Submarine Groundwater Discharge Flow Research Articles

Intercomparison of marine tracer and catchment-based submarine groundwater discharge estimates for a karst aquifer

Submarine groundwater discharge (SGD) is a major source of solutes to the ocean. Consequently, the accuracy of estimates of the magnitude and seasonal variability of SGD is critical for the management of coastal marine ecosystems. However, very few intercomparison studies between different conceptual approaches have been made to assess the reliability and consistency of SGD flow estimates while simultaneously gauging the applicability of underlying models and budget assumptions in real-world scenarios. To date, intercomparison efforts have focused on homogeneous aquifers, and less so on highly heterogeneous aquifers such as karst systems. Here we compare the results of tracer-based (salt mass balance, tidal prism and radon mass balances) and catchment-based methods (water balance, semi-distributed karst network model) to assess SGD at a data-rich coastal location. All approaches tested here reveal a positive relationship between SGD fluxes and groundwater level measured close to the preferential groundwater flow pathways in the phreatic karst aquifer (e.g. water-saturated conduits or fracture network). We use this finding to derive site-specific relationships between groundwater level and SGD flow rates. We derive these relationships from least square fitting of Darcian and Non-Darcian models and regression analysis. The relationships validate the findings derived independently from hydraulic/hydrogeological modelling of the complex karst networks. By combining various estimates made with distinct assumptions, this method produces robust estimates of the SGD seasonal variability, essential to develop optimal management strategies of SGD resources in coastal karstic regions. Where direct measurements of SGD are not possible, monitoring of coastal groundwater level close to preferential phreatic groundwater pathways can be thus used as a useful indicator to compare past groundwater flow measurements and derive site-specific relationships, particularly in settings where large flow rate changes are present such as karst aquifers.

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.

Applicability of Landsat 8 thermal infrared sensor for identifying submarine groundwater discharge springs in the Mediterranean Sea basin

Abstract. Submarine groundwater discharge (SGD) has received increasing attention over the past 2 decades as a source of nutrients, trace elements and ocean pollutants that may alter coastal biogeochemical cycles. Assessing SGD flows and their impact on coastal marine environments is a difficult task, since it is not easy to identify and measure these water flows discharging into the sea. The aim of this study is to demonstrate the significant usefulness of the freely available thermal infrared (TIR) imagery of the Landsat 8 thermal infrared sensor (TIRS) as an exploratory tool for identifying SGD springs worldwide, from local to regional scales, for long-term analysis. The use of satellite thermal data as a technique for identifying SGD springs in seawater is based on the identification of thermally anomalous plumes obtained from the thermal contrasts between groundwater and sea surface water. In this study, we use the TIR remote sensing (TIR-RS) imagery provided by Landsat 8 at a regional scale and discuss the principle limiting factors of using this technique in SGD studies. The study was developed in karstic coastal aquifers in the Mediterranean Sea basin during different seasons and under diverse meteorological conditions. Although this study demonstrates that freely available satellite TIR remote sensing is a useful method for identifying coastal springs in karst aquifers both locally and regionally, the limiting factors include technical limitations, geological and hydrogeological characteristics, environmental and marine conditions and coastal geomorphology.

A case study of freshwater discharge in porous calcarenite formation in coastal terrace at Manapad, South India

