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Large Groundwater Inputs to Coastal Waters Revealed by Ra-226 Enrichments
Abstract
THE flow of ground water directly into the coastal ocean has been studied previously by in situ measurements, seep meters and diffusion gradient models1. Although these techniques provide ample evidence that such flows occur, they do not provide a means of quantifying the groundwater flux on a regional scale. Here I report large enrichments of 226Ra in coastal waters of the South Atlantic Bight, and demonstrate that groundwater discharge is the main source of the 226Ra surplus. Using 226Ra data for brackish ground waters with estimates of residence times of nearshore waters, I conclude that the groundwater flux to these coastal waters must be about 40% of the river-water flux during the study period. Besides Ra, other metals, nutrients and organic compounds are expected to be enriched in brackish ground waters, so these findings require an upward revision of terrestrial fluxes of dissolved materials to these coastal waters, and perhaps a re-evaluation of such fluxes to the global ocean. These fluxes may be sensitive to hydrological factors, groundwater usage, dredging and sea-level change.
... Submarine groundwater discharge (SGD) is an important contributor of chemical species in many coastal settings (Moore 1996(Moore , 2008), yet is understudied in the Arctic (Lecher 2017). Two forms of SGD exist in the Arctic: subpermafrost SGD and supra-permafrost SGD. ...
... For this study, we define SGD as the mix of fresh, brackish, and saline water originating from coastal sediments and discharging into nearby waters. Fluxes of SGD are often estimated using the four Ra isotopes (Moore 1996(Moore , 2008Garcia-Orellana et al. 2021): 223 Ra (t 1/2 $ 11.4 d), 224 Ra (t 1/2 $ 3.6 d), 226 Ra (t 1/2 $ 1600 yr), and 228 Ra (t 1/2 $ 5.75 yr). As Ra isotopes are part of the U/Th decay series, they are sourced from crustal materials including rock and sediment. ...
... We employed the Ra isotope mass balance box model approach to quantify SGD (Moore 1996;Moore and Krest 2004;Garcia-Orellana et al. 2021), using the short-lived isotopes ( 224 Ra and 223 Ra) only owing to high background activities for the long-lived isotopes in the Beaufort waters immediately outside the lagoon (Fig. 2, Supporting Information Fig. S1). The relative fraction of daily inputs of 224 Ra and 223 Ra for all sampling periods are shown in Fig. 4. Detailed discussion of the model and all parameters used can be found in the Supporting Information Data S1. ...
Supra‐permafrost submarine groundwater discharge (SGD) in the Arctic is potentially important for coastal biogeochemistry and will likely increase over the coming decades owing to climate change. Despite this, land‐to‐ocean material fluxes via SGD in Arctic environments have seldom been quantified. This study used radium (Ra) isotopes to quantify SGD fluxes to an Arctic coastal lagoon (Simpson Lagoon, Alaska) during five sampling periods between 2021 and 2023. Using a Ra mass balance model, we found that the SGD water flux was substantial and dependent on environmental conditions. No measurable SGD was detected during the spring sampling period (June 2022), when the lagoon was partially ice‐covered. During ice‐free periods, the main driver of SGD in this location is wind‐driven lagoon water level changes, not tides, which control surface water recirculation through sediments along the lagoon boundary. A combination of wind strength and direction led to low SGD fluxes in July 2022, with an SGD flux of (6 ± 3) × 10⁶ m³ d⁻¹, moderate fluxes in August 2021 and July 2023, which had an average flux of (17 ± 9) × 10⁶ m³ d⁻¹, and high fluxes in October 2022, at (79 ± 16) × 10⁶ m³ d⁻¹. This work demonstrates how soil and environmental conditions in the Arctic impact Ra mobilization, laying a foundation for future SGD studies in the Arctic and shedding light on the major processes driving Ra fluxes in this important environment.
... On a much larger scale, chemical tracer studies suggest that saline groundwater discharge may contribute as much water to the coastal ocean as river discharge, with important implications for the nutrient budgets in coastal seawater [6]. It is now apparent that the majority of this saline discharge occurs in pulses [7,8], and new observations show that this discharge can occur kilometers offshore, likely controlled by the sub-seafloor stratigraphic architecture [9]. Overall, several scales of groundwater flow and submarine groundwater discharge (SGD) have been recognized [10]. ...
... The study site is a 150 km 2 area approximately 2-20 km offshore of the Isle of Palms, which is a barrier island on the north side of Charleston Harbor, South Carolina ( Figure 1). This area is ideal to investigate the linkage of sedimentary architecture and SGD because it is easily accessible; previous studies have characterized the underlying pre-Quaternary stratigraphy, regional geologic structures, and bedforms; and it is one of the few where SGD is measured through geochemical tracers and modeling [7,17,[34][35][36][37][38]. ...
... Broader studies of SGD in the Georgia Bight have shown that the discharge of saline groundwater from the seafloor is similar in volume to river discharge in the region [7,8], and this SGD carries nutrient fluxes that are significant for primary productivity [5]. The mechanism for this discharge was unclear for decades, but recently, pulses of discharge were identified in an offshore well field, using heat as a tracer [9], showing that significant discharge occurs far from shore during persistent wind-driven lows in sea level. ...
The spatial variations in Quaternary sediments on the inner continental shelf are produced by the progression of depositional environments during the latest sea-level rise, and this sedimentary architecture plays a fundamental role in controlling groundwater discharge. However, coincident seismic mapping, sediment cores, and hydrological studies are rare. Here, we combine high-resolution, 0.5–10 kHz, high-frequency seismic profiles with sediment cores to examine the nature of the sediment deposits, including paleochannels, where submarine groundwater discharge has also been studied in a 150 km2 area of the inner shelf north of Charleston, South Carolina. We used high-frequency seismic reflection to interpret seismic facies boundaries, including 16 paleochannel crossings, to 20 km offshore. From 13 vibracores taken at the intersections of the seismic lines, we defined seven lithofacies representative of specific depositional environments. The paleochannels that we cored contain thick layers of structureless mud sometimes interbedded with silt, and mud is common in several of the nearshore cores. Our results indicate that paleochannels are often mud-lined or filled in this area and were most likely former estuarine channels. Neither the paleochannels nor a mud layer were found farther than 11 km off the present shoreline. This offshore distance coincides with the strongest pulses of groundwater discharge, emerging just beyond the paleochannels. This suggests that the muddy paleochannel system acts as a confining layer for submarine groundwater flow.
... Radium activities of HG, TG, and DG in spring and autumn are presented in Table 1. The results indicated that the variation range of radium activities in groundwater is extensive, reaching 2−3 orders of magnitude, mainly due to the different lithology of the aquifer in the groundwater environment (Moore et al., 1995;Moore, 1996). The spring's average activities of 223 Ra, 224 Ra and 228 Ra in the HG (15 m buried depth, the same as below) were 4.14 dpm/(100 L) (1 Bq=60 dpm), 286.91 dpm/(100 L), and 140.96 dpm/(100 L), respectively. ...
... Sin., 2023 With the acceleration of urbanization in Tianjin's estuarine and coastal zone, especially since the 1960s, human activities have significantly affected the features of radium isotopes in groundwater and surface water. On the contrary, many scientific studies have shown that radium isotopes can be used as a novel tool to reflect the impact of urbanization on surface water-groundwater systems, for instance, water flushing times (Moore et al., 2006), horizontal and vertical diffusion processes (Moore, 2000a;Ku et al., 1980), exchange processes between surface water and groundwater (Baskaran et al., 2009), and the fluxes of submarine groundwater discharge (SGD) (Moore, 1996). Features of radium in the surface water (HH and DLJ) are affected by the combined effects of industry and agriculture. ...
... On the other hand, radium isotopes can be used to reflect the impact of urbanization on surface water-groundwater systems. For example, the "radium isotope ( 223 Ra and 224 Ra) apparent age model" proposed by Moore (1996) was used in this study to calculate the water residence time of groundwater (HG, TG and DG). The results showed that the average residence time of HG, TG and DG in spring is 11.50 d, 15.50 d and 18.80 d, respectively, which is higher than 11.36 d, 14.99 d and 15.73 d in autumn. ...
In order to characterize the features of radium isotopes in estuaries of Tianjin, a continuous survey and sampling of typical estuaries were conducted from 2013 to 2017 in this study. The activities of natural radioactive radium isotopes (223Ra, 224Ra, and 228Ra) in groundwater and surface water were measured by the radium-delayed coincidence counting (RaDeCC) system. The non-conservative behavior of the radium isotopes was investigated under hydrogeochemical conditions and urbanization. The results indicated that in terms of horizontal distribution, the activities of radium in groundwater (Hangu, Tanggu, and Dagang) showed an upward trend from north to south and demonstrated a higher figure than surface water (Haihe River and Duliujian River). Concerning the vertical distribution, the activitives of radium at a 15 m burial depth was higher than that at a 30 m burial depth in all measurements. The activities of radium isotopes in the study area increased with the increase of total dissolved solids, and their desorption behavior on Fe-Mn oxides was constrained by the redox intensity. Different hydrogeological conditions resulted in variations in the vertical profile of radium activities. The activity of radium was regulated by seasonal variation and precipitation in groundwater and surface water. In addition, the rapid urbanization has caused a significant impact on the features of radium isotopes in typical estuaries of Tianjin. Meanwhile, radium isotopes can be applied to reflect the impact of urbanization on surface water-groundwater systems. Clarifying and cleverly utilizing the relationship between behavior of radium isotopes and urbanization will promote the development of the Tianjin Binhai New Area in a healthy way.
... Currently, direct measurements (Lee, 1977), hydrologic modeling (Hou et al., 2016;Lee et al., 2012) and environmental isotope tracing (Cable et al., 1996;Kwon et al., 2014;Liu et al., 2015;Moore, 1996;Swarzenski et al., 2004) are commonly used in SGD studies. In addition, methods such as physical exploration and remote sensing are also included (Schubert et al., 2014;Taniguchi et al., 2003). ...
