinson and Gallagher, 1999). To model the groundwater discharge process at the field scale, the tide and associated boundary conditions along the beach must be included in the model. ...
The groundwater in coastal region has got immense attention by different researchers through mainly emphasizing on the dynamic interaction between groundwater and seawater. Due to the increase in development in coastal regions, it is important to ensure proper coastal management and coastal protection techniques in order to avoid various coastal hydro-geological, engineering, and environmental problems such as seawater intrusion, stability of coastal engineering structures, beach dewatering for construction purposes, and deterioration of the marine environment. The study area was selected as Kazibacha under Bathiaghata upazila of Khulna district. The surface water data was collected from Institute of water Modeling (IWM) for Rupsha river, Kazibacha point. Whereas two groundwater well such as well21 (N22 0 41.241, E89 0 31.691) and well22 (N22 0 41.038, E89 0 31.604) were considered for the analysis. Both surface and groundwater data were taken for the duration of 25-03-12 to 31-03-12 with an interval of 10 minutes. The datum for all the data was taken as mean sea level. Two tide components M2 and S2 were used. MATLAB-based software named TIDAL is used in order to analyze the interaction of ground water and surface water based on analytical solutions suggested by Li et. al. (2007). By applying moving average introduced by Serfes (1991) the mean water level of Rupsa river at Kazibacha point was computed as 2.27 m. Based on the software prediction the aquifer properties such as the transmissibility of well21 and well22 was found as 51.8 and 129.5 m 2 /h respectively and the hydraulic conductivity of well21 and well22 was found as 0.690 m/h and 1.73 m/h respectively which was very close to BWDB report. Moreover the amplitude analysis suggests that the tidal influence of the Rupsha river on the nearby groundwater of the eastern bank might be active within 100 m range. It can further be stated that if the surface water of Rupsha river in any case gets contaminated through salinity intrution or something else then the nearby groundwater within 100 m range might also get effected in different range. Another study is made on salinity intrusion modeling. For this we have choosen the lower Meghna near Raipur upazila (23° 3' 0" North, 90° 46' 0" East) of Lakshmipur district. By using Visual MODFLOW and secondary data a salinity intrusion model is made. Visual Modflow SEAWAT was used as experimental basis to simulate the salt water transport to shallow aquifers. To remove complexity a constant head boundary was assigned to entire model domain. The model simulation was run for 20 years' time step. Model result shows a rapid transport of salt for first eight years. After eight years salt transport speed is rather slow but gradually the concentration of salt in the next twelve years (in the time scale of 20 years) will reaches the amount which is alarming compared to national standards. From the analysis it is seen that the salinity may cover almost half the area of the aquifer.
... Meanwhile, this study also examined temporal variation in SGD rates. Several studies have shown that the magnitude of SGD is mainly controlled by semidiurnal, diurnal, and semimonthly (spring-neap) tidal fluctuations (Kim and Hwang 2002;Taniguchi 2002;Taniguchi et al. 2008b), while monthly variation in SGD has been shown where the hydraulic gradient has a well-defined seasonal cycle (Robinson et al. 1998;Kelly and Moran 2002;Michael et al. 2005;Charette 2007). Coastal zones in the Sea of Japan are generally characterized by a tideless regime (Nishida 1980), wherein sea surface height shows clear seasonal trends with respective highs and lows in Fig. 1 a The location of the study area. ...
We carried out a seasonal study of fresh submarine groundwater discharge (SGD) and associated nutrient fluxes in a semi-enclosed bay along a tideless coastal zone using a 222Rn and salinity mass balance model for a whole bay scale. The resulting SGD rates showed large intra-annual variability from 0.05 × 106 to 0.77 × 106 m3 day−1, which were controlled by seasonal changes in the interaction of multiple driving forces, including water table height and seawater level. The highest SGD rate in early spring was induced by heavy snow and low sea level, whereas the seasonal increase in sea level gradually suppressed fresh SGD rates. In summer, an elevated water table may induce higher SGD rates (approximately 0.4 × 106 m3 day−1) regardless of high sea levels. The highest SGD fraction in total terrestrial freshwater fluxes also occurred in summer (>40 %), due to the decreasing rate of surface river discharge. The seasonally averaged SGD rate was 0.36 × 106 m3 day−1. This value was similar to the annual groundwater recharge rate (0.33 × 106 m3 day−1) estimated by the water balance method in the basin. Nutrient fluxes from SGD were approximately 42, 65, and 33 % of all terrestrial fluxes of dissolved inorganic nitrogen, phosphorous, and silicate, respectively. The average fraction of SGD in the water fluxes including terrestrial and oceanic water was low (0.3 %), but that of nutrient fluxes increased to 20-38 %. Higher nutrient concentrations in groundwater compensated for the lower volumetric flux of groundwater. Because primary production was mostly restricted by phosphorous throughout the year, phosphorous-enriched nutrient transport via SGD would play an important role in biological production.
... Seepage meters showed a consistent spatial pattern with highest discharge intermediate offshore and lowest discharge farthest offshore (Fig. 2a). Groundwater discharge measured by seepage meters was consistently higher than predicted by Darcy's Law (i.e., Q=−K dh/dl, with K=~0.001 cm s −1 ; Robinson et al. 1998;, except at the farthest offshore location (SM5) where flow rates were lower than predicted (Fig. 3). The salinity trend observed in the seepage water matched well with the subsurface salinity section (Fig. 5). ...
