Role Of Submarine Groundwater Discharge Research Articles

The Role of Submarine Groundwater Discharge to the Input of Macronutrients Within a Macrotidal Subpolar Estuary

The major sources of macronutrients (nitrate, ammonium, phosphate, and silicic acid) in Jakolof Bay, Alaska are submarine groundwater discharge (SGD), rivers, and offshore water. We estimated SGD using natural geochemical tracers (radon and radium), a salt mass balance, and a two-component salinity mixing equation based on the change in groundwater salinity on falling lower low tide. Previous studies have hypothesized that the major macronutrient input into Jakolof Bay is offshore water. This study challenges that assumption by determining the relative contribution of macronutrients from SGD relative to offshore water and rivers. Here, SGD is tidally driven and, as the Northern Gulf of Alaska experiences some of the largest tidal ranges in the world, the SGD fluxes from this region are high relative to the global average regardless of local sediment type. The fluxes ranged from 596 ± 85 cm day−1 at low tide to 97 ± 83 cm day−1 at high tide and are predominantly composed of recirculated seawater (89%) rather than freshwater (11%). The major macronutrients in seawater had different input mechanisms into the semi-enclosed bay. SGD and offshore waters contend as the primary sources of nitrate, which is shown to be the limiting nutrient in this coastal area, while SGD dominates the input of silicic acid. Conversely, the aquifer is found to be a sink for phosphate, indicating that the nutrient is primarily sourced from offshore water.

Arsenic enriched groundwater discharge to a tropical ocean: Understanding controls and processes

The role of submarine groundwater discharge (SGD) in transporting terrestrial-sourced arsenic (As) to the global oceans is not well documented. In the present study, executed on a coast adjoining the extensive groundwater As-contaminated Ganges river delta, we hypothesize that As-enriched groundwater discharges to the adjoining Bay of Bengal (BoB) through SGD flow paths. We conducted high-resolution, field-based investigations and thermodynamic modeling to understand the SGD-sourced As discharge and geochemical cycling of As and other redox-sensitive solutes along the discharge path under varying redox conditions and water sediment interactions. The As distribution and other solutes were measured in a series of multi-depth observation wells and sediment cores, extending from the high tide line (HTL) to 100 m toward the sea, for pre- and post-monsoon seasons. Results reveal the presence of a plume carrying up to 30 μg/L dissolved load of As toward the sea. Arsenic is associated with a plume of Fe and exhibits similar shore-perpendicular variability. Arsenic distribution and transport is controlled by the Fe-Mn redox cycle and influenced by terrestrial groundwater discharge. Field-observations and geochemical modeling demonstrate that Fe-hydroxide precipitates in the subterranean estuary and acts as an interim sink for As , which is eventually mobilized on alteration of geochemical conditions with the season. Fluctuating plume size can be attributed to seasonal variation in fresh groundwater input to the site. Estimates indicate up to 55mg/m2/d As is released to BoB from the site. Based on physicochemical observations this study demonstrates the yet to be studied SGD derived As cycles and the role of SGD dynamics in controlling the fate of redox-sensitive contaminants and their discharge into global oceans.

Geochemical ratios mediated understanding of estuarine dynamics in submarine groundwater discharge prevalent basaltic aquifer

Submarine groundwater discharge (SGD) has been recognized as an integral pathway of hydrological cycle. The role of SGD as a mechanism for material transport from terrestrial to marine ecosystems has also been investigated, considering the large hydrological flux and a massive load of solutes is found to be carried as SGD to the coastal oceans. However, there are challenges in recognizing the process and delineating specific areas which are susceptible to SGD. Considering the unique geochemical signature of groundwaters found in varied lithology, this paper investigates the possibility of using geochemical ratios and their variability in coastal areas as a tracer for the identification of SGD. The Br−/Cl− ratio, prevalently used for identification of seawater-groundwater mixing is investigated. The study raises questions over the viability of Br−/Cl− as a tracer as Br− rich groundwater in the SGD prevalent area may be a cause of elevated Br−/Cl− ratio in the coastal oceans. Furthermore, the peculiar observation of Fe+2 enrichment in coastal seawater also indicated the influence of differential weathering on the subterranean estuarine (STE) sediments releasing Fe+2 as the groundwater in the area is not enriched in groundwater. This observation is very important in the context of rising sea levels as a larger STE due to rising sea levels can cause higher mobilization of Fe+2 in coastal oceans causing a direct influence on the coastal ecosystem.

