Saline Submarine Groundwater Discharge Research Articles

Seasonality of Nutrient Flux and Biogeochemistry in an Intertidal Aquifer

AbstractBiogeochemical reactions in coastal aquifers highly affect the nutrient flux associated with submarine groundwater discharge (SGD) to the ocean, which subsequently influences the oceanic environment and ecology. This study investigates a seasonal variation of SGD‐derived nutrient flux to the ocean and nutrient biogeochemistry in an intertidal aquifer of Tolo Harbor, Hong Kong. The results show that the inventory of NOx− and PO43− in the intertidal aquifer has a clear seasonality with a large inventory in summer, a small inventory in spring, and a median inventory in autumn and winter, respectively. Differently, the inventory of NH4+ is large in winter and summer and small in spring and autumn, which results from the coupled effects of seasonal change of both production and removal of NH4+ in the aquifer. The SGD‐derived nutrient (NH4+, NOx−, and PO43−) flux is the highest in summer (271.71, 24.86, and 116.66 mmol·day−1·(m coastline)−1) and is the lowest in spring (114.83, 1.70, and 20.26 mmol·day−1·(m coastline)−1). The majority of SGD‐derived nutrient flux is supported by the local remineralization of organic matter along with seawater infiltration. In autumn, the recharge of seawater induced by tidal pumping significantly shifts the biogeochemical balance of nutrients and is the major source of groundwater nutrients in the intertidal aquifer. Among the various nutrient fluxes (SGD, river discharge, atmospheric deposition, and benthic sediment diffusion) to Tolo Harbor, SGD‐derived PO43− flux is the second major source of seawater PO43− in addition to benthic sediment diffusion. The PO43− loading via SGD is of significance to the primary production in the phosphorus‐limited environment in Tolo Harbor. After considering the natural attenuation of nutrients in a sandy/silty beach aquifer, this study suggests the overestimation of SGD‐derived nutrient loading estimated previously that simply use average nutrient concentration of fresh SGD endmember and saline SGD endmember as the nutrient concentration of total SGD endmember.

Tracing submarine groundwater discharge flux in Tolo Harbor, Hong Kong (China)

Submarine groundwater discharge (SGD) is an important pathway for groundwater and associated chemicals to discharge to the sea. Groundwater levels monitored along a transect perpendicular to the shoreline are used to calculate SGD flux from the nearshore aquifer to Tolo Harbor, Hong Kong (China). The calculated SGD flux—recharge/discharge measured with Darcy’s Law methods—agrees well with estimates based on geo-tracer techniques and seepage meter in Tolo Harbor during previous studies. The estimated freshwater SGD is 1.69–2.0 m2/d at the study site and 0.3 ± 0.04 cm/d for the whole of Tolo Harbor, which is comparable to the river discharge (0.25 ± 0.07 cm/d) and precipitation (0.45 ± 0.15 cm/d). The tide-driven SGD in the intertidal zone is 13.98–17.59 m2/d at the study site and 2.42 ± 0.56 cm/d for the whole of Tolo Harbor. The SGD occurring in the subtidal zone and the bottom of Tolo Harbor is 3.12 ± 4.63 cm/d. Fresh SGD accounts for ~5% of the total SGD, while the rest (~95%) is contributed by saline SGD driven by various forces. About 96% of the tide-driven SGD in the intertidal zone occurs in the ebbing tide period because the head difference between the groundwater level and sea level is great during this period. Tide-driven SGD in the spring tide is ~1.2 times that during neap tide. The tidal fluctuation amplitude and tide-driven SGD in the intertidal zone are positively correlated to each other; thus, a spring neap variation of the tide-driven SGD is observed.

