Magnitude Of Submarine Groundwater Discharge Research Articles

Pockmarks and associated fresh submarine groundwater discharge in the seafloor of Puck Bay, southern Baltic Sea

This is the first well-documented report on the occurrence of pockmarks in Puck Bay. Pockmarks in the seafloor of Puck Bay were discovered during a hydroacoustic survey carried out in 2020. They are located at a depth of 25–27 m in the southwestern part of the bay. Significant depletion of chloride (Cl−) concentrations in sediment pore water was found within the depressions. Most likely, the formation of pockmarks was due to groundwater flow through the Miocene–Pleistocene system of aquifers, which extends from land to the bay area. One-dimensional modeling of vertical Cl− concentration profiles in pore water revealed the upward flow of freshened groundwater within the pockmarks. The magnitude of submarine groundwater discharge (SGD) was estimated to vary from 1.53·10−2 to 18·10−2 L·m−2·h−1. The effect of groundwater seepage was also observed at 3 cm above the seafloor within the pockmarks, which was identified as a decrease in salinity of approximately 0.12 PSU compared to reference sites.Furthermore, due to the effect of water advection, SGD can be detected even several meters above the seafloor as a decrease in salinity values within the thermocline layer.

Extreme precipitation events induce high fluxes of groundwater and associated nutrients to coastal ocean

Abstract. Current studies of submarine groundwater discharge (SGD) are commonly conducted under aquifer baseflow conditions, neglecting the influence of episodic events that can significantly increase the supply of nutrients and water. This limits our understanding of the social, biogeochemical, and ecological impacts of SGD. In this study, we evaluated the influence of an extreme precipitation event (EPE) on the magnitude of SGD. To do so, three seawater sampling campaigns were performed at a Mediterranean ephemeral stream-dominated basin after an EPE (∼ 90 mm in few hours) and in baseflow conditions. Results indicate that the groundwater flows after the EPE were 1 order of magnitude higher than those in baseflow conditions. The SGD induced by EPEs, which only take place a few days per year, represented up to one third of the annual discharge of groundwater and associated nutrients at the study site. This work accentuates the need to account for episodic increases in the supply of water and nutrients when aiming to provide reliable annual SGD estimates, particularly in the current context of climate change, since the occurrence of such events is expected to increase worldwide.

Computation of submarine groundwater discharge from geomorphologically different coastal catchments of SW India using numerical modeling

In this study, we investigate submarine groundwater discharge (SGD) and the influence which coastal geomorphology and geology can have on its behavior. Numerical models have been developed to estimate the magnitude of submarine groundwater discharge (SGD) from catchments with different geomorphology on the Southwest coast of India. Two coastal catchments of Thiruvananthapuram district, Kerala, India namely Varkala (184 km2) and Kadinamkulam (186 km2), have been selected to compute SGD. The results of the study confirmed that the SGD flux from the catchments differs in a great way across its multiple layers as well as in comparison to one another. The fresh SGD flux from Varkala catchment is found to be 0.1 m3/day/m and from Kadinamkulam catchment is 6 m3/m/day. In the present study, it is concluded that SGD is reflected in the heterogeneity of aquifers owing to the differences in geomorphology, land use-land cover, hydrogeology, and lithology. A numerical modeling approach that can yield reliable results with low computational cost and time is a potent method for quantifying SGD. The inferences drawn will be useful for designing aquifer recharge options and optimum pumping quantities for the region.

Submarine Groundwater Discharge From Non-Tidal Coastal Peatlands Along the Baltic Sea