The social implications of Submarine Groundwater Discharge from an Ecosystem Services perspective: A systematic review

Submarine Groundwater Discharge (SGD) is recognized as a fundamental hydrological process that supports many coastal biogeochemical cycles and social-ecological systems. However, very little has been investigated about how SGD affects society and, specifically, human well-being. This study systematically examines the published scientific literature on the social implications of SGD by using an Ecosystem Service (ES) perspective. Coastal services provided by ecosystems dependent on SGD are analyzed and clustered in the four main categories of Ecosystem Services (i.e., Provisioning, Supporting, Regulating and Cultural), which are in turn divided into subcategories defined as outcomes. This allows identifying and discussing both benefits and threats to coastal societies resulting from SGD outcomes. From the 1532 articles initially reviewed, the most frequently mentioned category was the supporting services (835) due to the mainstream trend in scientific literature to focus on the role of SGD as a process influencing coastal biogeochemical cycles. Conversely, cultural ES were mentioned in only 49 cases, which should not necessarily be interpreted as a lack of research or interest in this topic, but that this type of references are often not found in the scientific literature but in the grey literature. A detailed publication review was additionally conducted, identifying 114 case-studies from 96 different locations worldwide that reported cases in which SGD had social implications on the well-being. Our review also shows how the different types of Ecosystem Services can have multiple synergies and trade-offs between them, resulting in unequal impacts among stakeholder groups. Overall, this study identifies research gaps related to Ecosystem Services provided by SGD as well as opportunities for further studies, while developing an analytical framework that relies on the Ecosystem Services approach to guide future research on the social implications of SGD.

Assessment of Submarine Groundwater Discharge (SGD) as a Source of Nutrient at Jakarta Bay, Indonesia

Submarine groundwater discharge (SGD), the direct discharge of groundwater to the sea, is a ubiquitous phenomenon worldwide. While SGD is defined as “any and all flow of water on continental margins”, this review focuses on the terrestrial fraction, also known as fresh SGD Jakarta Bay, which started from the late 1980s have had greatest impact on mangrove areas and coastal water quality due to the expanding of the city. In such an environment, inputs of nutrients will play an important role in sustaining primary productivity. Atmospheric deposition and riverine runoff have been traditionally considered the main external sources of nutrients to the bay, whereas the role of submarine groundwater discharge has been largely ignored. . In this study, we used radon tracers and salinity to investigate SGD and associated nutrient inputs to Jakarta Bay especially from Citarum Estuary as the main river. The result shows that SGD becomes an important role in understanding the nutrient cycle at Jakarta Bay.

Submarine groundwater discharge-driven nutrient fluxes in a typical mangrove and aquaculture bay of the Beibu Gulf, China

To understand the role of submarine groundwater discharge (SGD) in a mangrove combined aquaculture ecosystem, groundwater samples and timeseries observations (27 h) of 224Ra, 223Ra, and nutrients were obtained during wet and dry seasons in Zhenzhu Bay, a typical mangrove and aquaculture ecosystem along the Beibu Gulf in China. The SGD rates in Zhenzhu Bay were estimated to be 22.3–44.5 cm/d in the wet season and 41.1–58.1 cm/d in the dry season, which were 0.8–1.6 and 9.8–14.1 times higher than the corresponding river water discharge values, respectively. Furthermore, SGD-driven dissolved inorganic nitrogen, dissolved inorganic orthophosphate, and dissolved inorganic silicate accounted for 72%, 56%, and 60% of the total nutrient input, respectively, during wet season, which increased to 93%, 98%, and 89% during dry season. The findings highlight that SGD plays an important role in the dissolved inorganic nutrient sources of Zhenzhu Bay, and that the effective utilization and management of the bay should consider SGD.