Submarine groundwater discharge represents a major but poorly constrained component of coastal marine chemical budgets. In the current study, the geochemical behavior of 224Ra, inorganic nitrogen species, and Fe in shallow coastal groundwater was characterized to improve estimates of chemical flux via submarine groundwater discharge (SGD) at a site in the York River estuary, VA (USA). Directly measured SGD rates varied between 3.9 ± 1.2 cm day−1 offshore, and 8.9 ± 2.6 cm day−1 close to shore. A clear inverse relationship was observed between SGD and tidal height, reflecting the hydraulic gradient between groundwater and surface water. Discharge rates varied spatially in conjunction with the subterranean estuary location, and there was a strong inverse correlation between seepage rates and seepage salinity. Dissolved 224Ra activity in the mixing zone reached levels up to 6 dpm L−1 and co-varied with salinity in the groundwater but not in the surface water or seepage water. Instead, a consistent sigmoidal trend of Ra with pH was observed, which matched previous laboratory experiment results. Dissolved NH4 + reached concentrations up to 120 μM in the groundwater and appeared to mix conservatively with respect to salinity in the subterranean estuary. In contrast, NOx (NO2 − + NO3 −) was low in both fresh groundwater and surface water and showed non-conservative enrichment (up to 23 μM) within the subterranean estuary. Dissolved Fe also showed non-conservative excess in the subterranean estuary, reaching concentrations up to 50 μM. SGD-derived chemical fluxes were estimated using several different commonly used approaches: average groundwater concentrations, pore water constituent-salinity trends coupled with directly collected seepage salinity, constituent concentrations in directly collected seepage, and concentrations in shallowest groundwater samples. Different flux estimates were compared with a “variable endmember” approach based on the observed geochemical distribution and inferred behavior.
... The sum of the forces that generate benthic pressure gradients is a composite force, which spans both the surface water and ground water domains. Robinson et al. [1998], Uchiyama et al. [2000], Taniguchi and Iwakawa [2001], and Taniguchi [2002] discuss q bf forcing factors. ...
... These were initially developed in the 1940's to measure loss of water from irrigation channels (Israelson and Reeve, 1944) and resurrected in the 1970's for use in small lakes and estuaries (McBride and Pfannkuch, 1975;Lee, 1977;John and Lock, 1977;Lee and Cherry, 1978). Seepage meters have since been used in numerous studies of seepage fluxes in rivers (Lee and Hynes, 1978;Libelo and MacIntyre, 1994;Cey et al, 1998;Landon et al, 2001), the near-shore marine zone (Bokuniewicz and Pavlik, 1990;Valiela et al, 1990;Cable et al, 1997;Taniguchi et al, 2003), tidal zones (Belanger and Walker, 1990;Robinson et al, 1998), coral reefs (Simmons andLove, 1984, Lewis, 1987), large lakes (Cherkauer and McBride, 1988) and water-supply reservoirs (Woessner and Sullivan, 1984). A constant-head variant of the seepage meter (the Idaho meter) has been used to measure leakage from irrigation channels into aquifers under Australian conditions (ANCID 2000; Byrnes and Webster, 1981). ...
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... Fresh groundwater exits the aquifer through a narrow discharge zone between the shoreline and the intersection of the lower interface and the seabed (Process 1, Figure 1) [Glover, 1959]. The fresh discharge zone is bounded seaward by the lower interface and landward by an intertidal brackish-saline circulation cell [Robinson et al., 1998;Michael et al., 2005]. Saltwater-freshwater mixing along the lower interface creates a density gradient that drives convective saltwater circulation (Process 2, Figure 1) [Cooper, 1959;Kohout, 1960]. ...
The biogeochemical reactivity of sandy beach aquifers is closely linked to physical flow and solute transport processes. Thus, a clearer understanding of the hydrodynamics in the intertidal zone is needed to accurately estimate chemical fluxes to the marine environment. A field and numerical modeling study was conducted over a one-year timeframe to investigate the combined effects of tidal stage, spring-neap variability in tidal amplitude, and seasonal inland water table oscillations on intertidal salinity and flow dynamics within a tide-dominated micro-tidal sandy beach aquifer. Measured and simulated salinities revealed an intertidal saline circulation cell with a structure and cross-sectional mixing zone area that varied over tidal, spring-neap and seasonal timescales. The size of the circulation cell and area of the mixing zone were shown for the first time to be most affected by seasonal water table oscillations, followed by tidal amplitude and tidal stage. The intertidal circulation cell expanded horizontally and vertically as the inland water table declined, displacing the fresh discharge zone and lower interface seaward. Over monthly spring-neap cycles, the center of the circulation cell shifted from beneath the backshore and upper beachface to the base of the beach. Salinity variations in the intertidal zone over semidiurnal tidal cycles were minimal. The dynamics of the circulation cell were similar in simulations with and without a berm. The highly transient nature of intertidal salinity over multiple time scales may have important implications for the types and rates of chemical transformations that occur in groundwater prior to discharge to the ocean.
... This cyclic discharge/recharge results in increased mixing of surface water and groundwater in the near-surface littoral zone sediments. [20,21] On average, groundwater from the mainland flows the east-southeast and discharges to the littoral zone and subtidal channel. The average upflow velocity in the littoral zone is approximately 0.15 cm d ¡1 (0.1 to 0.2 cm d ¡1 ) based on a mass balance of water entering this area from zone 1 and the measured width of the seepage face. ...