Submarine groundwater discharge (SGD) is an important pathway for water and materials within the land-ocean transition zone that can impact coastal environments and marine life. Although research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas characterized by a low hydraulic conductivity, such as carbon-rich coastal peatlands. The objective of this study was to determine the magnitude and location of terrestrial SGD to be expected from a non-tidal low-lying coastal peatland located along the Baltic Sea and to understand the controlling factors using numerical modeling. We employed the HYDRUS-2D modeling package to simulate water movement under steady-state conditions in a transect that extends from the dune dike-separated rewetted fen to the shallow sea. Soil physical properties, hydraulic gradients, geological stratifications, and topography were varied to depict the range of properties encountered in coastal peatlands. Our results show that terrestrial SGD occurs at the study site at a flux of 0.080 m2 d−1, with seepage rates of 1.05 cm d−1 (upper discharge region) and 0.16 cm d−1 (lower discharge region above submerged peat layer). These calculated seepage rates compare to observations from other wetland environments and SGD sites in the Baltic Sea. The groundwater originates mainly from the dune dike—recharged by precipitation and infiltration from ponded peatland surface water—and to a lesser extent from the sand aquifer. The scenario simulations yielded a range of potential SGD fluxes of 0.008–0.293 m2 d−1. They revealed that the location of terrestrial SGD is determined by the barrier function of the peat layer extending under the sea. However, it has little impact on volume flux as most SGD occurs near the shoreline. Magnitude of SGD is mainly driven by hydraulic gradient and the hydraulic conductivity of peat and beach/dune sands. Anisotropy in the horizontal direction, aquifer and peat thickness, and peatland elevation have little impacts on SGD. We conclude that SGD is most probable from coastal peatlands with high water levels, large Ks and/or a dune dike or belt, which could be an essential source for carbon and other materials via the SGD pathway.

Fresh and saline submarine groundwater discharge in a large coastal inlet affected by seasonal upwelling

AbstractSubmarine groundwater discharge is recognized as a major source of chemicals to the global ocean, exerting large control over coastal water composition. Radon and 226Ra are used to evaluate, for the first time, the occurrence and magnitude of submarine groundwater discharge in the Ría de Vigo, a large, highly productive embayment affected by seasonal, wind‐driven upwelling. The system is naturally enriched in 222Rn due to the regional granitic basement geology: high 222Rn activities (up to 106 Bq m−3) are detected in wells and boreholes in the drainage basin of the embayment. High 222Rn activities (>400 Bq m−3) are also measured in certain areas of the embayment. Comparatively lower 226Ra activities (<4 103 Bq m−3) were measured in the freshwater sources to the bay. Mass balances obtained with a box model are used to perform a volumetric estimate of fresh and saline submarine groundwater discharge in the Ría de Vigo under contrasting circulation patterns. Fresh groundwater is shown to be a relevant hydrological component of the Ria de Vigo water balance, equivalent to 9% ± 4% and 23% ± 9% of the volume discharged by tributary rivers during winter and summer, respectively. On the other hand, recirculation of seawater through permeable sediments is capable of filtering the entire upper volume of the Ria de Vigo through its seafloor in <100 days and might thus be a previously overlooked major source of regenerated solutes to the system.

Submarine groundwater discharge drives coastal water quality and nutrient budgets at small and large scales

Submarine groundwater discharge (SGD) can be a significant source of chemical pollutants from land to ocean. Here, we first estimated SGD using radium isotopes and related nutrient fluxes at the local scale in Jiaozhou Bay (JZB), a typical Chinese system that is experiencing rapid urban and industrial development. We then summarized SGD studies off China to assess the large-scale implications of SGD to nutrient budgets. In JZB, the location of contaminated nearshore waters revealed by an integrative water quality index (WQI) coincided with the SGD hotspots. The total (fresh and saline) SGD flux in JZB was estimated to be (0.64–1.67) × 107 m3/d or (2.12–5.59) cm/d based on 224Ra and 228Ra mass balance models. This was approximately 8 times the discharge rate of local rivers. By combining these JZB results with the literature data, we provide the first estimate of SGD and associated nutrient fluxes off China. The magnitude of SGD at the China-scale was (5.40–10.2) × 1012 m3/yr, accounting for 5–9% of the global SGD flux. SGD-derived nutrient fluxes summarized from ∼40 previous studies were one order of magnitude higher than riverine inputs. These nutrients fluxes from SGD contributed >50% of the total dissolved inorganic nitrogen (DIN), phosphorous (DIP) and silicate (DSi) inputs into Chinese coastal waters, which can explain about 60% of the phosphorus required by primary production. The mean DIN/DIP ratio (121) in SGD was significantly higher than the Redfield ratio, with important implications for phytoplankton growth and structure. SGD can influence water quality, dominate nutrient budgets, and drive primary production not only at the local scale, but also at the regional and global scales.