Nitrogen and oxygen isotopomeric constraints on the sources of nitrous oxide and the role of submarine groundwater discharge in a temperate eutrophic salt‐wedge estuary

AbstractEstuaries have been identified as sources of nitrous oxide (N2O) emissions to the atmosphere but questions remain as to which production pathway(s) govern the oversaturation of N2O observed in most estuaries worldwide. Here, we use a suite of nitrate and N2O isotopes, as well as the 15N site preference signatures of N2O to assess (1) the relative importance of different N2O production pathways in a eutrophic groundwater‐impacted salt‐wedge estuary and the aquifers underlying the estuary and (2) the influence of groundwater input on the overall N2O saturation in the estuary. This is one of the few studies to examine the effect of groundwater–surface water interaction on N2O cycling using N2O isotopes. The site preference values of N2O in the deep aquifer below the estuary were distinctive (83‰ ± 25‰) and were much higher than in either surface water (21‰ ± 6‰) or shallow groundwater (44‰ ± 8‰), suggesting the influence of multiple biotic and/or abiotic processes which proceed through multiple cycles, and/or the occurrence of a yet unidentified N2O production pathway. Isotope endmember considerations revealed that nitrifier‐denitrification was the major N2O production pathway within the shallow aquifer whereas within the estuarine water column, N2O saturation was governed by chemodenitrification and discharge of N2O‐laden submarine groundwater. Our study not only emphasizes the substantial, yet often underappreciated role of submarine groundwater discharge in estuarine N2O budgets, but also highlights the need to reevaluate the importance of the noncanonical denitrification pathways (i.e., chemodenitrification and nitrifier‐denitrification) in controlling the overall N2O production from estuarine and groundwater environments.

Remobilization of dissolved metals from a coastal mine tailing deposit driven by groundwater discharge and porewater exchange

Mining impacts on coastal environments have been extensively studied around the world. However, the role of Submarine Groundwater Discharge (SGD) and Porewater Exchange (PEX) as pathways for pollutants from mining waste deposits into seawater has been largely overlooked. Portmán Bay is located in the Cartagena-La Unión Pb-Zn sulphur mining district in Murcia, SE of Spain. The disposal of about 60 million tons of metal-rich mine tailings from 1957 to 1990 led to the infill of most of the bay. Although the effects of metals on indicator organisms have been shown previously, there is a major lack of knowledge on the release of dissolved metals from the emerged tailing deposit into the sea, more than 25 years after the closure of the mining activities.Samples for Ra isotopes (223Ra, 224Ra, 226Ra and 228Ra) and dissolved metals (Ag, Cd, Co, Pb, Zn) were analyzed in porewaters and seawater in order to separately estimate SGD and PEX driven dissolved metal fluxes. Our results show a continuous release of dissolved metals into the sea driven by both PEX and SGD. Most of dissolved metals are remobilized and released into the water column by PEX, which is a ubiquitous mechanism acting along the shoreline. Although SGD only represents 13% of the water flow, it drives large fluxes of dissolved Fe into the sea, mainly restricted to the west side of the bay. Large inputs of dissolved Fe2+ from the anoxic tailings deposit trigger a massive precipitation of iron hydroxides that enables the removal of most dissolved metals from the water column. This study highlights the role of PEX and SGD as significant mechanisms for the land to ocean transfer of dissolved metals from coastal mine tailings deposits.

The important role of submarine groundwater discharge (SGD) to derive nutrient fluxes into River dominated Ocean Margins – The East China Sea

Abstract Submarine groundwater discharge (SGD), which has been recognized as an important pathway for the transport of terrestrial chemical components (i.e., nutrients, trace elements and other contaminants) to the ocean, plays an important role on the biogeochemical cycling in marine environment, especially coastal/marginal seas. In the present work, the initial results on the amount of SGD into the continental shelf of the East China Sea (ECS), one of the marginal seas with large riverine input from the Changjiang River (Yangtze River) are evaluated. According to the locations and features of water masses in the ECS, radium (Ra) isotopes mass balance model was built and the conservative and non-conservative (excess) components of Ra were evaluated. Using the inventories of excess 228Ra and 226Ra, the residence time of water in the ECS was estimated as 1.30 ± 0.27 years. Then the SGD flux was estimated to be (5.42 ± 0.14) × 1011 m3 yr−1, which were 47 ± 1% of the total river discharge into the ECS along the coast. Nutrient fluxes driven by SGD were estimated to be (7.32 ± 0.19) × 1010, (1.79 ± 0.05) × 109 and (1.59 ± 0.04) × 1011 mol yr−1 for dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved inorganic silicate (DSi), respectively, which were about 0.7, 2.2 and 1.4 times, respectively, of the riverine inputs. Furthermore, SGD-driven nutrient had obviously high DIN/DIP ratios, which could lead to a number of large-scale environmental problems to the ECS, such as the frequent harmful algal blooms and hypoxia especially in the estuary and coastal area.