Multiple lines of evidence were used to document the natural attenuation of perchlorate in a shallow alluvial aquifer. In the upgradient, aerobic portion of the aquifer, perchlorate did not biodegrade. However, natural flushing by groundwater flow is reducing perchlorate concentrations in the aquifer over time. Perchlorate concentrations in the source area are expected to meet cleanup criteria in 11 to 27 years without active remedial measures. At the distal end of the plume, perchlorate is rapidly degraded as it migrates upward through organic rich littoral zone sediments. Apparent first-order degradation rates in groundwater were about 0.20 d(-1) and are consistent with laboratory macrocosm rates (0.12 d(-1)). qPCR results show a distinct region of the littoral zone where perchlorate degraders are elevated. The Eh within this zone varies from +0.1 to +0.3 V indicating perchlorate degraders can thrive in moderately oxidizing conditions. The study has shown that (i) there was no apparent perchlorate biodegradation in aerobic aquifer; (ii) perchlorate declines over time in aerobic aquifer due to flushing; (iii) there was a rapid perchlorate attenuation in organic rich littoral zone; and, (iv) qPCR results show large increases in perchlorate degraders in the littoral zone.
... Estimates of SGD vary over several orders of magnitude (a few to 10 6 m 3 /yr per meter shoreline) due to the natural spatial variability in hydrological, geological and climatological parameters (Burnett et al., 2001;Taniguchi et al., 2002;Gallardo and Marui, 2006). In particular, the extent of seawater circulation could vary considerably de-pending on the studied scale, from wave setup and tidal-driven short-term circulation (Robinson et al., 1998;Li et al., 1999), through seasonal (Michael et al., 2005;Gonneea et al., 2013a), and up to the multi-annual dispersive circulation along the freshsaline water interface (Cooper, 1959). Determining the circulation type and time scale is highly important to the characterization of water-rock interaction and to the assessment of its impact on oceanic mass balances. ...
... The infiltration of seawater by waves and tides alters the salinity distribution in the beach aquifer, the directions and rates of groundwater flow, and the rates of submarine groundwater discharge (SGD) (Michael et al., 2005;Vandenbohede and Lebbe, 2005;Robinson et al., 2007;Li et al., 2008). As a result of this infiltration, seawater circulating through beach aquifers can comprise a significant portion of SGD (Gallagher et al., 1996;Robinson et al., 1998;Boehm et al., 2006;Sieyes et al., 2008). Furthermore, mixing between through-flowing freshwater and circulating saltwater in the beach aquifer has been shown to impact biogeochemical processes that affect the transport of land-derived solutes to the sea (Charette and Sholkovitz, 2002;Kroeger and Charette, 2008;Santos et al., 2008;Spiteri et al., 2008;Loveless and Oldham, 2009;Santoro, 2009). ...
The intertidal zone of coastal aquifers is a dynamic region of mixing between saline surface water and fresh groundwater. Groundwater circulation in this zone is affected by complex forcing mechanisms that operate on a range of timescales and can regulate chemical fluxes to marine environments. We evaluated wave swash-induced infiltration and associated flow dynamics in the unsaturated region of two sandy beach aquifers with differing wave conditions and beach morphologies. High frequency (5 Hz) moisture and pressure sensors were used to measure fluctuations in water content, water table elevation, and hydraulic gradients in the swash zone at mean lower low water, mean sea level , and mean higher high water. Water content in the unsaturated region of the swash zone responded to wave overtopping and swash infiltration, with a rapid rise in water content followed by a slower decline. Swash-induced unsaturated infiltration rates, calculated from water content response, were lowest near low tide and increased up the beachface with inflow highest near high tide at both sites, consistent with an increase in water table depth up the beach. Unsaturated infiltration at the wave dominated beach was 1.6 m3/m per tidal cycle at the wave-dominated beach and 0.4 m3/m per tidal cycle at the tide-dominated beach. Saturated pore pressure measurements show that a water table mound formed as a consequence of swash infiltration that migrated up the beach during rising tide, leading to divergent seaward/landward groundwater flow. The results demonstrate the significant and spatially variable effects of wave swash on moisture dynamics in the unsaturated zone of beach aquifers, and show that these effects depend on wave conditions and beach characteristics. Results have implications for understanding transport and reaction of solutes in this biogeochemically active zone.
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.
Beach recovery describes the processes by which there is a natural restoration of beach material and coastal morphology following storm events, and these processes are common across the globe. However, the effects of beach recovery on salinity distribution and solute transport in unconfined coastal aquifers are poorly understood. This study examined the changes in salinity distribution in tidally influenced aquifers in response to beach recovery, based on numerical modeling. The extent and location of the upper saline plume and saltwater wedge were found to vary with the beach recovery. The variations in salinity distribution directly changed the particle travel times in the aquifers. Compared with the erosion profile after the storm (storm profile), an increase of up to 743% of the particle travel time in the intertidal zone was observed when the beach recovered to a berm (silting) profile. The berm profile increased the residence time and peak concentration of the land-sourced solute plume in the beach aquifer compared with the storm profile. The berm profile also enhanced the aquifer–ocean mass exchange, resulting in increased intertidal saltwater infiltration and submarine groundwater discharge. On the other hand, the storm profile can generate much higher solute efflux than the berm profile. The storm profile is more favorable in diluting the land-sourced conservative solute and shortening its residence time in an aquifer.
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.