... Through the establishment of a steady-state mass balance model based on radium, where the input flux equals the loss flux, we can estimate the radium flux associated with SGD input. Subsequently, by selecting appropriate radium-groundwater endmembers, the SGD flux can be quantified (e.g., Moore, 1996;Moore et al., 2006). The principles governing radon evaluation for SGD are fundamentally akin to radium, with subtle differences in source and sink terms (e.g., Santos et al., 2009;Zhang et al., 2016). ...
Ocean acidification (OA) is a global environmental concern, and submarine groundwater discharge (SGD) is a potentially process that enhances OA. This review summarizes the relationship between two types of constituents carried by SGD into China's seawater and OA. 1) Current research predominantly concentrates on constituent fluxes from SGD, neglecting its ecological impacts on carbon and nutrients budgets, as well as the mechanisms between carbon and nutrients. 2) Uncertainties persist in SGD research methods and acidification characterization. 3) There's a need to enhance quantitative research methods of SGD-OA, particularly in areas with intricate biogeochemical processes. Effective identification methods are crucial to quantify SGD's contribution to OA. Investigating core scientific questions, including SGD's impact on OA rates and scales, is paramount. While the primary focus is on SGD-OA research in China, insights gained from novel perspectives could have broader value for coastal management globally.
... The combined analysis of stable hydrogen and oxygen isotopic compositions of seawater, salinity, and temperature data is effective in identifying the origin and mixing patterns of water masses at the surface of the ocean, with isotopic patterns in surface seawater typically influenced by processes such as precipitation, evaporation, mixing, and freezing, thawing (Craig and Gordon, 1965). Radioactive radium isotopes are enriched in saline groundwater and conservative in seawater once discharged from groundwater (Burnett et al., 2006), making them excellent natural tracers for quantifying SGD at various scales, from hundreds of kilometers or less (Moore, 1996;Peterson et al., 2008), to thousands of kilometers (Rodellas et al., 2015) and even on global scales (Kwon et al., 2014;Cho and Kim, 2016). The Bohai Sea, acting as a vital buffering and attenuation zone for anthropogenic and natural pollutants between the coastal regions and open sea, often experiences severe environmental pollution challenges (Gao et al., 2014). ...
... Upon returning to the laboratory, these samples were kept at − 20 • C for less than three days before measurements. For radium extraction, water was collected in a large volume of 60 L for seawater, 30 L for river water, and 2 ~ 10 L for coastal groundwater and drained slowly (with a flow rate of less than 1 L min − 1 ) through manganese-coated acrylic fiber (Mn-fiber) that absorb dissolved radium from water (Moore, 1996). ...
... Previous studies have utilized radon to determine the presence of SGD (Burnett and Dulaiova, 2003); and recent studies have taken a more quantitative approach by using radon and radium to quantify SGD rates (Burmett et al., 2006;Colbert et al., 2008;Dulaiova et al., 2010;Rodellas et al., 2017). The general methodology adopted for the assessment of SGD flux through the radium isotope concentration includes following procedures; identification, mapping and evaluation of coastal aquifer derived tracers which are not recycled through coastal waters; determination of rate of exchange between coastal ocean and open ocean; estimation of tracer flux from aquifer to coastal ocean; estimating the concentration of tracers between aquifer and coastal ocean; the fluxes of other components of coastal system such as nutrients, carbon, metals or the ratios to the tracer are measured (Moore, 1996). For the local scale assessment of SGD flux is established through a geochemical model developed by . ...
... It is important to note, however, that while airborne TIR has the ability to detect temperature differences in surface water, in order to detect SGD, the groundwater must be more buoyant than the body of water it is discharging into (Becker, 2006). Fresh water is more buoyant than saline water, and will float along the surface; however cold SDG will sink when entering into a fresh-water environment (Moore, 1996). The long term monitoring of SGD flux in a particular coastal area can be conducted precisely using remote sensing methods apart from the traditional methods. ...
Submarine groundwater discharge (SGD) is the combination of fresh and saline groundwater flux to marine system through continental boundaries regardless of its chemical composition and factors influencing the flow. We have discussed the SGD studies in the Asian context; SGD has been studied in various parts of Asia, including China, Japan, South Korea, and Southeast Asia. In China, SGD has been studied in several coastal regions, including the Yellow Sea, the East China Sea, and the South China Sea. In Japan, SGD has been studied in the Pacific coast, where it has been found to be an important source of fresh water to the coastal ocean. In South Korea, SGD has been studied in the Yellow Sea, where it has been found to be an important source of fresh water to the coastal ocean. In Southeast Asia, SGD has been studied in several countries, including Thailand, Vietnam, and Indonesia. Recently the SGD studies acquired much development India, the research on SGD in India is limited, and more studies are needed to understand the SGD process, its impact on the coastal environment, and the management strategies, Groundwater extraction for irrigation, industry, and domestic use is increasing in India, which can affect the SGD process in coastal aquifers. Overall, the studies suggest that SGD is an important process in Asian coastal regions, playing a role in the supply of fresh water and the transport of pollutants and nutrients.
... However, the water masses with nutrients derived from coastal sediments often do not have low salinity but can be laterally transported to the offshore oceans by nearshore ocean currents and associated mesoscale eddies (Ueno et al., 2023). The transport of sediment-derived nutrients can be traced via chemical parameters such as N*, a tracer for denitrification and nitrogen fixation (Gruber & Sarmiento, 1997;Yoshikawa et al., 2006), and radon and radium isotopes (Moore, 1996;Sugimoto et al., 2016). Although salinity can be determined easily and continuously by using a conductivity sensor, the determination of chemical parameters usually requires water sampling and chemical analysis. ...
The lateral transport of nutrient‐rich water masses from coastal areas to offshore areas is an important factor controlling the primary production of phytoplankton in offshore regions. If the coastal low‐salinity waters have high nutrient levels, lateral transport can be traced via salinity observations. However, if high level of nutrients in the waters are not coupled with low salinity, for example, input from coastal sediments, water transport cannot be traced via salinity observations. In this study, visible fluorescent organic matter (FOMvis) was monitored by an in situ sensor as a tracer for sediment‐affected water masses from coastal areas. The FOMvis distribution in the upper 500 m water layer in offshore Japan along the 143°E transect was generally controlled by the distributions of subtropical water (i.e., Kuroshio water and Tsugaru warm current water) and subarctic water (i.e., Oyashio water). However, anomalously high levels of FOMvis were observed at depths of 20–60 m at an observational station, which could not be explained by physical mixing. This likely originated from the lateral transport of water masses from coastal areas. In and around Sendai Bay, where observations were carried out as an example of the Japanese coast, high levels of FOMvis were observed, accompanied by negative N*, an indicator of denitrification, and phosphate and silicic acid inputs from anoxic sediments. These observational results imply that FOMvis observation via in situ sensors can be useful for tracing the input of nutrient‐rich sediment‐affected water masses from coastal areas to offshore oceans.
... Fresh and saline SGD results in brackish SGD which are having high concentrations of nutrients, carbon, metals and greenhouse gases (Santos et al., 2021). The process of SGD was acknowledged by oceanographers and hydrologists for ages, but after the nineties more attention were given when its volumetric and chemical importance were recognized (Burnett, 1999;Moore, 1996). Although the rate of discharge through SGD is volumetrically small compared to riverine discharge; its nutrient, carbon, trace metal and other land derived pollutant load affects the coastal-marine ecosystem significantly and may create hypoxic zones (Correa et al., 2020;George et al., 2021;Misra et al., 2022;Xue et al., 2023). ...
Submarine Groundwater Discharge (SGD) and Seawater Intrusion (SWI) are two contrary hydrological processes that occur across the land-sea continuum and understanding their nature is essential for management and development of coastal groundwater resource. Present study has attempted to demarcate probable zones of SGD and SWI along highly populated Odisha coastal plains which is water stressed due to indiscriminate-exploitation of groundwater leading to salinization and fresh groundwater loss from the alluvial aquifers. A multi-proxy investigation approach including decadal groundwater level dynamics, LANDSAT derived sea surface temperature (SST) anomalies and in-situ physicochemical analysis (pH, EC, TDS, salinity and temperature) of porewater, groundwater and seawater were used to locate the SGD and SWI sites. A total of 340 samples for four seasons (85 samples i.e., 30 porewater, 30 seawater and 25 groundwater in each season) were collected and their in-situ parameters were measured at every 1–2 km gap along ~ 145 km coastline of central Odisha (excluding the estuarine region). Considering high groundwater EC values (> 3000 μS/cm), three probable SWI and low porewater salinities (< 32 ppt in pre- and < 25 ppt in post-monsoons), four probable SGD zones were identified. The identified zones were validated with observed high positive hydraulic gradient (> 10 m) at SGD and negative hydraulic gradient (< 0 m) at SWI sites along with anomalous SST (colder in pre- and warmer in post-monsoon) near probable SGD locations. This study is first of its kind along the Odisha coast and may act as initial basis for subsequent investigations on fresh-saline interaction along the coastal plains where environmental integrity supports the livelihood of coastal communities and the ecosystem.
... The variability of 226 Ra and Ba concentrations in surface ocean water is small except in coastal regions, where values may increase significantly due to inputs from various sources. These inputs include desorption of 226 Ra and Ba from sediment delivered by rivers (Li & Chan, 1979) and submarine groundwater discharge (SGD; Moore, 1996;Shaw et al., 1998). Bullock et al. (2022) estimated (dissolved plus desorbed) 226 Ra riverine inputs to the Pacific of (+17.5 ± 11.6) 10 14 dpm y 1 . ...