The focus of this study is the discharge of freshwater from coastal terraces to coastal areas. A large amount of submarine groundwater discharge (SGD) flow moves toward the sea. The study to determine the flux rate uses geology, grain size, magneto telluric, resistivity and water geochemistry for quality analysis. The research is on the structure of calcite and sandstone composed of high-porosity rocks in the entire terrace. The height of the platform is 45 m, and the high gradient of SGD flows through the porous medium. Rock types and grain studies are used to distinguish estuarine, beach and shallow sea environments. Round quartz grains were found at the top of the terrace in the form of nodules, which is evidence of the uplift of coastal landforms. In the microscopic study, the carbonate rock in the terrace contains calcium and other substances. The calcium substance has fine, medium, coarse grain and ionic properties. Resistivity sounding, 2D resistivity imaging, and magneto telluric technology are to use resistivity changes to determine the connectivity of permeable calcarinite rock and SGD in land and ocean underground discharges. Geochemical phase evolution maps are used to support freshwater and seawater intrusions in research areas. Water flow is calculated using 2D ERI and MT images. Due to the porosity and permeability of the rock, five open-hole wells in the Saint Xavier Cave were discovered, forming a calcarenite structure with a high water flow gradient. This is a useful case study for the purpose of drinking water for coastal people in the system of seepage aquifers.

Microbial community composition across a coastal hydrological system affected by submarine groundwater discharge (SGD).

Mobile Bay, the fourth largest estuary in the USA located in the northern Gulf of Mexico, is known for extreme hypoxia in the water column during dry season caused by NH4+-rich and anoxic submarine groundwater discharge (SGD). Nutrient dynamics in the coastal ecosystem point to potentially elevated microbial activities; however, little is known about microbial community composition and their functional roles in this area. In this study, we investigated microbial community composition, distribution, and metabolic prediction along the coastal hydrological compartment of Mobile Bay using 16S rRNA gene sequencing. We collected microbial samples from surface (river and bay water) and subsurface water (groundwater and coastal pore water from two SGD sites with peat and sandy lithology, respectively). Salinity was identified as the primary factor affecting the distribution of microbial communities across surface water samples, while DON and PO43- were the major predictor of community shift within subsurface water samples. Higher microbial diversity was found in coastal pore water in comparison to surface water samples. Gammaproteobacteria, Bacteroidia, and Oxyphotobacteria dominated the bacterial community. Among the archaea, methanogens were prevalent in the peat-dominated SGD site, while the sandy SGD site was characterized by a higher proportion of ammonia-oxidizing archaea. Cyanobium PCC-6307 and unclassified Thermodesulfovibrionia were identified as dominant taxa strongly associated with trends in environmental parameters in surface and subsurface samples, respectively. Microbial communities found in the groundwater and peat layer consisted of taxa known for denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This finding suggested that microbial communities might also play a significant role in mediating nitrogen transformation in the SGD flow path and in affecting the chemical composition of SGD discharging to the water column. Given the ecological importance of microorganisms, further studies at higher taxonomic and functional resolution are needed to accurately predict chemical biotransformation processes along the coastal hydrological continuum, which influence water quality and environmental condition in Mobile Bay.

Detection and quantification of low submarine groundwater discharge flows by radionuclides to support conceptual hydrogeological model of porous aquifers

Submarine groundwater discharge (SGD) is considered as a good indicator of the natural behavior of coastal aquifers especially in the case of intense groundwater withdrawal or when the aquifer recharge conditions are limited. Indeed, a decrease in SGD flows would result in saline intrusion compromising the groundwater reservoir use. The need for SGD studies in Mediterranean and semi-arid environments have long been recognized and little is known on low and diffuse flows. For this reason, the detection and quantification of low and diffuse SGD flows have been attempted in a Mediterranean porous aquifer with low potential of recharge and high touristic pressure from using a multi-tracing approach combining aerial thermal infrared images (TIR), temperature (T), electric conductivity (E.C.), major ions, selected trace elements, 2H, 18O, 222Rn, 223Ra and 224Ra. The TIR images allowed the detection of slight temperature variations at the seawater surface. Contrasted lithology and strong water-rock interactions due to groundwater residence time of many decades favor the 222Rn, 223Ra and 224Ra contents in groundwater that reach the seashores. Low temperature and E.C, as well as high radon and radium activities, evidenced SGD flows in agreement with the known flow conditions within the aquifer. The quantification of SGD flows revealed in very good agreement with the known recharge rates of the aquifer that validates its conceptual hydrogeological model. For the first time in the Mediterranean or in semi-arid context, a natural tracing experiment based on geochemistry and isotope hydrology tools allowed the quantification of SGD flows to validate the hydrogeological conceptual model and to solve the water balance at the whole aquifer scale. Such a methodology could be applied worldwide to other similar coastal aquifers with low and diffuse SGD flow conditions.