Submarine fresh groundwater discharging from discrete zones such as conduits overlain by thick sediment remains poorly documented and understood despite common anecdotal accounts of it. We analyzed this phenomenon by a suite of variable-density, variably-saturated groundwater flow simulations guided by field studies. The field observations included point flux and salinity measurements using seepage meters spread across the intertidal and the subtidal zones. The discretely measured seepage fluxes in the subtidal region were as high and the salinity was as fresh as those from the intertidal zone. Both fluxes and salinities varied with tides. Two-dimensional simulations of the idealized beach-ocean vertical section with discrete freshwater springs at the base showed the development of seawater recirculation cells along the edge of freshwater discharge plumes emanating from the outlets, causing mixing. Brackish water discharges at the sediment-water interface above where the plume mixes with the recirculating seawater. Sensitivity analyses showed that stronger terrestrial fresh groundwater flow increases the freshwater plume size but does not affect the mixing zone width. Tidal oscillations and a dynamic fresh groundwater flow widened the mixing zones and increased porewater flux across the sediment-water interface. Sediment thickness had a minor effect on the width of both the freshwater plume and its surrounding mixing zone. The subtidal freshwater plume and mixing zone likely represent a unique but potentially ubiquitous setting which hosts dramatic ecological and chemical gradients. Their local biogeochemical niche and coastal scale ecosystem impacts will need to be considered in investigations of coastlines.
Many recent investigations of groundwater dynamics in beaches employed groundwater models that assumed isotropic, numerically-convenient hydrogeological conditions. Real beaches exhibit local variability with respect to stratigraphy, sediment grain size and associated topographic profile, so that groundwater flow may diverge significantly from idealized models. We used a combination of hydrogeologic field methods and a variable-density, saturated-unsaturated, transient groundwater flow model to investigate SGD and solute transport under Cabretta Beach, a small transgressive barrier island seaward of Sapelo Island, Georgia. We found that the inclusion of real beach heterogeneity drove important deviations from predictions based on theoretical beaches. Cabretta Beach sustained a stronger upper saline plume than predicted due to the presence of a buried silty mud layer beneath the surface. Infiltration of seawater was greater for neap tides than for spring tides due to variations in beach slope. The strength of the upper saline plume was greatest during spring tides, contrary to recent model predictions. The position and width of the upper saline plume was highly dynamic through the lunar cycle. Our results suggest that field measurements of salinity gradients may be useful for estimating rates of tidally and density driven recirculation through the beach. Finally, our results indicate that several important biogeochemical cycles recently studied at Cabretta Beach were heavily influenced by groundwater flow and associated solute transport.
Tidal motions of the water table height inside a sloping beach are
investigated via field measurements and theoretical considerations. Only
the movements forced by the tide are considered, so a beach with
negligible wave activity was chosen for the field measurements. The data
show that even in the absence of precipitation the time averaged inland
water table stands considerably above the mean sea level. Also the water
table at a fixed point inside the beach is far from sinusoidal even
though its variation is forced by an essentially sinusoidal tide. This
latter effect is due to the boundary condition along the sloping beach
face which acts as a highly nonlinear filter. The observed behavior of
the water table is explained in terms of perturbation extensions to the
classical "deep aquifer solution." One extension deals with the
nonlinearity in the interior, the other with the boundary condition at
the sloping beach face.
A procedure is presented for calculating the hydraulic conductivity of an aquifer near a well from the rate of rise of the water level in the well after a certain volume of water is suddenly removed. The calculation is based on the Thiem equation of steady state flow to a well. The effective radius over which the head difference between the equilibrium water table in the aquifer and the water level in the well is dissipated was evaluated with a resistance network analog for a wide range of system geometries. The technique is applicable to completely or partially penetrating wells in unconfined aquifers. It can also be used for confined aquifers that receive water from the upper confining layer.
Fresh-water and environmental-water heads are shown to be useful in studying movement of ground water of variable density, such as in a system of fresh, diffused, and salt water. Fresh-water head at a given point in ground water of variable density is defined as the water level in a well filled with fresh water from that point to a level high enough to balance the existing pressure at the point. Fresh-water heads define hydraulic gradients along a horizontal. An environmental-water head at a given point in ground water of variable density is defined as a fresh-water head reduced by an amount corresponding to the difference of salt mass in fresh water and that in the environmental water between that point and the top of the zone of saturation. Environmental-water heads define hydraulic gradients along a vertical. Vertical and horizontal components of velocity in an anisotropic system with ground water of variable density are computed from hydraulic gradients defined by environmental-water and fresh-water heads, respectively, and from appropriate components of the permeability tensor. Equations for the component velocities are based on a particular generalized form of the Darcy equation. An equation showing a relation between the head observed in fresh water overlying diffused water and the elevation of the contact between fresh water and diffused water is given. The equation is based on the concept of environmental head. It is found to be a suitable basis for defining the specific limitations of the Ghyben-Herzberg and the Hubbert equations when they are used for fresh-diffused-salt water environments.
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
A new piston sampler allows the collection of high-quality core samples from sand, silt or clay, up to depths of 18 meters. The sampler is operated by a one- or two-person crew without a drilling rig. The sampler and ancillary equipment fit easily into a half-ton truck, making this a highly portable sampling system. Other advantages include minimal mechanical disturbance and precisely known sample depth. Casing is not required to maintain an open corehole below the water table and drilling fluid is not used in the corehole, so the solids and pore water of the sample should not be contaminated by foreign fluids. High-quality samples for physical, geochemical, and microbiological characterization of the subsurface are easily obtained with this new device.