Radium‐226(²²⁶Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full‐depth ocean section of ²²⁶Ra activities (T1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of ²²⁶Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher ²²⁶Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with ²²⁶Ra inputs primarily resulting from the decay of ²³⁰Th in sediments. Further, dissolved ²²⁶Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non‐conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non‐conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of ²²⁶Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non‐conservative processes that shape their respective distributions.
... Submarine groundwater discharge (SGD) is a key process driving chemical species exchange at the land-ocean boundary (Jeandel, 2016). About two decades ago, the flux of SGD into the South Atlantic Bight was proposed to be at least 40% of the river flux (Moore, 1996). Since then, SGD has attracted increasing attention, and has been now acknowledged as an important source of biogenic elements in the marine environment, including carbon, nutrients, and trace-elements required by marine organisms Garcia-Orellana et al., 2021;Luo et al., 2023;Moore, 2010a;Santos et al., 2021;Zhang et al., 2020Zhang et al., , 2024 However, most of these studies are concerned with the quantification of the total groundwater discharge without distinguishing different types of SGD processes. ...
As a significant source of nutrients and other components into the coastal ocean, submarine groundwater discharge (SGD) is a combination of multiple spatial‐temporal scale processes, including fresh terrestrial groundwater (FSGD), recirculating seawater (RSGD), and porewater exchange (PEX). Quantifying the different types of SGD is extremely important for understanding the coastal biogeochemical cycles of various materials. The green tide bloom in the southern Yellow Sea (China) has attracted increasing attention during the past decade. Haizhou Bay is thought to be an important green tide proliferating area, but the source of an apparent high supply of nutrients has not been identified yet. We report here on our investigations of the distribution patterns of Ra and Rn isotopes in groundwater and seawater in Haizhou Bay. By solving a combined mass balance for ²²⁴Ra, ²²³Ra, ²²⁶Ra, and ²²²Rn, we estimated that the bay's water residence time is 28.8 days, FSGD is highest at 3.6 cm d⁻¹, RSGD is 2.7 cm d⁻¹, and PEX is lowest at 0.6 cm d⁻¹. The total SGD into Haizhou Bay is estimated at 9.40 × 10⁷ m³ d⁻¹ (6.9 cm d⁻¹), about 12 times that of the local river discharge into the bay. SGD‐derived nutrients are shown to play an important role when considered among all known nutrient sources. Our results suggest that dissolved inorganic nitrogen (DIN) and silicate (DSi) are transported by SGD mainly via FSGD. We also note that the phosphorus (DIP) budget is heavily influenced by RSGD.
... Particle-reactive elements are scavenged by SPM and settle to coastal bottom sediments, from which they can be released into coastal seawater during early diagenesis (Audry et al., 2006;Homoky et al., 2016). In addition, submarine groundwater discharge may also contribute a significant quantity of dissolved elements and nutrients to the coastal ocean (Burnett et al., 2001;Kim and Kim, 2014;Moore, 1996). ...
... For example, most studies have determined groundwater Ra end-members based on the mean Ra activity in groundwater (e.g., Moore et al., 2008;Kwon et al., 2014;Liu et al., 2017). Moore (1996) and Stewart et al. (2015) used the maximum 226 Ra activity value of available groundwater samples for groundwater endmember to avoid SGD overestimation. Kim et al. (2005) calculated SGD by considering the entire value range of radium activity in groundwater samples, such that the groundwater radium end-member corresponds to a range. ...
... The last few decades have witnessed the advent of several approaches and techniques to quantify SGD including seepage meters studies (Belanger and Walker, 1990;Michael et al., 2003;Taniguchi et al., 2006), radon and radium isotopes such as 223,224,226,228 Ra and 220,222 Rn as geochemical tracers (Cable et al., 1996;Moore, 1996;Burnett et al., 2001;Chanyotha et al., 2014;Cabral et al., 2023), hydrogeological groundwater modeling (Oberdorfer, 2003;Al-Taliby et al., 2016;Russoniello et al., 2018;Evans et al., 2020;Al-Taliby and Pandit, 2023), thermal imaging using infrared sensors (Johnson et al., 2008;Wilson and Rocha, 2012), and geophysical methods like electrical resistivity Dimova et al., 2012). The majority of SGD flux measurement research employed Ra and Rn isotopes and seepage meters Santos et al., 2021). ...
The Indian River Lagoon System (IRLS) has been impacted by the surrounding development, leading to excessive nutrient loads that have resulted in frequent and prolonged phytoplankton blooms in the northern reaches. Our study focused on estimating terrestrial groundwater discharge (TGD) and associated nutrient loads by combining field measurements and hydrogeologic modeling at four transects: Eau Gallie (EGT), River Walk (RWT), Banana River (BRT), and Mosquito Lagoon (MLT) across the IRLS. Multiple monitoring stations were installed to collect groundwater and surface water levels, salinity, and nutrient concentrations during 2014-2015. Samples were analyzed for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). Numerical modeling was accomplished using SEAWAT to simulate TGD rates, whereas nutrient loads were calculated by multiplying simulated TGD by measured concentrations. TGD rates and nutrient loads were also estimated specifically for the “near-shore zone” along each transect. The effect of recharge from underlying Hawthorn Formation was also evaluated by incorporating estimated recharge rates into the models. Porewater and lagoon water samples showed that ammonium predominated over (NO2+NO3) and PO4 at all sites, resulting in DIN/DIP ratio surpassing the Redfield ratio. Low nitrite/nitrate, coupled with elevated ammonium concentrations at RWT, BRT, and MLT, may be attributed to biogeochemical transformations catalyzed by mangroves and wetlands. Simulated TGD showed mild temporal but significant spatial variation, especially between EGT and RWT compared to BRT and MLT. The highest average TGD of 0.73 and 0.77 m³/d.m occurred at RWT and EGT, respectively, whereas the lowest rates were predicted at BRT and MLT. The highest estimated average DIN loads of 507 and 428 g/yr.m were received at EGT and RWT, respectively, whereas MLT and BRT exhibited lower loads. The DIP loads were remarkably lower than the DIN loads and were significantly different in space and time between sites. Elevated DIN combined with reduced DIP resulted in DIN/DIP exceeding the Redfield ratio, thereby encouraging the blooming of harmful algae. Although the majority of seepage occurs through the near-shore zone, small amounts are received along the entire transect at all sites. The Hawthorn Formation does not contribute significant recharge to the aquifer at the transect locations.
... In this study, we utilized V/Ca ratios data from Porites coral skeletons in Xiaodonghai Bay near Sanya, northern SCS to assess the long-term history of variations in surface seawater V levels over a continuous record of coral growth spanning 137 years . Considering the significance of submarine groundwater discharge (SGD) in providing trace metals to the ocean (Moore, 1996(Moore, , 2010Moore et al., 2008), and the chemical conditions of the local aquifer, we deliberately evaluated the potential impacts of SGD on surface seawater V levels, based on the SGD rates constructed by rare earth element (REE) data from the same coral core as this study reported by our previous study (X. Jiang, Ren, et al., 2018;W. ...
In recent decades, vanadium (V) levels in the environment have dramatically increased and will continue to rise alarmingly in the future. Conventional methods for assessing oceanic V typically only involved the collection of seawater and sediments, which significantly limited our understanding of the dynamics of oceanic V and its relationship to environment and climate. Here, the interannual temporal variations in surface seawater V levels in the northern South China Sea, were reconstructed utilizing a 137‐year record of V/Ca ratios from Porites lutea coral from Xiaodonghai Bay off Sanya. We observed that anthropogenic activities had a limited impact on the V levels of surface seawater in the study area, whereas V variability exhibited significant disparities across different time periods. Precipitation‐related submarine groundwater discharge (SGD) recharge was the primary driver of V variation prior to 1990, with tropical cyclones (TCs) playing a detectable role in total precipitation during this period. Conversely, changes in surface seawater V levels after 1990 were predominantly linked to increasingly intense upwelling driven by summer winds. The recharge processes in unconfined groundwater systems within the study area might generate favorable oxic and alkaline conditions, which facilitated mineral dissolution and/or desorption reactions that elevated V concentrations along the groundwater flow path, which were recorded in coral skeletal V/Ca ratios. This study confirmed the seawater V was dominated by SGD and upwelling near Sanya, providing essential evidence for comprehending the geochemical behavior of V and the impact of ocean‐atmospheric climate interactions on long‐term variations in seawater V levels.
... Based on their suggestion, we choose the average activities obtained at near-shore stations as representative values of the recirculated SGD. Assuming that the ratio of fresh SGD flux to SGD flux (i.e., R F ) would be considered as 10% (Moore, 1996), the corrected groundwater 224 Ra and 228 Ra activities of each station are calculated, as shown in Table S2. ...
Submarine groundwater discharge (SGD), which can be traced using naturally occurring radium isotopes, has been recognized as a significant nutrient source and land-ocean interaction passage for the coastal waters of the Daya Bay, China. However, uncertainties in assessing SGD fluxes must still be discussed in detail. In this study, we attempted to utilize the Monte Carlo method to evaluate the uncertainties of radium-derived SGD flux in the northeast and entirety of the Daya Bay. The results show that the uncertainties of the SGD estimate in the northeast bay are very sensitive to variations in excess radium inventories as well as radium inputs from bottom sediments, while the uncertainties of the SGD estimate for the entire bay are strongly affected by fluctuations in radium inputs from bottom sediments and radium end-members of SGD. This study will help to distinguish the key factors controlling the accuracy of SGD estimates in similar coastal waters.