Ecostructuring of marine nematode communities by submarine groundwater discharge

Inputs of submarine groundwater discharge (SGD) to the coastal ocean may alter local and regional-scale biology. Here, we report on nematode assemblages along the north shore of Long Island, NY. We test if nematode communities differed between sites impacted by mixed fresh-saline SGD and where SGD is exclusively saline. Diversity of nematodes was low at sites impacted by fresh SGD and communities were dominated by a few opportunistic genera. Moreover, a set of typical freshwater nematode genera restricted to impacted sites was observed. Their presence in the marine coastal zone is exceptional and underlines the structuring role that fresh SGD plays in the local ecosystem. Saline SGD structured nematode assemblages differently compared to sites impacted by fresh SGD. The number of nematode genera was markedly higher at saline SGD sites, with a different community structure. This study highlights the importance to which inputs of fresh SGD may have on local ecosystem diversity in marine coastal environments.

Applications of radon and radium isotopes to determine submarine groundwater discharge and flushing times in Todos os Santos Bay, Brazil

Todos os Santos Bay (BTS) is the 2nd largest bay in Brazil and an important resource for the people of the State of Bahia. We made measurements of radon and radium in selected areas of the bay to evaluate if these tracers could provide estimates of submarine groundwater discharge (SGD) and flushing times of the Paraguaçu Estuary and BTS. We found that there were a few areas along the eastern and northeastern shorelines that displayed relatively high radon and low salinities, indicating possible sites of enhanced SGD. A time-series mooring over a tidal cycle at Marina do Bonfim showed a systematic enrichment of the short-lived radium isotopes 223Ra and 224Ra during the falling tide. Assuming that the elevated radium isotopes were related to SGD and using measured radium activities from a shallow well at the site, we estimated groundwater seepage at about 70 m3/day per unit width of shoreline. Extrapolating to an estimated total shoreline length provided a first approximation of total (fresh + saline) SGD into BTS of 300 m3/s, about 3 times the average river discharge into the bay. Just applying the shoreline lengths from areas identified with high radon and reduced salinity results in a lower SGD estimate of 20 m3/s. Flushing times of the Paraguaçu Estuary were estimated at about 3–4 days based on changing radium isotope ratios from low to high salinities. The flushing time for the entire BTS was also attempted using the same approach and resulted in a surprisingly low value of only 6–8 days. Although physical oceanographic models have proposed flushing times on the order of months, a simple tidal prism calculation provided results in the range of 4–7 days, consistent with the radium approach. Based on these initial results, we recommend a strategy for refining both SGD and flushing time estimates.

Terrestrial groundwater and nutrient discharge along the 240-km-long Aquitanian coast

We collected samples from sea water, runnel water, beach pore waters, water from the unconfined surficial aquifer discharging at the beach face, groundwater, and rainwater from the Aquitanian coast in order to determine the flux of dissolved inorganic nitrogen (DIN), phosphorus and silica from terrestrial submarine groundwater discharge (SGD). The flux of fresh groundwater was obtained from a water balance calculation based on precipitation and evapotranspiration and assessment of the coastal watershed from hydrograph separation. Waters with intermediate salinities between sea water and freshwaters are found all along the 240-km-long coast, indicating that SGD is ubiquitous. The estimated fresh water flux is 2.25m3d−1m−1 longshore. Terrestrial SGD provides a DIN flux of 9·106mol each year to the adjacent coastal zone. This flux is about four times lower than the release of DIN due to tidally driven saline SGD. The freshwater DIN flux is low because the upland land use consists almost exclusively of pine forest. Dissolved organic nitrogen represents more than 60% of the total dissolved nitrogen flux. Dissolved iron, phosphorus and silica have much higher concentrations in the anoxic forest aquifer than in the fresh-water end-member of the subterranean estuary sampled in the upper beach aquifer. This suggests that the salinity gradient of the estuary does not correspond to a redox gradient. The redox front between anoxic groundwater and fresh oxic waters occurs below the soil-depleted foredune/yellow dune. Anoxic P- and Si-rich waters seep directly on the beach face only in the north Gironde, where the foredunes are eroded. This study reveals the role of the sandy foredune aquifer in biogeochemical fluxes from SGD, which is to dilute and oxidize waters from the unconfined surficial upland aquifer.