Studies conducted in the vicinity of an industrial outfall in the Calcasieu River estuary, Louisiana, have shown that water, bottom and suspended sediment, and four different species of biota are contaminated with halogenated organic compounds (HOC) including haloarenes. A salting-out effect in the estuary moderately enhanced the partitioning tendency of the contaminants into biota and sediments. Contaminant concentrations in water, suspended sediments, and biota were found to be far below the values predicted on the basis of the assumption of phase equilibria with respect to concentrations in bottom sediment. Relative concentration factors of HOC between biota and bottom sediment increased with increasing octanol/estuarine water partition coefficients (K/sub ow*/), maximizing at log K/sub ow*/ of about 5, although these ratios were considerably less than equilibrium values. In contrast, contaminant concentrations in water, biota, and suspended sediments were much closer to equilibrium values. Bioconcentration factors of HOC determined on the basis of lipid content for four different biotic species correlated reasonably well with equilibrium triolein/water partition coefficients (K/sub tw/).
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.
The hydraulic potentiomanometer described herein consists of a potentiometer connected to a manometer by a flexible tube. The device is used to directly measure the direction of seepage as well as the hydraulic-head difference between groundwater and surface water. The device works most effectively in sandy materials. For accurate measurements the device must be free of air leaks. -Authors
Rates of groundwater discharge onto coral reefs at Barbados, West Indies, were measured with seepage meters and miniature piezometers. Seepage flux varied spatially, was correlated with water depth, and was about twice as high during the wet season as during the dry. Groundwater nitrogen concentrations were correlated with salinity but phosphate concentrations were not. Nitrate content of the discharge was much higher than was phosphate content. Measured fluxes were consistent with groundwater discharge estimates from aquifer models, but a large data set would be required to make accurate predictions of areal groundwater discharge and nutrient loading.
Contamination of groundwater is a pervasive and serious problem in many developed coastal areas1–3, but potential interactions of groundwaters with, and their impact on, adjoining coastal waters have generally not been considered. Although submarine ground-water discharge could be an important source of nitrogen to coastal marine environments4, there has been no direct evidence for this. Recently, subsurface discharge has been shown to account for about 10–20% of the freshwater input to Great South Bay, New York5. The upper aquifer is the presumed source of this discharge and, because it is heavily contaminated with nitrate6, we suggest that groundwater is an important source of nitrate to the bay. Our determinations of the interstitial nutrient chemistry of freshly collected cores from nearshore sediments support this hypothesis. Although highly variable in time, cores taken after substantial rainfall showed increasing nitrate concentrations and decreasing salinities with depth. We conclude that submarine groundwater discharge is indeed a source of nitrate which should be considered in estimates of nitrogen influx to coastal marine ecosystems.
Changes of land use in coastal watersheds to residential development with on-site sewage disposal represent a potential change in both the quantity and quality of nutrient inputs to coastal marine systems. Measurements of dissolved N and phosphate P in septic system effluent indicated initial concentrations 100-1000-fold greater than receiving coastal waters, with inorganic N/P ratios (17/1) similar to phytoplankton growth requirements. Transformations of organic and inorganic N and retention of inorganic P occurred in the initial meters of groundwater transport with substantial (almost-equal-to 70%) nitrification of effluent ammonium to nitrate and retention of phosphate by the soil (almost-equal-to 60%). The degree of initial transformation and retention was directly related to unsaturated infiltration distance and is consistent with the requirements of these processes for oxidizing conditions. At greater distances (10-100 m), over 99% of the total dissolved N occurred as nitrate, phosphate concentrations were reduced to background levels, and groundwater N/P ratios exceeded 2500/1. The greater the importance of high-N, low-P groundwater inputs to the nutrient balance of a coastal water body, the greater the potential for shifts in the nutrient which limits primary production.
Great South Bay (New York) is a large lagoon on the northeast coast of the United States. The flow of groundwater across the floor of Great South Bay has been reported to account for as much as 2/3 of the total freshwater inflow. In situ measurements of this seepage flow have been made along four offshore transects in the Bay. These measurements show that the flow rate decreases rapidly offshore; within 30 m of the shoreline, the submarine outflow rates were typically 40 l (day m2)−1 and decreased to less than 10 l (day m2)−1 at a distance of 100 m from shore. The Bay floor at the study locations was sand or silty sand with vertical intrinsic permeabilities ranging from 14 to 78 darcys.The flow rate across the Bay floor may be described by an exponentially decreasing function. The flow distribution may, therefore, be specified with two parameters—the flow value at the shoreline, A, and a ‘decay’ constant, c, that governs the rate of decrease of the flow offshore. The calculated total flows along the four transects were 2·1 × 103, 1·1 × 103, 8·5 × 103 and 3·9 × 103 l (day m)−1. Between 40% and 98% of this flow enters the Bay within 100 m from shore. The total flow of groundwater across the Bay floor was calculated to be about 2 × 108 l day−1 or 10–20% of the total freshwater inflow.