The Influence of Basaltic Islands on the Oceanic REE Distribution: A Case Study From the Tropical South Pacific

The Rare Earth Elements (REEs) have been widely used to investigate marine biogeochemical processes as well as the sources and mixing of water masses. However, there are still important uncertainties about the global aqueous REE cycle with respect to the contributions of highly reactive basaltic minerals originating from volcanic islands and the role of Submarine Groundwater Discharge (SGD). Here we present dissolved REE concentrations obtained from waters at the island-ocean interface (including SGD, river, lagoon and coastal waters) from the island of Tahiti and from three detailed open ocean profiles on the Manihiki Plateau (including neodymium (Nd) isotope compositions), which are located in ocean currents downstream of Tahiti. Tahitian fresh waters have highly variable REE concentrations that likely result from variable water–rock interaction and removal by secondary minerals. In contrast to studies on other islands, the SGD samples do not exhibit elevated REE concentrations but have distinctive REE distributions and Y/Ho ratios. The basaltic Tahitian rocks impart a REE pattern to the waters characterized by a middle REE enrichment, with a peak at europium similar to groundwaters and coastal waters of other volcanic islands in the Pacific. However, the basaltic island REE characteristics (with the exception of elevated Y/Ho ratios) are lost during transport to the Manihiki Plateau within surface waters that also exhibit highly radiogenic Nd isotope signatures. Our new data demonstrate that REE concentrations are enriched in Tahitian coastal water, but without multidimensional sampling, basaltic island Nd flux estimates range over orders of magnitude from relatively small to globally significant. Antarctic Intermediate Water (AAIW) loses its characteristic Nd isotopic signature (-6 to-9) around the Manihiki Plateau as a consequence of mixing with South Equatorial Pacific Intermediate Water (SEqPIW), which shows more positive values (-1 to -2). However, an additional Nd input/exchange along the pathway of AAIW, eventually originating from the volcanic Society, Tuamotu and Tubuai Islands (including Tahiti), is indicated by an offset from the mixing array of AAIW and SEqPIW to more radiogenic Nd isotope compositions.

Assessing the role of submarine groundwater discharge as a source of Sr to the Mediterranean Sea

Submarine groundwater discharge (SGD) has been identified as an important source of Sr to the ocean and the SGD-driven Sr flux to the global ocean has been recently re-evaluated (Beck et al., 2013). However, the uncertainty of this value is still high because of the uncertainties related to the determination of SGD flow rates and the paucity of 87Sr/86Sr data in SGD end-members. As carbonates have high Sr concentrations and are subjected to intense heightened weathering, they might significantly influence the SGD input of Sr to the ocean. Here we present data on Sr concentrations and 87Sr/86Sr ratios in three carbonate dominated sites of the western area of the Mediterranean Sea, a semi-enclosed basin characterized by abundant coastal carbonates. The 87Sr/86Sr ratios in groundwater were lower compared to modern seawater (∼0.70916), as expected for areas dominated by carbonate lithologies. Concentrations of Sr and 87Sr/86Sr ratios in groundwater showed conservative mixing in the studied subterranean estuaries. By using SGD flow rates reported in the literature for the study areas, a flow-weighted fresh SGD end-member characterized by a Sr concentration of 27–30μM and a 87Sr/86Sr ratio of 0.707834–0.708020 was calculated for the eastern coast of the Iberian Peninsula.Integrating these Sr data with literature data (i.e. values of Sr concentration and 87Sr/86Sr ratio from other lithologies as well as SGD flow rates), we also calculated the fresh SGD-driven Sr flux to the entire Mediterranean Sea, obtaining a value of (0.34–0.83)·109moly−1, with a 87Sr/86Sr of 0.7081–0.7086. Thus, for the entire Mediterranean basin, SGD is globally a source of Sr less radiogenic compared to seawater. The SGD Sr flux to the Mediterranean Sea represents 5–6% of the SGD Sr flux to the global ocean and the Mediterranean SGD end-member has higher Sr concentration (5–12μM) than the global SGD end-member (2.9μM). This confirms the significant role of carbonate lithologies on SGD-driven Sr fluxes to seawater.The fresh SGD-driven Sr flux to the Mediterranean Sea is about 20–50% of the riverine Sr input and significantly higher than the input through atmospheric dust deposition. Therefore SGD should be considered as an important continental source of Sr to the basin.