Hydrogeochemistry and magnitude of SGD in the Bay of Puck, southern Baltic Sea

Summary This work reports the hydrogeochemistry of submarine groundwater discharge (SGD) in the Bay of Puck, southern Baltic Sea. To understand the seasonal and spatial variability of SGD, groundwater and seawater-based SGD samples were collected in several sites in November 2017, March 2018, May 2018 and July 2018. Additionally, a vertical, one-dimensional, advection-diffusion model was used to estimate SGD in each site. The obtained results ranged from to 1.8 × 10−7 L cm−2 s − 1 to 2.8 × 10−7 L cm−2 s − 1 and depended on both: short-timescale factors (wind direction and monthly precipitation) and long-timescale factors (total precipitation and large-scale sea level variations). The calculated rates were further extrapolated to the entire Bay of Puck and ranged from 16.0 m3 s − 1 to 127.7 m3 s−1. The estimated SGD fluxes were significantly higher than results including only the freshwater component of SGD. In the Baltic Sea the importance of SGD, as a source of water and accompanying chemical substances, is still neglected, however, the present findings indicate that locally SGD can be higher than rivers runoff.

Relating estuarine geology to groundwater discharge at an oyster reef in Copano Bay, TX

Coastal bays with fresh submarine groundwater discharge (SGD) have been found to provide a favorable environment to shellfish (e.g., oysters). Significant variability in the spatial distribution and magnitude of SGD may occur because of subsurface heterogeneity. We investigated the relationship between stratigraphic controls (i.e. paleovalley systems) and the spatial distribution of SGD in relation to oyster reefs. In Copano Bay, South Texas, major oyster reefs have been shown to be oriented perpendicular to shore, along interfluves, and near paleovalley margins. A novel combination approach of radioactive geochemical tracers, stable isotopes, geophysical and petrophysical techniques were used to identify estuarine depositional environments for individual hydrogeologic characterization and evaluation of SGD distribution with implications to ecological features such as oyster reefs. A 1-km2 area within a paleovalley system, in Copano Bay, yielded discernible differences in sediment type, apparent resistivity, and SGD rates among four different depositional environments: paleovalley, paleovalley margin, oyster reef and interfluve. Electrical resistivity tomography revealed a direct connection between the oyster reef and the underlying, more permeable interfluve, and the presence of a fresher SGD component in the subsurface. Measurements of SGD from stationary and mobile continuous radon-222 surveys across each depositional environment showed that, by and large, areas in proximity to reefs have larger SGD rates (62.6 ± 11.3 cm/d in January and 28.7 ± 10.8 cm/d in July) when compared to paleovalleys (31.9 ± 8.9 cm/d in January and 19.5 ± 7.2 cm/d in July). The role of depositional environments as hydrologic controls on SGD to oyster reefs revealed in this study is further confirmed by significant differences in stable and radioactive isotopes observed over small spatial scales offshore when compared to the entire estuary. Although constant mixing and radon-222 degassing of bay waters are likely altering the relationships between the different depositional environments and magnitude of SGD, this study provides an improved understanding of possible pathways of freshwater and solute inputs to oyster reefs for more informed decisions on reef restoration efforts.

Retention and fate of groundwater-borne nitrogen in a coastal bay (Kinvara Bay, Western Ireland) during summer