Surface water, groundwater, and groundwater discharge quality surveys were conducted in Cherrystone Inlet, on Virginia's Eastern Shore. Shallow groundwater below agricultural fields had nitrate concentrations significantly higher than inlet surface waters and shallow groundwater underlying forested land. This elevated nitrate groundwater discharged to adjacent surface waters. Nearshore discharge rates of water across the sediment-water interface ranged from 0.02 to 3.69 liters·m−2·hr−1 during the surveys. The discharge was greatest nearshore at low tide periods, and decreased markedly with increasing distance offshore. Vertical hydraulic heads, Eh, and inorganic nitrogen flux in the sediments followed similar patterns. Nitrate was the predominant nitrogen species discharged nearshore adjacent to agricultural land use, changing to ammonium farther offshore. Sediment nitrogen fluxes were sufficient to cause observable impacts on surface water quality; nitrate concentrations were up to 20 times greater in areas of groundwater discharge than in the main stem inlet water. Based on DIN:DIP ratios, nitrogen contributions from direct groundwater discharge and tidal creek inputs appear to be of significant ecological importance. This groundwater discharge links land use activity and the quality of surface water, and therefore must be considered in selection of best management practices and water quality management strategies.
The seepage meter and the mini-piezometer were utilized in an attempt to evaluate ground-water reservoir interactions over a 12-month period at Echo Bay in Lake Mead. In conjunction with these techniques three standard piezometers, refraction seismic data, and water chemistry data were utilized to interpret seepage device results. During a four-month period, from December 1979 to March 1980, an 8 ft (2.5 m) rise in reservoir stage, the reservoir contributed water to Echo Wash bank storage at rates of up to 0.29 gpd/ft2 (12 lpd/m2). Ground-water discharge occurred for the remainder of the project, during a stage decline from April 1980 to May 1980, a rise in June 1980, and leveling off and slight decline for the remainder of the year, July 1980 to December 1980. The maximum seepage meter ground-water discharge rate of 3.0 gpd/ft2 (122 lpd/m2) was recorded in December 1980. Seepage meter water chemistry data for June were similar to Lake Mead water chemistry and were interpreted to be previously recharged Lake Mead water. September water chemistry data showed two possible components of ground-water discharge, a high SpC calcium sulfate Echo Wash ground water and a lower SpC Lake Mead recharged bank storage water. December ground-water chemistry data showed discharge to be a high SpC calcium sulfate water similar to Echo Wash ground-water quality which was apparently unaffected by Lake Mead inflow. Mini-piezometer data were collected at each seepage meter site. However, these data usually did not provide correlative results with seepage meter data probably because of suspended sediment in the piezometer water column and plugging of the perforated tip. Seepage meters were successfully utilized to characterize reservoir ground-water interaction in Echo Bay.
This research investigated the transport of land-applied nutrients and pesticides from unconfined aquifers to tidal surface waters of Virginia's coastal plain. Ground water, estuarine surface water, ground water discharge, upland soil, and offshore sediment samples were collected from May 1992 until February 1993 from four agricultural sites. Samples were analyzed for inorganic nitrogen and phosphorus and five pesticides: atrazine, cyanazine, alachlor, metolachlor, and carbofuran. Pesticides from aqueous samples were determined by liquid-solid phase extraction followed by gas chromatography-electron capture detection (GC-ECD) and/or by pesticide-specific immunoassay. Soil and sediment samples were analyzed by extraction and gas chromatography/mass spectrometry (GC/MS). Nutrient measurements indicated that fertilizer nitrogen was moving from the ground water to the surface water, and nitrogen fluxes across the sediment-water interface were correlated with fresh water discharge rates. Mean nitrate-N flux was 2.48 mg/m2hr, with a maximum value of 30.98 mg/m2hr. Pesticides were detected in more than half of the upland soil samples, in approximately 40 percent of the ground water samples, and in just under 20 percent of the seepage meter samples. Pesticides were not detected in any of the offshore sediment samples or surface water samples. Alachlor and metolachlor were detected in upland soil samples at concentrations ranging from 10 to almost 500 μg/kg. All five pesticides were found in ground water samples at concentrations generally below 1 μg/L, with alachlor, atrazine, and metolachlor most frequently found. Alachlor, atrazine, cyanazine, and metolachlor were detected in water discharging across the sediment-water interface and entering estuarine waters at concentrations ranging from 0.05 to 0.5 μg/L. These levels were generally consistent with the amount of dilution due to the mixing of fresh ground water and saline pore waters prior to discharge across the sediment-water interface. Based on all positive detections of pesticides in ground water discharge, which represented approximately 18 percent of all samples, average flux rates of cyanazine, metolachlor, alachlor, and atrazine were 0.32, 0.37, 0.80, and 1.12 μg/m2hr, respectively. These findings indicate that submarine ground water transport of both nutrients and pesticides does occur, and this transport route should be considered when implementing agricultural management practices. The levels of nitrogen transport to surface water appears significant. The overall levels of pesticide movement through ground water, although generally quite low, represent a transport route that is commonly neglected in watershed management.