... This likely results in a conservative SGD estimate since radionuclide activities of the parent isotopes have been shown to decrease by approximately a factor of 4 offshore in the GBR. 25 Previous studies have used the maximum radium activity for the groundwater endmember to provide conservative estimates. 29,51 If that same approach was applied to our porewater exchange estimates, porewater exchange rates would decrease between 30 and 51% but would still be an order of magnitude greater than river inputs. However, because both the mean concentration of 224 Ra (124.0 ± 25.3 Bq m −3 ) and 223 Ra (3.8 ± 0.8 Bq m −3 ) and median concentrations (125.1 and 3.2 Bq m −3 , respectively) were similar, we use the average concentration, which is likely the most representative of porewater entering the GBR. ...
Rivers are often assumed to be the main source of nutrients triggering eutrophication in the Great Barrier Reef (GBR). However, existing nutrient budgets suggest a major missing source of nitrogen and phosphorus sustaining primary production. Here, we used radium isotopes to resolve submarine groundwater discharge (SGD)-derived, shelf-scale nutrient inputs to the GBR. The total SGD was ∼10−15 times greater than average river inputs, with nearshore groundwater discharge accounting for ∼30% of this. Total SGD accounted for >30% of all known dissolved inorganic N and >60% of inorganic P inputs and exceeded regional river inputs. However, SGD was only a small proportion of the nutrients necessary to sustain primary productivity, suggesting that internal recycling processes still dominate the nutrient budget. With millions of dollars spent managing surface water nutrient inputs to reef systems globally, we argue for a shift in the focus of management to safeguard reefs from the impacts of excess nutrients.
... Submarine groundwater discharge (SGD) is the total water flux from coastal aquifers to the ocean and has been well known as an important component of the hydrologic cycle and a major seaward carrier of terrestrial materials (Moore 1996;Burnett and Dulaiova 2003). SGD is driven by different forces including terrestrial hydraulic gradients, density, tides, wave, and so on (Robinson et al. 2018). ...
Salt marshes can export considerable nutrients and carbon to the ocean through submarine groundwater discharge (SGD). However, the complicated SGD processes in salt marshes remain poorly understood. Here, we first report the phenomenon of numerous highly saline artesian springs found in a salt marsh system of East China. Multiple methods including time‐series thermal monitoring, isotope signatures, and high‐resolution electrical resistivity tomography were combined to determine their origin and trajectory. Strong evidence suggests that these springs keep discharging even during high tide and represent a long‐term re‐distribution process of the ancient marine water trapped in the unconfined aquifer. This new pattern of spring‐derived groundwater flow indicates a hidden SGD pathway and has significant implications for studies concerning SGD‐derived fluxes in similar multi‐aquifer‐aquitard coastal systems.
... The point measurements cannot be upscaled directly due to the heterogeneity of the coastal aquifers (Duque et al., 2020). The isotope tracer-based mass balance modeling, due to its explicit principles, feasibility, and suitability for multiscale regions, has been popular in recent decades since Moore (1996). Case studies have been conducted in many of the eutrophication hotspots in coastal waters, including those in China (Gu et al., 2012;Kim et al., 2005;Liu et al., 2017;Luo et al., 2014;, the east coast of Australia (Correa et al., 2020;Paytan et al., 2006;Stewart et al., 2015), the east coast of the United States (Beck et al., 2008;Charette & Buesseler, 2004;Fear et al., 2007;Heiss et al., 2020;Heiss & Michael, 2014;Porubsky et al., 2014), Europe (Bejannin et al., 2020;Montiel et al., 2018;Oehler et al., 2017), and the Arctic (Dimova et al., 2015). ...
River deltas typically have high population density and support a wide range of intensive and prosperous socioeconomic activities. The hydrological processes in these regions are complex, primarily due to the interactions among the river, aquifer, and sea. However, a systematic and quantitative elaboration of the river-aquifer-sea interactions is still lacking. Here we developed an integrated hydrological flow model for the Pearl River Delta (PRD), which contains the world's largest urban area in both size and population, to gain a deeper understanding of the complexities in the river-aquifer-sea interactions. The model performance was validated and cross-checked via observations at gauging stations and independent remote-sensing products (e.g., soil moisture, ET and total water storage anomalies). Based on the 10-year simulation results (2004-2013), the major findings of this study are as follows: 1) accurate representation of the tidal effect is important not only for simulating short-term flow dynamics but also for capturing the characteristics of long-term hydrological fluxes and states; 2) the flow-model-computed average groundwater discharge rate per unit length of the coastline for the PRD is 3.01 m 3 /d/m, which is comparable with those derived from water budget approaches but 1-2 orders of magnitude lower than the total submarine groundwater discharge (SGD) estimated by using isotope tracer-based methods; 3) the temporal variation of SGD is controlled by tidal forcing on an hourly time scale, but by terrestrial hydrological processes on monthly and annual time scales; and 4) an integrated hydrological flow model can be used to identify distinct and large subsurface zones sensitive to tidal fluctuations, quantifying the pivotal role of ocean tides in shaping the coastal groundwater system. This study represents a first step in using an integrated hydrological model to explore river-aquifer-sea interactions and their effects on the regional groundwater system simultaneously driven by meteorological and tidal forcings.
... Although global SGD rate assessments are difficult to establish, it is estimated that the magnitude of fresh SGD in the ocean is between 1 and 10% of annual river discharge (Burnett et al., 2003;Taniguchi et al., 2019). Moore (1996) determined that SGD, mainly saline SGD, to the ocean shelf must be about 40% of the river flux to the same area. In the Baltic Sea, fresh SGD represents~4% of river runoff (Peltonen, 2002). ...
Submarine groundwater discharge (SGD) can be a significant source of dissolved nutrients, inorganic and organic carbon, and trace metals in the ocean and therefore can be a driver for the benthic-pelagic coupling. However, the influence of hypoxic or anoxic SGD on the carbonate system of coastal seawater is still poorly understood. In the present study, the production of dissolved inorganic carbon (DIC) and alkalinity (AT) in coastal sediments has been investigated under the impact of oxygen-deficient SGD and was estimated based on the offset between the measured data and the conservative mixing of the end members. Production of AT and DIC was primarily caused by denitrification and sulphate reduction. The AT and DIC concentrations in SGD decreased by approximately 32% and 37% mainly due to mixing with seawater counterbalanced by reoxidation and CO2 release into the atmosphere. Total SGD-AT and SGD-DIC fluxes ranged from 0.1 to 0.2mol m⁻² d⁻¹ and from 0.2 to 0.3mol m⁻² d⁻¹, respectively. These fluxes are probably the reason why the seawater in the Bay of Puck is enriched in AT and DIC compared to the open waters of the Baltic Sea. Additionally, SGD had low pH and was undersaturated with respect to the forms of the aragonite and calcite minerals of CaCO3. The seawater of the Bay of Puck also turned out to be undersaturated in summer (Inner Bay) and fall (Outer Bay). We hypoth e size that SGD can potentially contribute to ocean acidification and affect the functioning of the calcifying invertebrates.
... Different pockmark morphologies indicate different fluid flow regimes and the persistent flow 1 Introduction Submarine groundwater discharge (SGD) is a common global phenomenon that describes the flow of fresh or saline waters from the subsurface into the ocean. Fresh submarine groundwater discharge (FSGD) generally occurs in regions where onshore aquifers with higher terrestrial hydraulic heads compared to the sea-level are hydraulically connected to the ocean (Moore, 1996). Where confined aquifers extend beneath the ocean, offshore freshened groundwater systems (OFGs) can form. ...
Fresh submarine groundwater discharge (FSGD) influences the biogeochemistry of coastal areas and can be a proxy for potential untapped resources of offshore freshened groundwater (OFG). In most areas however, the onshore-offshore connection and the recharge characteristics of offshore aquifers are poorly constrained, making a potential exploitation of this resource challenging. Offshore Wellington (New Zealand), a well-defined onshore aquifer system extends beneath the harbour, where substantial amounts of freshwater seep out from the ocean floor. The aquifer system has been studied in detail and recently the first attempts worldwide have been made here to use the offshore groundwater as a future source of drinking water. However, the locations and extent of FSGD as well as its influence on seafloor morphology are still poorly understood. Exact localisation of FSGD sites is essential to sample and quantify discharging waters but remains challenging due to a lack of robust and appropriate measurement procedures. Novel sensing strategies, such as the influence of seeping groundwater on hydroacoustic water column reflectivity could greatly improve the identification of groundwater discharge locations worldwide. Therefore, we use a multidisciplinary dataset and evaluate different methodologies to map the spatial extent of FSGD sites and determine their geomorphologic expressions on the seafloor of Wellington Harbour. In this study, single and multibeam hydroacoustics and towfish (temperature, salinity and turbidity) transects were combined with remotely operated vehicle (ROV) dives and sediment cores to better characterise FSGD sites. We observed several hundred seafloor depressions (pockmarks) that we attribute to continuous seepage of gas and groundwater from the seafloor. Different pockmark morphologies indicate different fluid flow regimes and the persistent flow allows even small pockmarks to remain unchanged over time, while the geomorphologic expressions of anchor scours on the seafloor diminish in the same region. Enhanced hydroacoustic reflections in the water column within and above the pockmarks indicate suspended sediment particles, which are likely kept in suspension by discharging groundwater and density boundaries.
... Data on composition and chemistry of bedrock and groundwater are often unavailable, flow paths from terrestrial nutrient sources to the seafloor can be heterogeneous, and fluxes are difficult to measure in situ (Lino et al., 2023). Tracer techniques to estimate volume flows into coastal waters have been used, including radium (Moore, 1996), radon , methane Chanton et al., 1996) DSi (Oehler et al., 2019) and multiple tracers (Godoy et al., 2013;Rocha et al., 2016). In many regions, groundwater fluxes are simply ignored, but in some areas, especially arid regions with little surface runoff or geology that favors subsurface flows (e.g., regions of karst topography such as the Yucatan), these inputs to coastal waters can be significant. ...