Submarine groundwater discharge as a source of dissolved nutrients to an arid coastal embayment (La Paz, Mexico)

Submarine groundwater discharge (SGD) was investigated in the southeastern portion of La Paz Bay (Baja California, Mexico) using radon (222Rn) as a natural tracer. In the absence of permanent surface flows in this arid region, we hypothesize SGD is a major regional source of dissolved nutrients. Four spatial surveys showed higher radon and nutrient (NH4, NO3, NO2, PO4 and SiO4) values in the winter than summer lagging rainfall by 3–4 months. The surveys revealed two sites (Balandra and Merito) with a stronger radon signal. Intensive time series (12–24 h) measurements at those sites were used to estimate SGD fluxes using a radon mass balance approach. In Balandra, SGD was estimated to be 0.18 m3 m−2 day−1 and significant correlations between radon and nutrients (NO3 and SiO4) were observed. In Merito, SGD rates were estimated to be 0.10 m3 m−2 day−1 and no correlations between nutrients and 222Rn were observed. The difference between the two sites was interpreted to be related to different components dominating SGD (i.e., fresh SGD in Balandra and saline SGD in Merito). The estimated SGD-derived nutrients fluxes were 2–52, 0.04–0.94, 7–164 mmol m2 day−1 for dissolved inorganic nitrogen, phosphate, and silicate, respectively. These fluxes could explain between 5 and 20 % of the regional marine primary productivity values.

Non-conservative behavior of fluorescent dissolved organic matter (FDOM) within a subterranean estuary

The role of submarine groundwater discharge (SGD) in releasing fluorescent dissolved organic matter (FDOM) to the coastal ocean and the possibility of using FDOM as a proxy for dissolved organic carbon (DOC) was investigated in a subterranean estuary in the northeastern Gulf of Mexico (Turkey Point, Florida). FDOM was continuously monitored for three weeks in shallow beach groundwater and in the adjacent coastal ocean. Radon (222Rn) was used as a natural groundwater tracer. FDOM and DOC correlated in groundwater and seawater samples, implying that FDOM may be a proxy of DOC in waters influenced by SGD. A mixing model using salinity as a seawater tracer revealed FDOM production in the high salinity region of the subterranean estuary. This production was probably a result of infiltration and transformation of labile marine organic matter in the beach sediments. The non-conservative FDOM behavior in this subterranean estuary differs from most surface estuaries where FDOM typically behaves conservatively. At the study site, fresh and saline SGD delivered about 1800mgd−1 of FDOM (quinine equivalents) to the coastal ocean per meter of shoreline. About 11% of this input was related to fresh SGD, while 89% were related to saline SGD resulting from FDOM production within the shallow aquifer. If these fluxes are representative of the Florida Gulf Coast, SGD-derived FDOM fluxes would be equivalent to at least 18% of the potential regional riverine FDOM inputs. To reduce uncertainties related to the scarcity of FDOM data, further investigations of river and groundwater FDOM inputs in Florida and elsewhere are necessary.

An evaluation of the efficacy of radium isotopes as tracers of submarine groundwater discharge to southern Rhode Island's coastal ponds

Submarine groundwater discharge (SGD) represents a significant flux of water and chemical constituents to coastal regions. Because 226Ra and 228Ra are highly concentrated in groundwater relative to surface water, and their signal is well-integrated throughout coastal surface waters, these isotopes have been increasingly utilized to quantify SGD over the past several decades. In this study, a combined radium and salt balance is used to yield estimates of fresh and saline SGD to four of Rhode Island's coastal ponds (Winnapaug, Quonochontaug, Ninigret, and Point Judith Ponds). Fresh SGD rates determined for each pond range between 0 and 17Lm−2d−1. These values largely fall in line with previous estimates made using hydrologic models. The uncertainty of saline SGD estimates for each pond is high; minimum and maximum saline SGD estimates differ by as much as three orders of magnitude. These results echo the discrepancy between two previous radium-based studies from this location. The variability of dissolved radium in the Rhode Island coastal aquifer plays a large role in the uncertainty of these saline SGD estimates. Groundwater dissolved radium activity is dependent upon many different factors, including groundwater salinity and redox conditions, sediment thorium and radium activities, and the amount of time to which the sediment has been exposed to saline groundwater. Due to the heterogeneous nature of glacial outwash sediments, none of these parameters are likely consistent throughout the aquifer in this location. Groundwater 226Ra and 228Ra activities in the coastal ponds vary significantly both laterally and with sediment depth. For example, groundwater samples collected in Ninigret Pond separated by a vertical distance of only 5cm differ by as much as 3×103dpm100L−1 for both 226Ra and 228Ra. The results from this study indicate that a single groundwater sample or suite of samples may not accurately reflect the average radium activity of the groundwater actually discharging into each pond. Targeted sampling in areas where SGD is qualitatively observed can potentially remedy this problem.