Degraded water quality due to water column availability of nitrogen and phosphorus to algal species has been identified as
the primary cause of the decline of submersed aquatic vegetation in Chesapeake Bay and its subestuaries. Determining the relative
impacts of various nutrient delivery pathways on estuarine water quality is critical for developing effective strategies for
reducing anthropogenic nutrient inputs to estuarine waters. This study investigated temporal and spatial patterns of nutrient
inputs along an 80-km transect in the Choptank River, a coastal plain tributary and subestuary of Chesapeake Bay, from 1986
through 1991. The study period encompassed a wide range in freshwater discharge conditions that resulted in major changes
in estuarine water quality. Watershed nitrogen loads to the Choptank River estuary are dominated by diffuse-source inputs,
and are highly correlated to freshwater discharge volume. in years of below-average freshwater discharge, reduced nitrogen
availability results in improved water quality throughout most of the Choptank River. Diffuse-source inputs are highly enriched
in nitrogen relative to phosphorus, but point-source inputs of phosphorus from sewage treatment plants in the upper estuary
reduce this imbalance, particularly during summer periods of low freshwater discharge. Diffuse-source nitrogen inputs result
primarily from the discharge of groundwater contaminated by nitrate. Contamination is attributable to agricultural practices
in the drainage basin where agricultural land use predominates. Groundwater discharge provides base flow to perennial streams
in the upper regions of the watershed and seeps directly into tidal waters. Diffuse-source phosphorus inputs are highly episodic,
occurring primarily via overland flow during storm events. Major reductions in diffuse-source nitrogen inputs under current
landuse conditions will require modification of agricultural practices in the drainage basin to reduce entry rates of nitrate
into shallow groundwater. Rates of subsurface nitrate delivery to tidal waters are generally lower from poorly-drained versus
well-drained regions of the watershed, suggesting greater potential reductions of diffuse-source nitrogen loads per unit effort
in the well-drained region of the watershed. Reductions in diffuse-source phosphorus loads will require long-term management
of phosphorus levels in upper soil horizons. *** DIRECT SUPPORT *** A01BY074 00021
The Delmarva Peninsula is an extensively farmed region in which nitrate from commercial fertilizers and poultry has entered
the ground water and streams. The peninsula contains forested wetlands in a variety of settings, and their size and location
are a result of the surrounding hydrologic and soil conditions. Three regions, here referred to as hydrogeomorphic regions,
were selected for study. Each region has characteristic geologic and geomorphic features, soils, drainage patterns, and distribution
of farmland, forests, and forested wetlands. In all three regions, forested wetlands generally occupy poorly drained areas
whereas farmlands generally occupy well-drained areas. The three hydrogeomorphic regions studied are the well-drained uplands,
the poorly drained uplands, and the surficial-confined region. The well-drained uplands have the largest amount of farmland
and the smallest amount of forested wetlands of the three regions; here the forested wetlands are generally restricted to
narrow riparian zones. The poorly drained uplands contain forested wetlands in headwater depressions and riparian zones that
are interspersed among well-drained farmlands. The surficial-confined region has the smallest amount of farmland and largest
amount of forested wetlands of the three regions studied. Wetlands in this region occupy the same topographic settings as
in the poorly drained uplands. Much of the farmland in the surficial-confined region was previously wetland. Nitrate concentrations
in ground water and surface water on the peninsula range widely, and their distribution reflects (1) the interspersion of
forests among farmland, (2) hydrogeologic conditions, (3) types of soils, and (4) the ground-water hydrology of forested wetlands.
The well-drained uplands had higher median nitrate concentrations in ground water than the poorly drained uplands or the surficial-confined
region. The highest nitrate concentrations were in oxic parts of the aquifer, which are beneath well-drained soils that are
farmed, and the lowest were in anoxic parts of the aquifer, which are beneath poorly drained soils overlain by forested wetlands.
The effect of forested wetlands on water quality depends on the hydrogeologic conditions, extent of farming, and type of soils.
The three regions contain differing combinations of these factors and thus are useful for isolating the effects of forested
wetlands on water quality.
Key Wordswater quality-forested wetlands-hydrology
Direct measurements of groundwater seepage show the importance of subaqueous discharges as sources of fresh water and of dissolved
chemicals to lakes and the coastal ocean. The rate of seepage decreases rapidly offshore; an analytical solution was developed
that describes the discharge as Ki(In(coth πxk/4l))/k where i is the hydraulic gradient, K is the vertical hydraulic conductivity,l is the aquifer thickness, x is the distance from the shoreline, and k2 is K divided by the horizontal hydraulic conductivity. In addition to variations due to the inhomogeneities in the aquifer,
seepage into the coastal ocean involves some recirculation of the salt water. In Great South Bay, New York measured fluxes
were as great as 150 I m−2 d−1. The discharge near the shore was typically 50 I m−2 d−1, decreasing to 30 1 m−2 d−1 at a distance of 100 m offshore. Secondary convection due to an unstable density structure at the sediment-water interface
may also be superimposed on the seepage distribution. Fingers of salt should be capable of carrying marine water many decimenters
downward against the fresh groundwater advection. As a result, care must be exercised in interpreting direct measures of seepage
flux to recognize the contribution of recirculated seawater.
Intensive research in Chesapeake Bay has indicated that reductions in nitrogen inputs to the bay will be necessary to restore
water quality to levels needed for resurgence of bay living resources. Fall-line water quality monitoring efferts have characterized
diffuse-source nitrogen inputs from a large percentage of the bay drainage basin, but relatively little information exists
regarding rates of nitrogen delivery to tidal waters from coastal plain regions. Extensive nitrate contamination of shallow
groundwater due to agricultural activities, coupled with the dominant role of subsurface flow in discharge from Coastal Plain
regions of the drainage basin, creates the potential for high rates of nitrogen delivery to tidal waters via groundwater seepage.
This study utilized intensive hydrologic and water chemistry monitoring from April 1992 through September 1994 to determine
the spatial characteristics of the groundwater-estuarine interface, as well as the rates of subsurface nitrogen transport
from an agricultural field into nearshore waters of the Wye River, a subestuary of Chesapeake Bay. The hydrogeologic characteristics
of the study site resulted in groundwater discharge to the Wye River occurring almost exclusively within 15 m of the shoreline.