Submarine groundwater discharge as a source of nutrients to the North Pacific and Arctic coastal ocean

Primary productivity in both the Arctic Ocean and the Gulf of Alaska (GOA) is nitrogen limited in the summer when light limitation is relieved, and stratification in the GOA inhibits nutrient fluxes from deep water sources. Concentrations of nutrients and trace metals in these regions are higher closer to shore, and thus rivers have been attributed as the primary coastal source of nutrients and trace metals. Here we evaluate the role of Submarine Groundwater Discharge (SGD), a previously unquantified source of nutrients and trace metals to the coastal Arctic Ocean and GOA. SGD is an especially enriched in nitrate relative to other nutrients, contributing 1.2±0.4mol NO3 day−1m−1 of shoreline of the Arctic Ocean. In the GOA, both SGD-associated nitrate flux (4.3±2.1 NO3 day−1m−1 of shoreline) and silicate flux (13±6 SiO4 day−1m−1 of shoreline), are substantial when compared to other external nutrient sources. Conservative extrapolations indicate overall SGD supplies more nitrate (1.5–17.5 times) to the GOA than rivers.

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.

Submarine groundwater discharge as a major source of nutrients to the Mediterranean Sea

The Mediterranean Sea (MS) is a semienclosed basin that is considered one of the most oligotrophic seas in the world. In such an environment, inputs of allochthonous nutrients and micronutrients play an important role in sustaining primary productivity. Atmospheric deposition and riverine runoff have been traditionally considered the main external sources of nutrients to the MS, whereas the role of submarine groundwater discharge (SGD) has been largely ignored. However, given the large Mediterranean shore length relative to its surface area, SGD may be a major conveyor of dissolved compounds to the MS. Here, we used a (228)Ra mass balance to demonstrate that the total SGD contributes up to (0.3-4.8)⋅10(12) m(3) ⋅ y(-1) to the MS, which appears to be equal or larger by a factor of 16 to the riverine discharge. SGD is also a major source of dissolved inorganic nutrients to the MS, with median annual fluxes of 190⋅10(9), 0.7⋅10(9), and 110⋅10(9) mol for nitrogen, phosphorous, and silica, respectively, which are comparable to riverine and atmospheric inputs. This corroborates the profound implications that SGD may have for the biogeochemical cycles of the MS. Inputs of other dissolved compounds (e.g., iron, carbon) via SGD could also be significant and should be investigated.

Effects of submarine groundwater discharge on the present‐day extent of relict submarine permafrost and gas hydrate stability on the Beaufort Sea continental shelf

AbstractWe investigate the role of submarine groundwater discharge on the offshore temperature and salinity field and its effect on the present‐day extent of submarine permafrost and gas hydrate stability on the North American Beaufort Shelf with a two‐dimensional numerical model based on the finite volume method. This study finds that submarine groundwater discharge can play a large role in submarine permafrost evolution and gas hydrate stability, suggesting that local hydrology may control the evolution of submarine permafrost as strongly as does sea level or paleoclimatic conditions. Submarine permafrost evolution shows transient behavior over potentially long time scales (e.g., several glacial cycles) before a balance of density‐ and pressure‐driven flows is established with the permeability variations imposed by the overlying permafrost layer. The “detectable” offshore permafrost extent is related to the quasi‐stationary location of the saltwater‐freshwater transition. Larger values of submarine groundwater discharge allow permafrost to extend farther offshore because fresh pore water preserves relict ice. Therefore, differences in the permafrost extent at locations that share similar paleoclimatic history may be explained in part by differences in the local hydrology. Gas hydrate stability on the North American Beaufort Shelf may be more widespread than currently thought because low‐ice saturation, highly degraded submarine permafrost likely exists beyond the boundary detectable by common geophysical methods.