... Although groundwater discharge into coastal oceans was recognized as a significant component of the hydrologic cycle in the 19th century (Sonrel 1868), SGD remained unquantified until the mid-1990s (Moore 1996). This discharge is diffusive and spatially and temporally highly variable. ...
Chemical and isotopic processes occur in every segment of the hydrological cycle. Hydrogeochemistry-the subdiscipline that studies these processes-has seen a transformation from "witch's brew" to credible science since 2000. Going forward, hydrogeochemical research and applications are critical to meeting urgent societal needs of climate change mitigation and clean energy, such as 1) removing CO2 from the atmosphere and storing gigatons of CO2 in soils and aquifers to achieve net-zero emissions, 2) securing critical minerals in support of the transition from fossil fuels to renewable energies, and 3) protecting water resources by adapting to a warming climate. In the last two decades, we have seen extensive activity and progress in four research areas of hydrogeochemistry related to water-rock interactions: arsenic contamination of groundwater; the use of isotopic and chemical tracers to quantify groundwater recharge and submarine groundwater discharge; the kinetics of chemical reactions and the mineral-water interface's control of contaminant fate and transport; and the transformation of geochemical modeling from an expert-only exercise to a widely-accessible tool. In the future, embracing technological advances in machine learning, cyberinfrastructure, and isotope analytical tools will allow breakthrough research and expand the role of hydrogeochemistry in meeting society's needs for climate change mitigation and the transition from fossil fuels to renewable energies. This article is protected by copyright. All rights reserved.
... Submarine groundwater discharge (SGD) is an important process for the global hydrological cycle (Moore, 1996;Burnett et al., 2003;Santos et al., 2021). SGD has a significant influence on marine environments and elemental biogeochemical cycles, as it delivers large quantities of substances (e.g., nutrients, trace metals, carbon, and rare earth elements) to the coastal ocean (Kim et al., 2005;Rodellas et al., 2015;Garcia-Orellana et al., 2021;Santos et al., 2021). ...
Naturally occurring radium (223Ra, 224Ra, 226Ra, and 228Ra) isotopes have been widely applied as geochemical tracers in marine environments, especially when estimating the submarine groundwater discharge (SGD). In this sense, the influencing factors and transport mechanism of radium isotope activity in aquifers can be key information for SGD estimation. This work evaluates the adsorption/desorption behavior of 224Ra and 226Ra in the solid-liquid phase through a leaching experiment and analysis of field data. The results suggested that radium isotope activity was positively correlated with salinity and grain size, in the case of abundant sediments. Through ion analysis, we found that the ions (Na+, Ca2+, Mg2+, and Ba2+) exchanged with radium isotopes in the process of transport. A 1-D reactive transport model was established to simulate the transport process of radium isotope in aquifers. The model successfully simulated the variation of radium isotope desorption activity with salinity and was subsequently verified in the field. This study contributes to the understanding of the geochemical behavior of radium isotopes in aquifers and provides guidance for selecting a suitable groundwater endmember in SGD estimation.
... Offshore occurrence of freshwater, which may or may not discharge across the land-ocean interface, has also been highlighted recently (e.g., Post et al., 2013). Global estimates suggest that SGD could have a substantial impact on ocean chemistry by conveying trace elements (Charette et al., 2016;Church, 1996;Moore, 1996) and nutrients (Rodellas et al., 2015) from continental shelf sediments to the open oceans. Despite the low water flux of fSGD compared to global river runoff (a few percent at most; Zektser & Loaiciga, 1993;Berner & Berner, 2012;Zhou et al., 2019), its impact on the global geochemical cycle is evident. ...
... It was recognised by hydrologists for many years [6], but SGD drew little attention from the oceanographic community until after [2]. However, SGD has received increased attention during the last few decades since it was recognised that it may be both volumetrically and chemically important [5,7]. Although SGD is generally small compared to riverine flow into oceans, it contributes high concentrations of nutrients, trace metals and other land-derived contaminants [8,10]. ...
Submarine Groundwater Discharge (SGD) is the continuous flow of groundwater under favourable hydrogeological and hydrological settings, which carries nutrients, traces metals and other contaminants to the sea coast and influences the coastal ecosystem. Therefore, integrated site-specific field investigations are necessary for identifying the SGD zones and its magnitude assessment for sustainable development of the coastal ecosystem. An attempt has been made to identify the potential SGD zones along the 485 km long coastline of Odisha using the groundwater level data and physico-chemical analysis of groundwater, pore water and sea water samples. The areas having an occurrence of groundwater level above mean sea level (more than tidal amplitude) together with the absence of any significant seasonal/diurnal variation in groundwater levels were identified as probable SGD zones. In situ measurement of basic water quality parameters (temperature, pH, salinity, EC, DO, etc.) was carried out at every 3–5 km intervals along the coastline for sea water (44), pore water (43) and groundwater (36) samples using portable multiparameter water quality kit during the field investigation. The pore water samples, indicating low salinity (<25 ppt) and low EC values (<35 mS/cm), were considered to be suspected SGD. A total number of seven such suspected zones were identified along the coastal tract of Odisha. The pore water salinity in these seven locations is found to be 14.6 ppm (Chaumukh–Balasore), 3.05 ppt (Chandipur–Balasore), 3.71 ppt (Madanpur–Balasore), 19.79 ppt (Pentha–Kendrapara), 18.9 ppt (Beleshwar–Puri), 12.59 ppt (Golden Beach, Puri) and 0.46 ppt (Nolia Nuagaon, Ganjam) in the proximity of sea coast between low tide and high tide. The nutrients (Nitrate, Phosphate and Silica) analysed in the pore water samples also have shown elevated signatures for the identified zones, thus support possible SGD and need further conformation using stable isotopes, sea surface temperature and radon measurements.KeywordsUbmarine groundwater dischargeGroundwater levelPore waterOdisha coast
... The water resources on Earth are classified according to the feed concentration. Brackish water, which is defined based on its range of salinity between seawater and freshwater, includes river water, groundwater, and surface water [18]. The concentration of brackish water was the lowest among the other water resources. ...
High-energy consumption is a critical issue associated with seawater reverse osmosis (SWRO) desalination, although the SWRO has been regarded as one of the most energy-efficient processes for seawater desalination. This means that SWRO involves a larger amount of fossil fuel and other energy sources for water production, which imposes a negative impact on the environment such as greenhouse gas emission. Therefore, the high-energy consumption of SWRO should be addressed to minimize environmental impacts and to allow for sustainable exploitation of seawater. However, the recent trend of energy consumption in SWRO seems to have reached a saturation point, which is still higher than theoretical minimum energy. To find new and innovative strategies for lowering current energy consumption, a comprehensive understanding of energy use in SWRO plants from theoretical analysis to actual energy consumption in real SWRO plants is required. This book can provide readers with information about the current state of energy consumption in actual SWRO plants, the fundamental understanding of energy use of SWRO plants from theoretical point of view, and advanced technologies and processes that could be applied for future energy reduction. In addition, this book will offer a detailed methodology for analyzing energy issues in seawater desalination. Through this book, readers will obtain an insight into how to deal with and analyze the energy issues in SWRO desalination.
ISBN: 9781789061208 (paperback)
ISBN: 9781789061215 (eBook)
ISBN: 9781789061222 (ePub)
... Radium (Ra) isotopes are useful tracers of SGD because of their wide ranges of half-lives ( 223 Ra = 11.4 d, 224 Ra = 3.7 d, 226 Ra = 1600 y and 228 Ra = 5.75 y) and conservative behaviors (Moore, 1996;Povinec et al., 2012;Garcia-Orellana et al., 2021;Wang et al., 2021a;Xu et al., 2022). They have been successfully applied to trace SGD over different spatial and temporal scales (Taniguchi et al., 2019;Wang et al., 2019a). ...
Coastal terrestrial groundwater discharge and porewater exchange are two different submarine groundwater discharge (SGD) pathways. The contribution of both pathways in dissolved carbon export to the coastal water remains almost unknown. Here, we investigated terrestrial groundwater discharge, porewater exchange, and dissolved carbon export in a tropical estuary (Moyangjiang, China) using stable water (δ2H and δ18O) and radioactive (222Rn, 224Ra, and 228Th) isotopes. The average terrestrial groundwater discharge was estimated to be (3.2±0.2) × 106 m3 d− 1 by a three end-member mixing model of δ18O and salinity. The porewater exchange was estimated to be 11.3±0.14 cm d− 1 by the 224Ra/228Th disequilibrium method. Accordingly, dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) fluxes from terrestrial groundwater discharge were estimated to be (1.19±0.10) × 107 and (3.23±0.25) × 105 mol d−1, respectively. In comparison, when upscaling to the whole estuary, DIC and DOC export fluxes from porewater exchange were estimated to be (1.56±0.21) × 105 and (2.12±0.29) × 105 mol d−1, respectively. Terrestrial groundwater discharge derived DIC flux was two orders of magnitude greater than that from porewater exchange, and was three times greater than that from river discharge. Terrestrial groundwater discharge was recognized as a significant contributor to DIC export. Porewater exchange played a disproportionately important role in the delivery of DOC to the ocean, and its contribution to DOC needed to be emphasized. Overall, both pathways contributed a large amount of dissolved carbon export to the coastal water, and play an important role in delivering terrestrial carbon across land-ocean interface. Evaluating the contributions of different pathways of SGD to terrestrial solutes will help to improve the accuracy of SGD associated terrestrial solute fluxes.
... The contributions of Si fluxes driven by PEX to total Si fluxes differed in heterogeneous environments with the contributions of 14-32% in the Jiulong River Estuary, 5% in a salt marsh of the Changjiang River Estuary, China, and 58-90% in the Krka River Estuary characterized as oligotrophy (Wang et al., 2015;Liu et al., 2019;Chen et al., 2021). Tides are recognized as a major control for Si fluxes by affecting hydraulic gradient, hydraulic retention time and wave setup (Moore, 1996;Taniguchi, 2002). Studies based on high-resolution time-series observation suggested the water parameters such as salinity, pH, and redox potential varied with tides, and thus affected the microbial activities, nutrient cycling, the distribution of reduced species and the elemental adsorption and desorption (Taniguchi, 2002;Delgard et al., 2012). ...