The magnitude of submarine groundwater discharge (SGD) and its contribution to nitrogen biogeochemistry in a small embayment in the Western Coast of Ireland subject to occasional hypoxia were investigated during summer. Time series (24 h) of 222Rn, NO3−, NO2−, NH4+, dissolved reactive silicate (DRSi) and salinity (June 2010, July and September 2011 and July 2013), total dissolved phosphorus (TDP) (September 2011 and July 2013) and dissolved organic nitrogen (DON) (July 2013) were measured at the mouth of the bay and coupled with relevant sediment–water fluxes and input loadings to derive nutrient budgets. In-situ activity ratios of the naturally occurring radium isotopes 224Ra and 223Ra were employed in parallel to the freshwater fraction method to determine the timescale of freshwater retention in the system. Based on 222Rn mass balances (n = 4), the mean groundwater (±SE) discharge into Kinvara Bay was 10.4 ± 6.3×104 m3 days−1, delivering average loads of 376 ± 67 kg Si day−1 (as DRSi), 1.6 ± 0.2 kg P day−1 (as TDP) and ~280 kg N day−1 of dissolved nitrogen (272 ± 49 DIN, essentially as NO3−, and 8.2 ± 1.6 DON), which correspond to ~98.8, 49.1 and ~93.5 % of total allochthonous nutrient inputs respectively. Expressed on an areal basis and annual scale the exogenous N summer loading of Kinvara is equivalent to 25.9 g N m−2 year−1. Our biogeochemical budgets indicate that tight benthic-pelagic coupling contributes to the very high retention levels of N within the bay with subtidal sediments acting as a link in the internal N cycle via DNRA, while ~18 % of the exogenous N load is removed by benthic denitrification. Rapid cycling of DON into bioavailable forms of N within the timescale of freshwater retention in the system (~7 days) contributes ~50 % to local biological N fixation. Nutrient availability ratios are N:P ~173 and Si:P ~503, indicating that primary production is P-limited while the carbon yield (~3.01 × 105 mol C day−1, or ~0.313 kg C m−2 year−1) suggests the bay is eutrophic during the summer. SGD-borne Nitrogen loading is therefore the major driver of eutrophication in Kinvara Bay. Our biogeochemical characterization is consistent with the observed phytoplankton community composition and species succession and justifies the local observation of HAB’s. In addition, the relative magnitude of DNRA-promoted N retention compared to N removal by denitrification, coupled with seasonal hypoxia, suggests that the system is advanced in the chronological sequence of eutrophication effects on shallow coastal systems.

228Ra flux in the northwestern Pacific marginal seas: Implications for disproportionally large submarine groundwater discharge

To determine the fluxes of 228Ra (half life: 5.75 years) to the northwestern Pacific marginal seas, 228Ra data were compiled for the East China Sea, Yellow Sea, South Sea, and East/Japan Sea. Most of the 228Ra data in the East Sea were newly obtained through the onboard surveys in July 2012, October 2012, and March 2013. A 228Ra mass balance model was established by considering the input fluxes from the Pacific Ocean, river, atmosphere, sediments, and submarine groundwater discharge (SGD) and the output fluxes by radioactive decay and to the Pacific Ocean and the deep East Sea through vertical mixing. Using this model, the 228Ra flux through SGD was estimated to be at least 2.3 × 1015 dpm yr−1 in the entire area of these marginal seas, which is about 20% of the total 228Ra input flux to this region. This SGD-driven 228Ra flux to this region accounts for about 2% of that to the global oceans. This result implies that the magnitude of SGD in this region could be disproportionally larger than those in general oceanic margins. We suggest that SGD may play a critical role in the delivery of terrestrial materials to these large marginal seas, perhaps due to large tidal pumping.

Global estimate of submarine groundwater discharge based on an observationally constrained radium isotope model

AbstractAlong the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo‐Pacific Oceans between 60°S and 70°N is (12 ± 3) × 1013 m3 yr−1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo‐Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.

Isotopic, geophysical and biogeochemical investigation of submarine groundwater discharge: IAEA-UNESCO intercomparison exercise at Mauritius Island