Submarine groundwater discharge as a main source of rare earth elements in coastal waters

To evaluate the role of submarine groundwater discharge (SGD) as a source of rare earth elements (REEs) in the coastal ocean, we estimated the SGD associated discharge of REEs into two semi-enclosed coastal bays in the southern coast of Korean peninsula. The mass balances of REEs demonstrated that the REE fluxes through SGD were two to three orders of magnitude higher than those through the other sources, such as diffusion from bottom sediments and atmospheric dust fallout. The Nd inputs through the total SGD from the two small coastal bays, Gamak Bay (148km2) and Hampyeong Bay (85km2), were estimated to be 0.7×104–1.3×104moly−1, which is 0.06–0.3% of the total Nd fluxes from global rivers. In this region, coastal seawater was observed to have a substantially higher middle REE (MREE), which appears to be due to a large discharge of groundwater that is highly enriched with MREE. High MREE bearing waters were observed over the entire shore of the southern coast of Korea, implying that such high SGD-driven REE fluxes are common in coastal area. Our results suggest that the SGD-driven REE fluxes may contribute considerably to the global budget of REEs in the ocean.

Global land–ocean linkage: direct inputs of nitrogen to coastal waters via submarine groundwater discharge

The role of submarine groundwater discharge (SGD), the leakage of groundwater from aquifers into coastal waters, in coastal eutrophication has been demonstrated mostly for the North American and European coastlines, but poorly quantified in other regions. Here, we present the first spatially explicit global estimates of N inputs via SGD to coastal waters and show that it has increased from about 1.0 to 1.4 Tg of nitrate (NO3-N) per year over the second half of the 20th century. Since this increase is not accompanied by an equivalent increase of groundwater phosphorus (P) and silicon (Si), SGD transport of nitrate is an important factor for the development of harmful algal blooms in coastal waters. Groundwater fluxes of N are linked to areas with high runoff and intensive anthropogenic activity on land, with Southeast Asia, parts of North and Central America, and Europe being hot spots.

Detecting Freshwater Inputs via Groundwater Discharge to Marina Lagoon, Mediterranean Coast, Egypt

Much work in recent years has reported on the role of submarine groundwater discharge (SGD) on coastal biogeochemistry, but most of those studies have focused on temperate or tropical climates where year-round rainfall recharges surficial aquifers. The aim of this work, however, was to examine SGD behaviors in an arid setting—Marina Lagoon on the Egyptian Mediterranean coast. SGD was estimated via radon surveys and time-series measurements in lagoon waters during two campaigns (wet season in March 2009 and dry season in July 2010). Relatively higher values of radon were detected in March (maximum >30 dpm/L) compared to July (up to 16 dpm/L), which would indicate either enhanced input rates, or lower mixing/atmospheric losses during the wet season. Lower salinity waters within Marina Lagoon were characterized by higher radon and higher concentrations of dissolved inorganic nitrogen (DIN) and silica (DISi), characteristic of groundwater inputs. Based on lagoon and groundwater radon measurements and an advection-diffusion model, SGD average rates between 0.83 to 2.4 × 108 L/day were estimated for both surveys. Since no seasonal pattern was detected, we suspect that either (1) SGD is derived from a regional aquifer not influenced by recharge from local rainfall or (2) local water use for irrigation and domestic purposes artificially recharges the surficial aquifer in the dry summer (tourist) months, which compensates for the lack of rainfall occurring at that time.

How significant is submarine groundwater discharge and its associated dissolved inorganic carbon in a river-dominated shelf system?