Patterns and variability of groundwater flow and radium activity at the coast: A case study from Waquoit Bay, Massachusetts

Radium is widely used as a natural tracer of groundwater discharge to the ocean. To better understand how radium activities vary within the groundwater systems that contribute this discharge to coastal waters, we characterized the spatial patterns of porewater activities of the four radium isotopes in the context of the groundwater flow system of Waquoit Bay, Massachusetts. Radium activities were highly heterogeneous across locations, though qualitatively consistent with inferred groundwater flowpaths and simple models of radium evolution. A comparison of radium activity measurements in discharging groundwater, porewater, and baywater indicates that saline submarine groundwater discharge is likely a mixture of water with long and short subsurface residence times. Flow is driven by forcing mechanisms acting on a range of timescales, from waves, to tides, to seasons and longer. Estimates of radium flux along transects extending offshore are disproportionate to groundwater discharge, and individual isotopes have distinct patterns with peak activity occurring at different locations. Modeled sensitivity of porewater radium activities to production rates and sorptive properties of the subsurface indicates that variability in radium activity is caused by aquifer property heterogeneity, variations in groundwater flowpaths, and hydrologic transience. These results suggest that, in many cases, the complexity of coastal groundwater flow and its influence on radium activity present a challenge for using radium activity measured in seawater to quantify groundwater inputs. However, characterization of the spatial pattern of radium activities within the groundwater system may provide valuable qualitative understanding of the subsurface flow processes that drive groundwater discharge.

Breathing of a coral cay: Tracing tidally driven seawater recirculation in permeable coral reef sediments

Coral reefs are characterized by high gross productivity in spite of low nutrient concentrations. This apparent paradox may be partially reconciled if seawater recirculation in permeable sediments over large (meters) and long (hours to days) scales is an important source of recycled nitrogen and phosphorus to coral reefs. In this paper we use radon (222Rn, a natural tracer) to quantify tidally driven pore water (or groundwater) exchange between (1) an offshore coral cay island and its fringing reef lagoon and (2) a reef lagoon and the surrounding ocean. As seawater infiltrates Heron Island at high tide, it acquires a radon signal that can be detected when pore waters emerge from carbonate sands at low tide. A nonsteady state model indicated that vertical pore water upwelling rates (or saline submarine groundwater discharge) were >40 cm/d within the reef lagoon and >100 cm/d outside the lagoon at low tide. Within the lagoon, tidal pumping and temperature‐driven convection were the main driving forces of pore water advection. At low tide, the reef lagoon level is about 1 m higher than the surrounding ocean. As a result, a steep hydraulic gradient develops at the reef edge, driving unidirectional filtration through the reef framework. Groundwaters were highly enriched in nitrate (average of 530 μmol, likely influenced by bird guano) relative to lagoon waters (1.9 μmol). Rough but conservative estimates indicated that groundwater‐derived nitrate fluxes (7.9 mmol/m2/d) can replace the entire lagoon nitrate inventory every <19 days. We speculate that as offshore coral islands “breath” (inhale seawater at high tide and exhale groundwater at low tide), they release nutrients that lead to sustained productivity within coral reefs.