Calculated groundwater discharge rates were found to vary widely in the short term due to tidal fluctuations but in the long
term were driven by seasonal changes in groundwater recharge rates. The zone of groundwater discharge contracted shoreward
during summer months of low discharge, and expanded to a maximum width of approximately 15 m during high discharge periods
in late winter. Average discharge rates were more than five times higher in winter versus summer months. Groundwater nitrate
concentrations entering the discharge zone were relatively stable throughout the study period, with little evidence of denitrification
or nitrate uptake by riparian vegetation. Consequently, nitrogen discharge patterns reflected the strong seasonality in groundwater
discharge. Annual nitrate-N discharge was approximately 1.2 kg m−1 of shoreline, indicating drainage basin rates of nitrogen delivery to tidal waters of approximately 60 kg ha−1.
At 26 sites on the upper Great Lakes and their connecting channels where groundwater seepage measurements have been made, seepages decrease continually offshore at only seven (26%). Instead, 17 (63%) of the sites have offshore seepages which substantially exceed those at some locations closer to shore. The distributions fall into two consistent patterns, one where a nearshore decrease in seepage with increasing distance is succeeded offshore by an increase and a second where nearshore seepages are relatively constant and then increase farther offshore. The patterns have been observed in two independent studies using two totally different seepage meter designs and emplacement methods and are attributed to hydrogeologic heterogeneities beneath the surface-water body.Detailed marine geophysical measurements at seven sites on Green Bay show that the anomalously high offshore seepages there are associated with local thinning of the relatively low hydraulic conductivity glacial sediments separating the Bay from the underlying aquifer, a fractured dolomite. At these locations, heterogeneity in sediment thickness is a stronger factor in causing seepage anomalies than is heterogeneity of either sediment or bedrock hydraulic conductivity. Insufficient information exists to confirm a parallel cause-effect relation at the other sites, but evidence points in that direction.Similar seepage anomalies have been observed at other, smaller lakes by other researchers, but never with this frequency. It is argued that the cause of the difference is the size of the Great Lakes. Seepage occurs and measurements have been made over much greater offshore distances than in smaller lakes, and the likelihood that heterogeneities will occur within the zone of measurement is thus also much greater.
Submarine groundwater discharge (SGD) into a coastal lagoon off Perth, Western Australia, contains nitrate and silicate in concentrations two orders of magnitude higher than those of the receiving waters. This discharge delivers enough nitrate to replace that dissolved in the lagoon water mass about every eight days and enough silicate to replace the lagoon silicate in about 48 days. The delivery rate of nitrate nitrogen by SGD is equal to about 48% of that required for observed growth rates of lagoon macrophytes. Surface salinity is lower close to the shore as a result of SGD. During calm conditions a salinity front was observed in the lagoon, with a nearshore pool of nutrient-enriched water floating above the more saline ocean water.
Seepage flux can be measured and samples of groundwater flowing into lakes and estuaries collected by enclosing an area of bottom with a cylinder vented to a plastic bag. The method has the advantage of not requiring measurements of permeability of bottom sediments. Seepage velocities from −0.1–2.58 µ m s ⁻¹ were measured in Minnesota and Wisconsin lakes and in Nova Scotia and North Carolina estuaries.
Where seepage inflow was rapid (0.4–0.8 s ⁻¹ ), water collected with the seepage meter was chemically similar to water from wells on the same flow path, and the distribution and chemistry of the seepage concurred with a theoretical flow net. The rate and direction of seepage flux were correlated with water surface elevation during a tidal cycle.
Flow through porous media
- de Weist
New perspectives in the Chesapeake Bay system: A research and management partnership
- L. Libelo
- W.G. Maclntyre
- G.H. Johnson
Methods of soil analysis, Part I. Physical and mineralogical properties, including statistics of measurement and sampling
- A. Klute
Ground water nitrogen contributions to coastal waters of Virginia's Eastern Shore: Identification of high-risk discharge regions and remediation strategies Proceedings Watershed
- W G M A Reay
- C A Robinson
- Lunsford
Ground water discharge in coastal systems: Implications for the Chesapeake Bay
- W.G. Reay
- G.M. Simmons Jr
- S. Nelson
- P. Elliot
- B. Farquhar
- C. McManus
Submarine groundwater discharge quality in relation to land use patterns in the southern Chesapeake Bay
- G.M. Simmons Jr
- E. Miles
- W.G. Reay
- D.L. Gallagher
Soil Survey of Northampton County, Virginia
- P.R. Cobb
- D.W. Smith
Submarine groundwater discharge to the Patuxent River and Chesapeake Bay
- C.F. Zimmermann
- J.A. Mihursky
- A. Chaney
Submarine groundwater discharge to the Patuxent River and Chesapeake Bay New perspectives in the Chesapeake Bay System: A research and management
- C F Zimmermann
Ground water discharge in coastal systems: Implications for the Chesapeake Bay Perspectives on the Chesapeake Bay: Advances in estuarine sciences
- W G G M Reay
- Simmons
- Richardson D.L.
Ground-water quality assessment of the Delmarva Peninsula Delaware Maryland and Virginia-Analysis of available water-quality data through 1987
- P A R J Hamilton
- . J Shedlock
- Phillips
Ground water discharge in coastal systems: Implications for the Chesapeake Bay
- W G Reay
- . M Simmonsjr
InNew perspectives in the Chesapeake Bay System: A research and management Partnership
- C F Zimmermann