Abstract. In order to assess the role of submarine groundwater discharge (SGD) and its impact on the carbonate system on the northern South China Sea (NSCS) shelf, we measured seawater concentrations of four radium isotopes 223,224,226,228Ra along with carbonate system parameters in June–July, 2008. Complementary groundwater sampling was conducted in coastal areas in December 2008 and October 2010 to constrain the groundwater end-members. The distribution of Ra isotopes in the NSCS was largely controlled by the Pearl River plume and coastal upwelling. Long-lived Ra isotopes (228Ra and 226Ra) were enriched in the river plume but low in the offshore surface water and subsurface water/upwelling zone. In contrast, short-lived Ra isotopes (224Ra and 223Ra) were elevated in the subsurface water/upwelling zone as well as in the river plume but depleted in the offshore surface water. In order to quantify SGD, we adopted two independent mathematical approaches. Using a three end-member mixing model with total alkalinity (TAlk) and Ra isotopes, we derived a SGD flux into the NSCS shelf of 2.3–3.7 × 108 m3 day−1. Our second approach involved a simple mass balance of 228Ra and 226Ra and resulted in a first order but consistent SGD flux estimate of 2.2–3.7 × 108 m3 day−1. These fluxes were equivalent to 12–21 % of the Pearl River discharge, but the source of the SGD was mostly recirculated seawater. Despite the relatively small SGD volume flow compared to the river, the associated material fluxes were substantial given their elevated concentrations of dissolved inorganic solutes. In this case, dissolved inorganic carbon (DIC) flux through SGD was 153–347 × 109 mol yr−1, or ~23–53 % of the riverine DIC export flux. Our estimates of the groundwater-derived phosphate flux ranged 3–68 × 107 mol yr−1, which may be responsible for new production on the shelf up to 0.3–6.3 mmol C m−2 d−1. This rate of new production would at most consume 11 % of the DIC contribution delivered by SGD. Hence, SGD may play an important role in the carbon balance over the NSCS shelf.

Assessing hydrologic and biogeochemical controls on pore-water dissolved inorganic carbon cycling in a subterranean estuary: A 14C and 13C mass balance approach

Submarine groundwater discharge (SGD) studies have focused on nutrient fluxes to coastal zones in many regions of the world. However the role of SGD in carbon cycling and fluxes is less well studied. We traced SGD-derived dissolved inorganic carbon (DIC) through a subterranean estuary using Δ 14C and δ 13C to evaluate how water residence times and carbon sources affected cycling and potential DIC fluxes to coastal waters. Samples were collected from the Indian River Lagoon subterranean and surface estuaries for analysis of DIC concentrations and natural ∆ 14C-DIC and δ 13C-DIC abundances. Pore-water samples were collected from multiple depths at three piezometer stations along a 250-m long transect perpendicular to shore and extending across the seepage face (about 20–22 m from shore). This approach captured the gradient from rapid, fresh groundwater discharge at the shoreline, across the groundwater–seawater subterranean mixing zone, and into purely marine offshore sediments beyond the seepage face. Freshwater and marine end-member DIC samples were also collected from a beach well and overlying lagoon surface waters beyond the seepage face, respectively. Older ∆ 14C-DIC and more enriched δ 13C-DIC values were observed at the shoreline than at the beach well (∆ 14C: − 431‰ ± 31, δ 13C: − 9.1‰ ± 1.0 versus ∆ 14C: − 108‰, δ 13C: − 16.9‰, respectively), indicating the influence of carbonate dissolution along the groundwater flow path. Pore-water DIC isotopic values indicate the groundwater plume extends from the base of the shoreline profile (115cmbsf) to the distal end of the freshwater seepage face 20–22 m offshore. The application of dual isotope, two- and three-end-member mixing models shows DIC sources change with increasing distance from shore, transitioning from a groundwater dominated system nearshore, to a mixing zone, and then into the seawater recirculation system beyond the mixing zone seepage face. Contributions from organic carbon remineralization increased with distance from shore and had varying effects on carbonate dissolution, depending on flow paths, residence times, and redox conditions. Our study demonstrates the connectivity of unconfined coastal aquifers to surface water estuaries and illustrates how hydrologic and biogeochemical complexities within the subterranean estuary can significantly contribute to the variations in sources of DIC. Furthermore, these findings highlight the importance of qualitative tracing of DIC sources together with an in-depth consideration of in situ biogeochemistry and quantitative SGD estimates. Understanding how spatial changes in hydrology and biogeochemical conditions contribute to the composition and flux of SGD-derived older DIC to coastal waters elucidates the role subterranean estuaries may ultimately play in the global carbon budget.