Quantification of point‐source groundwater discharges to the ocean from the shoreline of the Big Island, Hawaii

Aerial thermal infrared imaging has revealed that submarine groundwater discharge (SGD) along the western coast of the Big Island of Hawaii is often focused as point‐source discharges that create buoyant groundwater plumes that mix into the coastal ocean. We quantified the SGD fluxes associated with several plumes using natural geochemical tracers. Offshore transects of 222Rn and 224Ra show elevated activities and corresponding low salinities in the nearshore waters within the plumes, indicating that these naturally occurring radionuclides can be useful tracers of groundwater inputs in this area. Using a series of simultaneous mass balance equations for water, salt, and radon, we determined the groundwater fluxes of six plumes near Kona, Hawaii. The average SGD fluxes ranged from 1100 m3 d−1 to 12,000 m3 d−1 of total (fresh + saline) SGD. The fresh (meteoric) groundwater equivalents for the same flows, modeled by adjusting the groundwater end member to reflect freshwater rather than brackish groundwater composition, ranged from 630 m3 d−1 to 8600 m3 d−1. These fluxes are in general agreement with earlier results obtained by hydrological water budgets and physical oceanographic analyses of fresh SGD rates in this region.

Land or ocean?: Assessing the driving forces of submarine groundwater discharge at a coastal site in the Gulf of Mexico

Submarine groundwater discharge (SGD) is an important pathway of trace element and nutrient cycling in the coastal ocean. Since terrestrial and marine SGD driving forces are usually superimposed, it is difficult to separate their relative contributions. This work uses continuous, long‐term (∼2 years) records of seawater 222Rn to estimate total SGD rates and hydrological observations to examine the driving forces of SGD at a coastal site on the Gulf of Mexico. While seasonal cycles cannot be clearly identified, short‐term fluctuations were ubiquitous. Radon‐derived SGD was strongly associated with the neap spring tidal cycle, at least during a period of extreme drought when minimum external disturbances were present. We applied multiple independent approaches to separate the relative contribution of fresh and saline SGD. A salinity mixing model indicated that the contribution of fresh SGD ranged between 3 and 12%. The rate of freshening of water within a seepage meter required vertical freshwater advection of ∼0.9 cm d−1 at low tide, which is <10% of the total SGD estimated from 222Rn (∼11 cm d−1). Darcy's law calculations and water table decline rate also suggested a fresh SGD contribution <5%. The temporal trends in total SGD and the reasonable agreement of our multiple approaches indicate that transient marine forces, likely tidal pumping, dominate benthic advective exchange in these permeable sediments. Compared to regional river inputs, our conservatively extrapolated saline SGD is on the same order of magnitude, but fresh groundwater only represents a small fraction. Future investigations should refine techniques for scaling up local observations.

Tidal pumping drives nutrient and dissolved organic matter dynamics in a Gulf of Mexico subterranean estuary

We hypothesize that nutrient cycling in a Gulf of Mexico subterranean estuary (STE) is fueled by oxygen and labile organic matter supplied by tidal pumping of seawater into the coastal aquifer. We estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions. The results indicate a large variability in nutrient concentrations over tidal and seasonal time scales. At high tide, nutrient concentrations in shallow beach groundwater were low as a result of dilution caused by seawater recirculation. During ebb tide, the concentrations increased until they reached a maximum just before the next high tide. The dominant form of nitrogen was dissolved organic nitrogen (DON) in freshwater, nitrate in brackish waters, and ammonium in saline waters. Dissolved organic carbon (DOC) production was two-fold higher in the summer than in the winter, while nitrate and DON production were one order of magnitude higher. Oxic remineralization and denitrification most likely explain these patterns. Even though fresh SGD accounted for only ∼5% of total volumetric additions, it was an important pathway of nutrients as a result of biogeochemical inputs in the mixing zone. Fresh SGD transported ∼25% of DOC and ∼50% of total dissolved nitrogen inputs into the coastal ocean, with the remainder associated with a one-dimensional vertical seawater exchange process. While SGD volumetric inputs are similar seasonally, changes in the biogeochemical conditions of this coastal plain STE led to higher summertime SGD nutrient fluxes (40% higher for DOC and 60% higher for nitrogen in the summer compared to the winter). We suggest that coastal primary production and nutrient dynamics in the STE are linked.