Submarine groundwater discharge (SGD) into a shallow lagoon on the west coast of Mauritius Island (Flic-en-Flac) was investigated using radioactive ( 3H, 222Rn, 223Ra, 224Ra, 226Ra, 228Ra) and stable ( 2H, 18O) isotopes and nutrients. SGD intercomparison exercises were carried out to validate the various approaches used to measure SGD including radium and radon measurements, seepage rate measurements using manual and automated meters, sediment bulk conductivity and salinity surveys. SGD measurements using benthic chambers placed on the floor of the Flic-en-Flac Lagoon showed discharge rates up to 500 cm/day. Large variability in SGD was observed over distances of a few meters, which were attributed to different geomorphological features. Deployments of automated seepage meters captured the spatial and temporal variability of SGD with a mean seepage rate of 10 cm/day. The stable isotopic composition of submarine waters was characterized by significant variability and heavy isotope enrichment and was used to predict the contribution of fresh terrestrially derived groundwater to SGD (range from a few % to almost 100%). The integrated SGD flux, estimated from seepage meters placed parallel to the shoreline, was 35 m 3/m day, which was in reasonable agreement with results obtained from a hydrologic water balance calculation (26 m 3/m day). SGD calculated from the radon inventory method using in situ radon measurements were between 5 and 56 m 3/m per day. Low concentrations of radium isotopes observed in the lagoon water reflected the low abundance of U and Th in the basalt that makes up the island. High SGD rates contribute to high nutrients loading to the lagoon, potentially leading to eutrophication. Each of the applied methods yielded unique information about the character and magnitude of SGD. The results of the intercomparison studies have resulted a better understanding of groundwater–seawater interactions in coastal regions. Such information is an important pre-requisite for the protection and management of coastal freshwater resources.

Nutrient input from submarine groundwater discharge versus intermittent river-water discharge through an artificial dam in the Yeongsan River estuary, Korea

The Yeongsan River estuary (YRE) is located downstream of a dam housing an artificial lake on the southwestern coast of Korea. Intermittent discharges of lake water through dam gates control the water level of the lake. This study compares fluxes of dissolved nutrients in discharged water with those of submarine groundwater discharge (SGD) occurring in the YRE in July and October 2008. With the exception of dissolved inorganic phosphorus (DIP) which limits primary production, nutrient concentrations in the YRE are controlled mainly by conservative mixing between lake water and open ocean water during periods of lake water discharge. In comparison with lake water discharge, the magnitude of SGD, based on a 222Rn mass balance model, is relatively small (about 1.5×105 m3 day−1 in July and about 30% higher in October) and dependent on the water elevation in the lake. However, SGD contributes considerably to the input of DIP when discharge of lake water is halted. Our study shows that SGD is more important in the delivery of biogeochemical components into estuaries that are obstructed by artificial dams.

Submarine groundwater discharge (SGD) as a main nutrient source for benthic and water-column primary production in a large intertidal environment of the Yellow Sea

The biogeochemistry and magnitude of submarine groundwater discharge (SGD) was investigated in one of the largest tidal flat ecosystems worldwide, along the Yellow Sea coast. A representative semi-enclosed embayment located in the south eastern Yellow Sea, Hampyeong Bay, was chosen for this purpose. Groundwater and seawater samples were collected in three seasons (May, July, and September) and analyzed for Ra isotopes, nutrients, and photosynthetic pigments. The biogeochemistry of SGD was strongly influenced by tidal oscillations and seasonal precipitation changes and switched from a brackish, nutrient-enriched regime in May and July to an exclusively saline regime, with lower nutrient concentrations, in September. SGD magnitudes, calculated by using a 226Ra mass balance model, were 0.14 m 3 m − 2 d − 1 in May and 0.35 m 3 m − 2 d − 1 in September. A nutrient mass balance was established for the two campaigns, which suggests that SGD causes the flushing of substantial amounts of pore water nutrients into this embayment; because of SGD, the embayment acts as a source of dissolved inorganic silicates (DSi) that are transported to the open ocean. Potential C fixation rates derived from this nutrient mass balance were compared with two different models for water-column phytoplankton productivity based on water-column Chl a and local irradiation levels. The Chl a-based models generally showed lower C fixation rates than the nutrient-based mass balance, indicating removal of up to 70% of the nutrients by other primary producers, such as benthic algae. During monsoon season, when benthic algal biomass is high and nutrient fluxes are substantial due to a terrestrial component, SGD — driven benthic primary production could play a significant role in this large tidal flat ecosystem.