Coastal wetlands are an important hotspot for nutrient cycling and transport from the land to the ocean. Silicon (Si) as a vital biogenic element affects plant growth and health of coastal ecosystems. The understanding of key factors and processes controlling dissolved silicate (DSi) exchange between the wetlands and coastal water has been limited due to the lack of measured data. We carried out intensive investigations of time-series DSi concentrations and porewater exchange across the Sediment-Water Interface (SWI) along a tidal creek with a mangrove-salt marsh gradient during neap and spring tides in 2020. Seasonal observations of surface water in a tidal creek and Zhangjiang Estuary (Fujian Province, China) were conducted from 2017 to 2020. The results showed that there was a net export of DSi from the mangroves to tidal creek with rates of 2.11 and 2.40 mmol m⁻² d⁻¹ in neap and spring tides respectively, suggesting the mangroves served as the source of DSi. However, the salt marshes had a net DSi import with one or two orders of magnitude lower than the export from the mangroves. DSi export across the wetland‐estuarine continuum was largely controlled by porewater exchange, groundwater geochemistry (pH, temperature) and plant root uptake. Groundwater in the mangroves has larger ratios of DSi : DIN (dissolved inorganic nitrogen) (2.5 ± 0.6) and DSi : DRP (dissolved reactive phosphorus) (1257 ± 35) compared with surface water. The net export of DSi from mangroves would modify the nutrient stoichiometry and mitigate the effects of reduced river DSi flux caused by damming on coastal ecosystem. This study provides new insights into the wetland Si cycling for sustaining coastal ecosystem health.
... a). Radium quartet has been widely employed in tracing submarine groundwater discharge and estimating residence time on nearshore, embayment, and shelf scales (Moore, 1996;Charette et al., 2003;Wang et al., 2014Wang et al., , 2015Wang et al., , 2018Tan et al., 2018;Zhang et al., 2020). The long-lived radium isotopes, 226 Ra and 228 Ra, have been used to trace water masses and to estimate vertical and horizontal mixing rates in the open ocean (Moore, 1972;Trier et al., 1972;Sanial et al., 2018). ...
In the open ocean, radium isotopes are useful tracers of residence time and water-mass mixing. However, limited by the measurement resolution of commonly used gamma counters, the low activity of radium in the open ocean makes it necessary to enrich radium from large volumes of seawater and pretreat radium-enriched carriers prior to measurements. The commonly applied method of radium enrichment and pretreatment, however, has limitations of uneven coating of MnO2 on cartridges, relatively expensive cartridges, time-consuming issues during cartridge-ashing, ash loss during transfer, and changes of gamma counters efficiency caused by different ash weights. To address these issues, in this study we optimized the enrichment and pretreatment of low-activity radium prior to measurements. Firstly, we replaced commonly used acrylic cartridges with cheaper polypropylene cartridges, which took 6 h to be ashed, 42 h shorter than for acrylic cartridges. Secondly, MnO2-coated cartridges were prepared with a circulating hot acidic KMnO4 solution to ensure homogeneous coating. The radium extraction efficiency of this MnO2-coated cartridge was 20%–61% higher than that prepared by directly immersing cartridges in the solution. The radium delayed coincidence counter efficiency for MnO2-coated cartridge was stable with a moisture content of 0.05–1. Lastly, after ashing cartridges, instead of directly transferring the ash to a measurement vial, a mixture of hydroxylamine hydrochloride and hydrochloric acid was used to completely leach the ash for long-lived radium, followed by coprecipitation by BaSO4, to avoid potential loss of ash during transfer and variations in measurement geometry due to different ash weights. And the recovery of long-lived radium pretreatment was 94%–102%, which improved by 11% compared with the common method. In addition, the radium extraction efficiency of the MnO2-coated cartridge varied from 3% to 4% within the in situ pump working flow rate of 4–7 L/min, which fell within the measurement errors.
... With the industry and population in this region growing rapidly (Gao et al., 2021), quantifying the sources of nutrients is essential in the sustainable ecosystem and recreational developments and management of Dongshan Bay. To investigate the role that SGD plays in the nutrient budget and distribution in Dongshan Bay and to further explore the relative importance of small bays along the entire Chinese coast, we chose 226 Ra (half-life of 1,600 years) and 228 Ra (half-life of 5.75 years) to trace SGD in this study as radium isotopes have been proven to be excellent natural tracers of SGD (Moore, 1996;Beck et al., 2007;Swarzenski et al., 2007). ...
Despite over 90% of China’s coastal bays have an area less than 500 km², the geochemical effects of SGD on those ecosystems are ambiguous. Based on mapping and time-series observations of Ra isotopes and nutrients, a case study of small bays (<500 km²), we revealed that submarine groundwater discharge (SGD) predominately regulated the distribution of nutrients and fueled algal growth in Dongshan Bay, China. On the bay-wide scale, the SGD rate was estimated to be 0.048 ± 0.022 m day⁻¹ and contributed over 95% of the nutrients. At the time-series site where the bay-wide highest Ra activities in the bottom water marked an SGD hotspot with an average rate an order of magnitude greater, the maximum chlorophyll concentration co-occurred, suggesting that SGD may support the algal bloom. The ever-most significant positive correlations between ²²⁸Ra and nutrients throughout the water column (P< 0.01, R² > 0.90 except for soluble reactive phosphorus in the surface) suggested the predominance of SGD in controlling nutrient distribution in the bay. Extrapolated to a national scale, the SGD-carried dissolved inorganic nitrogen flux in small bays was twice as much as those in large bays (>2,000 km²). Thus, the SGD-carried nutrients in small bays merit immediate attention in environmental monitoring and management.
... In addition, as we found many exposed clastic bedrocks near the bank embankment in the field, the base of the whole bank embankment can be regarded as a relatively permeable aquifer. Thus, as schematically shown in Fig. 10, there is likely a large exchange of groundwater and solutes associated with submarine groundwater discharge (i.e., SGD; Moore, 1996) between the estuarine tidal flats and aquaculture ponds. For example, we found that in subtidal zone Es-2, which was near the low tide line, DRP and TDP first increased from the overlying water to a 2 cm depth of sediment, then decreased from 2 cm to 8 cm, and then increased again with depth (Fig. 3). ...
Estuarine mangrove wetlands have gradually declined owing to the growing construction of aquaculture ponds. How the speciation, transition, and migration of phosphorus (P) adaptively change in the sediments of this pond-wetland ecosystem remains unclear. In this study, we used high-resolution devices to explore the contrasting P behaviors associated with the redox cycles of Fe-Mn-S-As in estuarine and pond sediments. The results showed that the construction of aquaculture ponds increased the content or percentage of the silt, organic carbon, and P fractions in sediments. Dissolved organic P (DOP) concentrations in pore water were fluctuant with depths, accounting for only 18±15% and 20±11% of total dissolved P (TDP) in estuarine and pond sediment, respectively. Furthermore, DOP was less strongly correlated with other P species, including Fe, Mn, and sulfide. The coupling of dissolved reactive P (DRP) and TDP with Fe and sulfide confirmed that P mobility is regulated by Fe redox cycling in estuarine sediments, whereas Fe(III) reduction and sulfate reduction co-regulate P remobilization in pond sediments. The apparent diffusion flux revealed all sediments acting as sources for TDP (0.04–0.1 mg m−2 d−1) to the overlying water, while mangrove sediments were sources of DOP, and pond sediments were major sources of DRP. The DIFS model overestimated the P kinetic resupply ability, which was evaluated using DRP rather than TDP. This study improves our understanding of P cycling and budget in aquaculture pond-mangrove ecosystems and has important implications for understanding water eutrophication more effectively.
Radioisotopes from the U/Th decay series are used routinely as tracers for submarine groundwater discharge (SGD) worldwide. We present the main principles of radon and radium mass-balance approaches used for quantifying SGD in coastal areas and discuss some challenges. For example, modeling exercises can substantially help interpret field measurements and reduce uncertainties. We showed how the stable isotopes of nitrogen, carbon, and pigments were used to determine the impacts of SGD's quality on coastal water and biota. Finally, we suggest that the novel deep-learning modeling approaches using radioisotopes are projected to be an important future direction in SGD research.
Keywords: Machine learning in future SGD studies; Modeling SGD; Radiotracers: radon, radium; SGD biological impacts; SGD-derived emerging contaminants; Submarine groundwater discharge (SGD)
Seawater intrusion has been a globally significant environmental issue. This paper comprehensively reviews and highlights the research methods of seawater intrusion in China, recommending the potential application of novel radioactive radium-radon isotopes. Geochemical and geophysical techniques have been extensively utilized in studying seawater intrusion in China, including methods such as hydrochemical analysis, groundwater level observations, geophysical survey techniques, and isotope tracing. The former three methodologies boast a lengthier historical application in seawater intrusion field, while the radium-radon tools in isotope tracing, as newcomers, can specifically indicate crucial scientific questions such as seawater intrusion rates, salt groundwater age, water-rock reactions, and preferential flow dynamics. However, it is imperative to acknowledge the limitations inherent in the utilization of radium-radon tools within the realm of seawater intrusion research, as with any other methodologies. Strategic integration of radium-radon tools with other methodologies will propel advancements in the investigation of seawater intrusion in China. While the primary focus is on research methods in China, insights gained from novel radium-radon tools could have broader value for seawater intrusion research and coastal management globally.