Geochemical and Geophysical Examination of Submarine Groundwater Discharge and Associated Nutrient Loading Estimates into Lynch Cove, Hood Canal, WA

Geochemical tracer data (i.e., 222Rn and four naturally occurring Ra isotopes), electromagnetic (EM) seepage meter results, and high-resolution, stationary electrical resistivity images were used to examine the bi-directional (i.e., submarine groundwater discharge and recharge) exchange of a coastal aquifer with seawater. Our study site for these experiments was Lynch Cove, the terminus of Hood Canal, WA, where fjord-like conditions dramatically limit water column circulation that can lead to recurring summer-time hypoxic events. In such a system a precise nutrient budget may be particularly sensitive to groundwater-derived nutrient loading. Shore-perpendicular time-series subsurface resistivity profiles show clear, decimeter-scale tidal modulation of the coastal aquifer in response to large, regional hydraulic gradients, hydrologically transmissive glacial terrain, and large (4-5 m) tidal amplitudes. A 5-day 222Rn time-series shows a strong inverse covariance between 222Rn activities (0.5-29 dpm L(-1)) and water level fluctuations, and provides compelling evidence for tidally modulated exchange of groundwater across the sediment/water interface. Mean Rn-derived submarine groundwater discharge (SGD) rates of 85 +/- 84 cm d(-1) agree closely in the timing and magnitude with EM seepage meter results that showed discharge during low tide and recharge during high tide events. To evaluate the importance of fresh versus saline SGD, Rn-derived SGD rates (as a proxy of total SGD) were compared to excess 226Ra-derived SGD rates (as a proxy for the saline contribution of SGD). The calculated SGD rates, which include a significant (>80%) component of recycled seawater, are used to estimate associated nutrient (NH4+, Si, PO4(3-), NO3 + NO2, TDN) loads to Lynch Cove. The dissolved inorganic nitrogen (DIN = NH4 + NO2 + NO3) SGD loading estimate of 5.9 x 10(4) mol d(-1) is 1-2 orders of magnitude larger than similar estimates derived from atmospheric deposition and surface water runoff, respectively.

Submarine groundwater discharge to Tampa Bay: Nutrient fluxes and biogeochemistry of the coastal aquifer

To separately quantify the roles of fresh and saline submarine groundwater discharge (SGD), relative to that of rivers, in transporting nutrients to Tampa Bay, Florida, we used three approaches (Darcy's Law calculations, a watershed water budget, and a 222Rn mass-balance) to estimate rate of SGD from the Pinellas peninsula. Groundwater samples were collected in 69 locations in the coastal aquifer to examine biogeochemical conditions, nutrient concentrations and stoichiometry, and salinity structure. Salinity structure was also examined using stationary electrical resistivity measurements. The coastal aquifer along the Pinellas peninsula was chemically reducing in all locations sampled, and that condition influences nitrogen (N) form and mobility of N and PO 4 3−. Concentrations of NH 4 +, PO 4 3− and ratio of dissolved inorganic N (DIN) to PO 4 3− were all related to measured oxidation/reduction potential (pε) of the groundwater. Ratio of DIN: PO 4 3− was below Redfield ratio in both fresh and saline groundwater. Nitrogen occurred almost exclusively in reduced forms, NH 4 + and dissolved organic nitrogen (DON), suggesting that anthropogenic N is exported from the watershed in those forms. In comparison to other SGD studies, rate of PO 4 3− flux in the seepage zone (μM m − 2 d − 1 ) in Tampa Bay was higher than previous estimates, likely due to 1) high watershed population density, 2) chemically reducing conditions, and 3) high ion concentrations in fresh groundwater. Estimates of freshwater groundwater flux indicate that the ratio of groundwater discharge to stream flow is ∼ 20 to 50%, and that the magnitudes of both the total dissolved nitrogen and PO 4 3− loads due to fresh SGD are ∼ 40 to 100% of loads carried by streams. Estimates of SGD based on radon inventories in near-shore waters were 2 to 5 times greater than the estimates of freshwater groundwater discharge, suggesting that brackish and saline SGD is also an important process in Tampa Bay and results in flux of regenerated N and P from sediment to surface water.