Benthic nitrate biogeochemistry affected by tidal modulation of Submarine Groundwater Discharge (SGD) through a sandy beach face, Ria Formosa, Southwestern Iberia

To investigate both the role of tides on the timing and magnitude of Submarine Groundwater Discharge (SGD), and the effect on benthic nitrogen biogeochemistry of nitrate-enriched brackish water percolating upwards at the seepage face, we conducted a study of SGD rates measured simultaneously with seepage meters and mini-piezometers, combined with sets ( n = 39) of high resolution in-situ porewater profiles describing NH 4 +, NO 3 −, Si(OH) 4 and salinity distribution with depth (0–20 cm). Sampling took place during two consecutive spring tidal cycles in four different months (November 2005, March, April and August 2006) at a backbarrier beach face in the Ria Formosa lagoon, southern Portugal. Our results show that the tide is one of the major agents controlling the timing and magnitude of SGD into the Ria Formosa. Intermittent pumping of brackish, nitrate-bearing water at the beach face through surface sediments changed both the magnitudes and depth distributions of porewater NH 4 + and NO 3 − concentrations. The most significant changes in nitrate and ammonium concentrations were observed in near-surface sediment horizons coinciding with increased fraction of N in benthic organic matter, as shown by the organic C:N ratio. On the basis of mass balance calculations executed on available benthic profiles, providing ratios of net Ammonium Production Rate (APR) to Nitrate Reduction Rate (NRR), coupled to stoichiometric calculations based on the composition of organic matter, potential pathways of nitrogen transformation were speculated upon. Although the seepage face occasionally contributes to reduce the groundwater-borne DIN loading of the lagoon, mass balance analysis suggests that a relatively high proportion of the SGD-borne nitrogen flowing into the lagoon may be enhanced by nitrification at the shallow (1–3 cm) subsurface and modulated by dissimilatory nitrate reduction to ammonium (DNRA).

Radon and radium isotope assessment of submarine groundwater discharge in the Yellow River delta, China

Naturally occurring chemical tracers were used to assess the magnitude of submarine groundwater discharge (SGD) during two different sampling periods at a coastal site south of the Yellow River delta, China. We used salinity and pH as indicators of the terrestrial and recirculated seawater components of discharging groundwater and radium isotopes to quantify offshore transport rates. We then used an hourly time series of multiple radium isotopes (224Ra, 223Ra, and 226Ra) to quantify SGD rates and also used 222Rn and seepage meters to independently quantify SGD rates as a comparison to the radium results. Offshore transport rates were found to range from 3.3 to 4.7 cm s−1. Modeled time series radium activities indicated average SGD rates ranging from 4.5 to 13.9 cm d−1 in September 2006 and from 5.2 to 11.8 cm d−1 in July 2007. Temporal trends associated with the radium approach agree with SGD patterns revealed by automated seepage meters deployed nearby, but the absolute fluxes are about 70% lower than those determined by the seepage meters. Modeled SGD rates based on 222Rn (mean = 13.8 cm d−1 in 2006 and 8.4 cm d−1 in 2007) agree with those determined by the radium analysis. Differences in derived SGD rates between the different radium isotopes (226Ra highest; 224Ra lowest) are likely results of uncertainties in the background activities and our limited selection of appropriate groundwater/pore water end‐member values. Scaling our results to the entire Yellow River delta, we find SGD fluxes (and corresponding nitrate fluxes) 2–3 times that of the Yellow River.

Nutrient inputs from submarine groundwater discharge (SGD) in Masan Bay, an embayment surrounded by heavily industrialized cities, Korea

In order to estimate the magnitude of submarine groundwater discharge (SGD) and the associated nutrient fluxes in Masan Bay on the southern coast of Korea, we measured the concentrations of 226Ra and nutrients in seawater, brackish groundwater, and stream water in May and August 2006. Gauging unidentified nutrient fluxes through SGD is very important in this bay since diatom red tides have been occurring from April to October every year since the 1980s. Based on a 226Ra mass balance model, the submarine inputs of coastal groundwater were estimated to be 4.8 × 10 6 and 5.7 × 10 6 m 3 d − 1 (61 and 71 L m − 2 d − 1 ) in May and August, respectively, which were approximately 840% and 540% of the surface water discharge into the bay. The fluxes of dissolved inorganic phosphorus (DIP) and silicate (DSi) through SGD were 2–3 fold higher than those via stream water, while the fluxes of dissolved inorganic nitrogen (DIN) were comparable to those from surface waters during both sampling periods. Nutrient fluxes through stream waters relative to those from SGD were more significant in the inner part of the bay, which appears to be due to the direct influence of heavily polluted stream waters. Our study shows that the large and continuous supply of inorganic nutrients through SGD may play an important role in eutrophication and the occurrence of red tides in this bay, which should be taken into consideration in the environmental management of the bay.