The present study aims to understand the impact of submarine groundwater discharge (SGD) on a coastal area with different lithology and degrees of SGD. Sampling cam- paigns took place in Puck Bay and the Gulf of Gdansk, southern Baltic Sea encompassing years between 2009 and 2021. The methodological approach combined geophysical characterization of the surface sediments with detailed spatial and temporal (isotope) biogeochemical investigations of pore and surface waters, and was supported by nearshore groundwater and river surveys. Acoustic investigations identified areas of disturbance that may indicate zones of preferential SGD release. The composition of porewater and the differences in the bay’s surface
waters disclosed SGD as common phenomenon in the study area. Regional SGD was estimated through a radium mass balance. Local estimation of SGD, based on porewater profiles, revealed
highest SGD fluxes at the sandy shoreline, but relatively low elemental fluxes. Though SGD was low at the muddy sites corresponding elemental fluxes of nutrients and dissolved carbon exceeded those determined at the sandy sites due to intense diagenesis in the top sediments. SGD appears to be sourced from different freshwater endmembers; however, diagenesis in surface
sediments substantially modified the composition of the mixed solutions that are finally discharged to coastal waters. Overall, this study provides a better understanding of the SGD dynamics
in the region by a multi-approach and emphasizes the need to understand the processes occurring at the sediment-water interface when estimating SGD.
Coastal seas play an important role in the cycling of materials such as nutrients, influencing the productivity of marine organisms. River inflow and submarine groundwater discharge (SGD) are important pathways by which terrestrial nutrients reach estuaries. The assessment of SGD fluxes and their impact on the near-shore marine environment is essential for understanding nutrient cycling and evaluating pollution of coastal seas. SGD-derived nutrients provide positive impacts on marine productivity but other terrestrial contaminants may be harmful to coastal ecosystems. Radon-222 is applicable as a tracer of SGD as groundwater contains much higher concentrations of 222Rn than river water and seawater. This chapter introduces a general tracer method using 222Rn to evaluate SGD in coastal seas, including sampling and physical observations, and a 222Rn mass-balance model.
Biogeochemical transformations within highly saline subterranean estuaries (STE) dramatically affect solute cycling, resulting in submarine groundwater discharge (SGD) with distinct chemical signatures. The study hypothesizes that biogeochemical processes within hypersaline bay porewaters (PW) simultaneously affect nitrogen, carbon, and radium cycling. We measured radium isotopes (226Ra, 224Ra, and 223Ra), nutrients (dissolved inorganic nitrogen [DIN: NH4+ + NO2− + NO3−], HPO42− [DIP], HSiO3− [DSi], dissolved organic carbon [DOC]), total alkalinity (TA), dissolved inorganic carbon (DIC), stable isotopes, and major cations in PW and surface water (SW) of Baffin Bay, a well-mixed, semi-enclosed estuary along the semiarid northwestern Gulf of Mexico coast, over three seasons in a characteristically dry year. This study's findings show a concurrent increase in NH4+, DIP, DSi, and TA/DIC with reduced metal species (e.g., Mn and Fe) and Ra during the hot and dry seasons, particularly in PW, under increasingly reducing conditions. Principal component analyses (PCA) suggest these increases are primarily driven by dissimilatory nitrate/nitrite reduction to ammonium (DNRA) and dissolution of lithogenic particles and biogenic CaCO3, modulated by organic matter degradation or remineralization. While more significant terrestrial groundwater inputs may contribute to solutes and Ra supply in the STE, the biogeochemically induced variability in solute concentrations in PW primarily drives larger SGD-derived fluxes, particularly notable in hot months. During a typically dry year, these fluxes, estimated as the average of 226Ra and 223Ra mass balance models (e.g., July/November fluxes in Mmol∙d−1: 0.093/0.092 of NO3−; 0.2/0.02 of NO2−; 72/16 of NH4+; 72.2/18 of DIN; 1.5/0.2 of HPO42−; 20/9 of HSiO3−; 42/37 of DOC; 503/399 of TA; 582/431 of DIC) are orders of magnitude (∼4 for DIN and DIC, ∼3 for DIP, DSi, and DOC, and ∼2 for TA) greater than surface runoff inputs. These substantial SGD inputs likely sustain phytoplankton growth and potentially fuel harmful algal blooms while countering estuarine acidification.
As an important land-ocean interaction process, submarine groundwater discharge (SGD) is composed of multiple dynamical processes at different scales and plays an important role in the study of coastal ocean geochemical budgets. However, most of the existing studies focus on the quantification of the total groundwater discharge, few studies are about the differentiation and quantification of groundwater discharge processes at different scales (i.e., short-scale SGD and long-scale SGD). As a world-class river, the Huanghe River is highly turbid and heavily regulated by humans. These natural and anthropogenic factors have a significant impact on groundwater discharge processes in the Huanghe River Estuary (HRE). In this study, the distribution patterns of the natural geochemical tracer radium isotopes (224Ra and 223Ra) and other hydrological parameters in the HRE were investigated during four cruises. By solving the mass balance of 224Ra and 223Ra in the HRE, the long-scale SGD flux was quantified as 0.01–0.19 m/d, and the short-scale SGD flux was 0.03–0.04 m/d. The rate of short-scale SGD remained essentially constant among seasons, while the rate of long-scale SGD varied considerably at different periods and showed a synchronous trend with the variation of river discharge. The results of this study are significant for understanding the SGD dynamics in the HRE and the contribution of SGD to the ocean geochemical budgets.
The increase in population and coastal infrastructure demands a rigorous assessment of coastal vulnerability for extreme events. The predictions by Intergovernmental Panel for Climate Change (IPCC) indicate that climate change will severely impact coastal regions, river systems, urban infrastructures and increase the frequency of inundation along coastal areas. This study emphasizes the need to use extreme events and socioeconomic data to evaluate the vulnerability of a given coast. This paper presents the vulnerability assessment of the Chennai coastal region using eight variables: coastal elevation, coastal slope, rate of shoreline change, tidal range, sea level rise, storm surge, Adyar flood and land use and land cover. A Coastal Vulnerability Index (CVI) is calculated by integrating the physical and social–economic variables. The Coastal Vulnerability Index for shoreline erosion ranges from 15 to 21 with a mean value of 18 and a standard deviation of 1.6, while the Coastal Vulnerability Index for inundation ranges from 4.1 to 12.7 with a mean value of 7.3 and a standard deviation of 1.4. The vulnerability maps show the exposed regions around river Adyar that are highly vulnerable to coastal erosion and floods, especially due to extreme events. The assessment indicated the variation in the vulnerability of inundation around the Chennai Port regions and the low vulnerability of the shoreline erosion in this region. The vulnerability assessment suggests that planning and adaptation of the coastal ecology for the future needs are to be performed with extreme caution.KeywordsCoastal Vulnerability IndexCoastal processesSocioeconomic parametersExtreme eventsClimate change
The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the
mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies
(redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw
Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ13Corg
values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity
levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to
65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during
deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously
reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature
shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both
Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation)
suggest predominantly brackish and marine settings with significant water exchange between the basin and open
ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local
bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ98Mo and δ238U values
for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ98Mo of the
Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-
Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation
and no correlation) of δ238U and U enrichment factors (EF) were observed for Exshaw black shales deposited
from locally euxinic bottom waters. The positive correlation group samples (–0.13‰ to 0.23‰) suggest U
isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic
basins. Higher δ238U values (>0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰)
from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the
above Mo and U isotope fractionations to the Exshaw shales, global seawater δ98Mo and δ238U at the Devonian-
Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and –0.9‰ to –0.5‰, respectively.
A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB,
which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore,
our data support expanded ocean euxinia at the DCB as an important contributor to the Hangenberg Crisis.
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.
Coastal beaches are the most important part of the marine environment and are highly influenced by the interactions taking place between groundwater and surface water of any form. Generally, the purity index of the groundwater is based on the concentration of Total Dissolved Solids (TDS) in it. Analytical solutions are the best tools for groundwater flow and transport modeling. However, contaminant transport along the sloping coastal beaches with complex boundaries cannot be addressed with available analytical solutions. In the present study, new analytical models are developed for groundwater flow and contaminant transport for sloping coastal beaches with continuity and tidal boundary conditions. To assess the performance of the new analytical solution and to validate the range of aquifer parameters, numerical simulation is performed using Du-Fort Frankel (DFF) Scheme. Numerical experimentation is carried out using the Tchebycheff and L2 norms. It is observed that the new analytical solution for the contaminant transport gives acceptable results over the wide range of the aquifer parameters. To show the effectiveness of the developed models, two case studies from Indian coastal aquifers namely, Kalpakkam, Tamil Nadu, and Bhavnagar, Gujarat are considered. The profiles of the contaminant concentration are obtained to study the TDS behavior along these complex coastal beaches in the spatiotemporal directions. The results are compared with numerical model results and found to be satisfactory. The combined effect of the initial rainfall and rainfall decay constants showed a significant impact on the concentration of TDS. TDS concentrations are observed to be varying highly with the variation in the tidal constituents and bed slope.
Radium 228 in surface ocean waters is derived from sedimentary fluxes generated in estuaries, salt marshes, and fine-grained sediments of the inner shelf. Thus the 228Ra activity of shelf waters is a measure of their interaction with these sediments. Mixing of the 228Ra signal offshore follows known circulation patterns and examination of the 228Ra field as determined by the 228Ra/226Ra activity ratio (AR) reveals details of these patterns. During February, sinking of chilled shelf waters carries high AR water to depths of 80-130 m just below the shelf break. In April, upwelling from 500-700 m depth brings low AR water to the shelf break, where it replaces the high AR waters present 2 months earlier. During August, low AR waters at the shelf break could be traced to the surface, where they separated higher AR waters both inshore and offshore of the shelf break. Because of such short-term variability, one-dimensional steady state models which balance 228Ra decay with mixing do not provide a true measure of horizontal mixing rates. Fluxes necessary to support the 228Ra standing crop in the South Atlantic Bight are of the order of 2.5×1014 dpm/yr. These are similar to estimates from the New York Bight but 25 times lower than the apparent flux from the Amazon system.