Hydrologic forcing of submarine groundwater discharge: Insight from a seasonal study of radium isotopes in a groundwater‐dominated salt marsh estuary

A seasonal study of radium‐derived submarine groundwater discharge (SGD) and associated nitrogen fluxes was carried out in a salt marsh estuary between 2001 and 2003 (Pamet River Estuary, Massachusetts). Twelvehour time series of salinity and radium at the estuary inlet were used to determine the relative importance of fresh versus saline SGD, respectively. The distinct radium (228Ra : 226Ra) isotopic signature of marsh peat pore water and aquifer‐derived brackish groundwater was used to further partition the Ra‐derived SGD estimate. Of these three groundwater sources, only the marsh‐derived groundwater was constant across time. The ratio of brackish to fresh SGD was inversely correlated with water table elevation in the aquifer, suggesting that Ra‐derived SGD was enhanced during dry periods. The various SGD fluxes were responsible for an average annual dissolved inorganic nitrogen (DIN) input of between 1.7 mol m−2 yr−1 and 7.1 mol m−2 yr−1 and a soluble reactive phosphate (SRP) flux of 0.13‐0.54 mol m−2 yr−1. Approximately 30% of the SGD‐derived DIN and SRP flux is exported to coastal waters (Cape Cod Bay), whereas 70% is retained by the salt marsh ecosystem.

The importance of submarine groundwater discharge to the near shore nutrient supply in the Gulf of Aqaba (Israel)

We used two short‐lived radium isotopes (223Ra, 224Ra) and a mass balance approach applied to the radium activities to determine the nutrient contribution of saline submarine groundwater discharge to the coastal waters of the northern Gulf of Aqaba (Israel). Radium isotope activities were measured along transects during two seasons at a site that lacked any obvious surficial water input. An onshore well and an offshore end member were also sampled. For all samples, nutrients and salinity data were collected. Radium isotope activities generally decreased with distance offshore and exhibited significant tidal variability, which is consistent with a shorederived tidally influenced source. Submarine groundwater contributes only 1–2% of the water along this coast, but this groundwater provides 8–46% of the nutrients. This saline groundwater is derived predominately from tidally pumped seawater percolating through the unconfined coastal aquifer and leaching radium and nutrients. This process represents a significant source of nutrients to the oligotrophic nearshore reef.

Evaluations of spatial distribution of submarine groundwater discharge

The paired catchments with the same basin size and meteorological condition but different geology were selected to compare the fresh submarine groundwater discharge (SGD) rate observed by seepage meters with value calculated from water balance method in Kumamoto, Japan. The fresh SGD rate decreases with the distance from the coast, on the other hands, recirculated saline SGD is higher at near shore due to wave set‐up and at offshore due to tidal pumping. The spatial integrations of fresh SGD agree well with the water budget in both paired catchments. The difference of fresh component of SGD between two catchments may be caused by the difference of geology in the study area.

Fresh and saline groundwater discharge to the ocean: A regional perspective

Studies of groundwater flow in coastal aquifers often focus strongly on freshwater and investigate flow in a narrow (<5 km) zone surrounding the coastline. This work was designed to place coastal flow in a regional context and to compare fresh and saline submarine groundwater discharge (SGD) for regional flow systems. Numerical flow and transport models were developed to estimate SGD associated with topography‐driven flow, seawater recirculation, and geothermal convection in a passive margin setting. Simulations were based on two cross sections of North Carolina, and sensitivity studies were used to explore the impact of varying hydraulic and transport parameters. Results suggest that saline flow associated with seawater recirculation and geothermal convection should be considered in studies of SGD. Studies limited to shallow topography‐driven flow may be justified in using small study areas, but flow systems contributing to SGD may span at least 20 km surrounding the coast and continental slope.