Radium mass-balance in Jamaica Bay, NY: Evidence for a substantial flux of submarine groundwater

A mass balance for the naturally-occurring radium isotopes ( 224Ra, 223Ra, 228Ra, and 226Ra) in Jamaica Bay, NY, was conducted by directly estimating the individual Ra contributions of wastewater discharge, diffusion from fine-grained subtidal sediments, water percolation through marshes, desorption from resuspended particles, and water exchange at the inlet. The mass balance revealed a major unknown source term accounting for 19–71% of the total Ra input, which could only be resolved by invoking a source from submarine groundwater. Shallow (< 2 m depth) groundwater from permeable sediments in Jamaica Bay was brackish and enriched in Ra relative to surface bay waters by over two orders of magnitude. To balance Ra fluxes, a submarine groundwater input of 0.8 × 10 9–9.0 × 10 9 L d − 1 was required. This flux was similar for all four isotopes, with individual estimates varying by less than a factor of 2. Our calculated groundwater flux was 6- to 70-fold higher than the fresh groundwater discharge to the bay estimated by hydrological methods, but closely matched direct flow rates measured with seepage meters. This suggests that a substantial portion of the discharge consisted of recirculated seawater. The magnitude of submarine groundwater discharge varied seasonally, in the order: summer > autumn > spring. Chemical analyses suggest that the recirculated seawater component of submarine groundwater delivers as much dissolved nitrogen to the bay as the fresh groundwater flux.

Hydrogeological modeling of submarine groundwater discharge on the continental shelf of Louisiana

A regional scale hydrogeologic model has been developed to estimate the magnitude of submarine groundwater discharge to the coastal waters of southeastern Louisiana. The model domain incorporates both the onshore recharge area of terrestrially derived freshwater, and fluid circulation within the sediments on the continental shelf. The hydrogeologic properties of these sediments, which form part of the Coastal Lowlands Aquifer System, have been well‐characterized in earlier studies. The low topographic relief of the coastal plain and an extensive zone of seawater intrusion are key features of the groundwater flow system. Model calculations suggest that no water containing a substantial component of terrestrial origin discharges on the continental shelf. Rather the near‐shore coastal zone serves as a groundwater recharge area of saline water that then forms the seawater recirculation system beneath the coastal plain. The modeling results are consistent with interpretations of the rates of submarine groundwater discharge derived from geochemical tracers, presented in a companion paper by McCoy et al. (2007).

Submarine groundwater discharge (SGD) into the Yellow Sea revealed by 228Ra and 226Ra isotopes: Implications for global silicate fluxes

We estimated the magnitude of submarine groundwater discharge (SGD) into the Yellow Sea, which is one of the largest continental shelves in the world, using 226Ra and 228Ra isotopes. On the basis of 228Ra and 226Ra mass-balance models, we estimated the advective flux of 226Ra through SGD to be ∼270×1012 dpm yr−1 in the Yellow Sea. Using this 226Ra flux and the measured 226Ra activity in coastal groundwater, the submarine discharge of groundwater (mostly brackish groundwater) was calculated to be at least 40% of the river-water input (∼2.3×1011 m3 yr−1). Then, on the basis of the SGD and the concentration of Si in coastal groundwater, we estimated the flux of Si through SGD to be 20–100% of that associated with river discharge (∼23×109 mol yr−1). This large SGD is likely to be due to the high tidal range (up to 10 m) and the wide distribution of sandy sediments in the Yellow Sea, favorable for the recirculation of seawater through bottom sediments and rocks. This result from such a large area implies that the Si flux through SGD may be significant on a global scale.