This thesis presents the results of a detailed study made of the local interstellar medium (ISM) in the region of the South Celestial Pole (SCP). The study lends support to the "cosmic bubble bath" concept of the ISM, which envisages the ISM as the result of material swept up by interstellar 'bubbles' blown out by evolving OB associations and by supernovae. The study is based on the results of detailed surveys of: nebulosity visible on Schmidt plates; neutral hydrogen at 21 cm; and reddening in the direction of SCP stars. The study of 1.2 m Schmidt J survey plates revealed an extensive covering of faint, broken nebulosity over the SCP, with a number of brighter patches and filaments. It appears that the nebulae are seen predominantly by reflection of the integrated light from the overlying Carina spiral arm. Distance estimates based on published polarization data and on UBV photometry of stars associated with reflection nebulae in the region suggest that the nebulosity forms a broken sheet (or sheets) lying parallel to the galactic plane at an altitude of 40 - 80 pc. The 21 cm high-resolution survey of low-velocity neutral hydrogen (HI) made with the 64 m Parkes radiotelescope confirmed the general features reported in previous, lower -resolution studies. The predominantly local gas contains considerable small-scale (< 1(DEGREES)) structure. The gas distribution is dominated by a ridge feature. There is evidence to support the suggestion that the ridge represents an HI 'bubble' blown by stars in the nearby Sco-Cen OB association, but evidence also that sections of it are more consistent with the passage of a blast wave through the local ISM. The reddening survey was carried out using an extensive table prepared from published photometric data. Results refer to the approximate distance interval (TURN) 60 - 300 pc. They show reddening gradients ranging from below 0.01 mag. kpc('-1) (E(,B-V)/d) to 4 mag. kpc('-1) and higher. The distribution is also very patchy, showing structure on a scale of 0(DEGREES)25.
The concentration of 226Ra in bottom sediments in Chesapeake Bay is lower than in incoming sediments and decreases systematically in the seaward direction, reflecting both a seaward increase in sediment grain size and a displacement of Ra from sediments as salinity increases. The amount of 226Ra lost from sediments approximately balances that gained by the water. In the seaward 150 km of this estuary, where atmospheric deposition is the dominant 210Pbex source, the 210Pbex/Org-C ratio is virtually constant in surface samples because the production of carbon is apparently regulated so as to be roughly constant per unit area. From independent productivity estimates, about 10% of the carbon is found to survive remineralization in the estuary long enough to be incorporated into surface sediments. Zinc is enriched in surface sediments of the northern two-thirds of the Bay by a factor of 2-3 over shales. The Znex/Org-C ratio decreases systematically southward, unlike the 210Pb/Org-C ratio. This reflects a regional decline southward in deposition of anthropogenic Zn from the atmosphere. A negative 210Pb anomaly is the most distinctive indicator of sediments deposited by a major flood in 1972 and testifies to the importance of such events in estuarine sedimentation.
Improvements in the analyses of 226Ra and 228Ra in seawater made possible by better extraction and processing techniques reduce significantly the errors associated with these measurements. These improvements and the extensive sampling for Ra isotopes conducted on the TTO North Atlantic Study should enable us to use the distribution of 228Ra to study mixing processes on a 3-15 year time scale in both the upper and deep North Atlantic. The 228Ra profiles already analyzed show a closer resemblance to GEOSECS tritium data than to TTO tritium data in the upper ocean. This is because the transient tracer tritium was responding on a 10-year time scale during GEOSECS and a 20-year time scale during TTO. The steady state tracer 228Ra should always respond on a time scale of 8 years. Thus the 228Ra data obtained on TTO should provide a means to extend the features of the GEOSECS tritium field to the regions of the TTO study. The 226Ra data are of high enough quality to identify features associated with different water masses. Changes in the positions of the deep-water masses since the GEOSECS cruise are revealed by the 226Radata.
We have analyzed 8 years of wind and salinity data from a frontal zone in a region of the inner continental shelf off the southeastern United States. The changes in low-salinity water stored in the frontal zone have been parameterized by analyzing the montly rate of change in freshwater content. When the rate of change in freshwater content was negative, we interpreted this as a loss of low-salinity water from the frontal zone. When this parameter was compared with seasonally averaged alongshore wind stress, the rate of loss was independent of the alongshore wind stress magnitude until threshold of about 0.1 dyne cmâ»Â² was reached. Above the thereshold there was a clear relationship between northward alongshore wind stress and rate of loss freshwater from the inner shelf. Experimental evidence suggests that horizontal currents in the inner-shelf frontal zone have cyclonic shear with increasing depth. When wind stres is northward and offshore, near-surface low-salinity water is transported offshore by Ekman transport while near-bottom high-salinity water is transported shoreward.
The first 224Ra (t1/2 = 3.64 days) measurements from mixing zones of estuarine systems are presented for the Pee Dee River-Winyah Bay and Delaware Bay Estuaries. High-resolution gamma-ray spectrometry was used to determine 224Ra, 228Ra (t1/2 = 5.7 years), and 226Ra (t1/2 = 1622 years) activity ratios. Desorption and diffusion from suspended and bottom sediments contributes to the non-conservative increases of the three isotopes in each systems. In Delaware Bay 224Ra concentrations were nearly constant over the 2.50/00 to 150/00 salinity range where two turbidity maximum zones are located. 228Th scavenging onto the suspended particles in the turbid zones may supply a regenerative source of 224Ra in this system. Samples collected on the ebb and flood tide from a salt marsh along Delaware Bay have a 5-fold increase in 224Ra from flood to ebb and 3- and 2-fold increases for 228Ra and 226Ra respectively, indicating salt marshes are another source of radium to estuarine waters.
Data on SGWD from sites in the Florida Keys and on the southeastern continental shelf of the US indicate that water movement across sediment/water interfaces is a common occcurrence at least to water depths of 30 to 35m. Discharge values from the Florida Keys were 8.9l m-2d-1 for depths <27m and 5.4 l m-2d-1 for depths of c27 to 39m. On the southeastern continental shelf, discharge ranged between c6 and 20 l m-2d-1. One site was found in 20m depth where there was a persistent negative hydraulic head and mean influx of seawater to the sediments (c10.8 l m-2d-1). Even though geohydrological models would predict coupling of SGWD with landbased hydraulic heads, definitive lower salinity SGWD could not be detected. The driving force seems to be subtidal pumping and much of the discharge measured was probably recycled seawater. SGWD serves to move dissolved solutes into the water column, and could be an important link in benthic-pelagic coupling in continental shelf ecosystems. -from Author
Radium isotopes provide unique and important information concerning water exchange in a salt marsh-tidal creek system. Seasonal radium data collected over five tidal cycles from a creek draining a South Carolina salt marsh, radium and thorium data from the adjacent marsh sediments and interstitial water radium data from the drainage basin are modeled to yield residence times of water in the upper 10 cm of the marsh sediments ranging from less than one to twenty-six hours. Water residence times derived from chambers which directly measure the flux of Ra isotopes to the creek waters agree well with these calculated interstitial water turnover times.Dissolved radium activities in the tidal creek were greater during the summer than during other times of the year. We suspect that seasonal changes in the storage of organic carbon and rates of bioturbation lead to net reduction of radium carrier phases in the marsh sediments during the summer.
Recent advances in sampling and counting techniques have provided the means of measuring 226Ra, 228Ra and 224Ra at low activities in natural waters. Samples are preconcentrated in the field by adsorbing radium on a fibre coated with manganese oxides. Absolute activities and activity ratios are measured using germanium detectors supplemented in some cases by alpha scintillation measurements of 222Rn. This paper describes tests and results obtained using a Ge(Li) crystal and an intrinsic germanium crystal with a 1 cm diameter well. The well detector has an efficiency two to three greater than the flat Ge(Li) system and thus has a considerably higher sensitivity. Results from ground waters and estuarine waters are presented which demonstrate the usefulness of the germanium detectors in studies of radium in natural waters.
Concentrations of dissolved226Ra in Winyah Bay, South Carolina, and in the adjacent Atlantic Ocean are augmented by the desorption of radium from sediments in the low-salinity area of the estuary and diffusion from bottom sediments. Desorption of226Ra is reflected by lower concentrations in suspended sediments from higher-salinity regions of the estuary. Bottom sediments from the high-salinity region have lower226Ra/230Th activity ratios than those from the low-salinity end.The shape of the dissolved226Ra vs. salinity profile is influenced by the river discharge. During average-discharge conditions, desorption of226Ra from suspended and bottom sediments increases the dissolved226Ra concentrations by a factor of 3.5 as the water passes through Winyah Bay. High river discharge produces an initial increase of dissolved226Ra by a factor of 2 to 3 and apparently reflects only desorption from suspended sediments. By driving the salt wedge down the estuary and reducing the zone of contact of salt water with bottom sediments, the high-flow conditions sharply reduce the flux of226Ra from bottom sediments.
Freshwater off the Georgia, South Carolina and northeast Florida coast varies from 5 to 12 km3 depending on the local runoff. The flushing rate calculated from the freshwater volume and river runoff rate is relatively invariant at 2.7 months. This invariance is thought to imply a constant flushing process and it is postulated that Gulf Stream meanders and eddies are important in that process. A simplified calculation involving Gulf Stream meander frequency and typical entrainment dimensions also produces a flushing rate of 2–3 months.
A pronounced desorption phenomenon of 226Ra from sediment was observed in the Hudson River estuary. Mass balance calculations indicate that the desorption of 226Ra from the river-borne sediment in estuarine environment is an important source of 226Ra to the oceans.