Submarine groundwater discharge revealed by 224Ra and 223Ra in Coffs Harbour, Australia

Submarine groundwater discharge (SGD) is an important link between the land and ocean. In this study, eddy diffusivity coefficients (Kh) and SGD rates were examined off the Jetty Beach in Coffs Harbour (Australia) in September 2017 and 2018. In order to estimate Kh, the apparent age and SGD, two short-lived radium isotopes 224Ra and 223Ra were measured along a shore-perpendicular transect and beach groundwater. Estimated SGD rates were 3.3 and 5.4 cm day−1 in 2017 and 12.7 and 9.3 cm day−1 in 2018, based on average 224Ra and 223Ra groundwater endmember activities. The SGD flow was higher in 2018 due to the combination of a spring tide and rainfall events that took place prior to sampling.

Nutrient input through submarine groundwater discharge in two major Chinese estuaries: the Pearl River Estuary and the Changjiang River Estuary

In this study, we used a 224Ra mass balance model to evaluate the importance of submarine groundwater discharge (SGD) for the budgets of biogenic elements in two major Chinese estuaries: the Pearl River Estuary (PRE) and the Changjiang River Estuary (CRE). The apparent water age in the PRE was estimated to be 4.8 ± 1.1 days in the dry season and 1.8 ± 0.6 days in the wet season using a physical model based on the tidal prism. In the dry season, the water age in the CRE was estimated to be 11.7 ± 3.0 days using the 224Ra/223Ra activities ratios apparent age model. By applying the 224Ra mass balance model, we obtained calculations of the SGD flow in the PRE of (4.5–10) × 108 m3 d−1 (0.23–0.50 m3 m−2 d−1) and (1.2–2.7) × 108 m3 d−1 (0.06–0.14 m3 m−2 d−1) in the dry season and wet season, respectively, and the estimated SGD flux was (4.6–11) × 109 m3 d−1 (0.18–0.45 m3 m−2 d−1) in the dry season of the CRE. In comparison with the nutrient fluxes from the rivers, the SGD-derived nutrient fluxes may play a vital role in controlling the nutrient budgets and stoichiometry in the study areas. The large amount of dissolved inorganic nitrogen and phosphorus fluxes together with high N: P ratios into the PRE and CRE would potentially contribute to eutrophication and the occurrence of red tides along the adjacent waters.

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.

Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from 224Ra/228Th disequilibrium

Pore water exchange (PEX) and submarine groundwater discharge (SGD) represent two mechanisms for solute transport from the seabed into the coastal ocean. However, their relative importance remains to be assessed. In this study, we pursued the recently developed 224Ra/228Th disequilibrium approach to quantify PEX fluxes of 224Ra into the Jiulong River estuary, China. By constructing a full mass balance of water column 224Ra, we were allowed to put various source terms, i.e., SGD, diffusive and advective pore water flow (PEX), and river input in a single context. This led to the first quantitative assessment of the relative importance of PEX vs. SGD in the delivery of solutes into an estuary. We carried out two surveys in the Jiulong River estuary: one in January 2014 (winter survey), the other in August 2014 (summer survey). By virtue of a 1-D mass balance model of 224Ra in the sediment column, we demonstrated that PEX fluxes of 224Ra were highly variable, both temporally and spatially, and can change by 1–2 orders of magnitude in our study area. Moreover, we identified a strong correlation between 224Ra-based irrigation rate and 234Th-based sediment mixing rate. Our results highlighted irrigation as the predominant PEX process for solute transfer across the sediment–water interface.Total PEX flux of 224Ra (in 1010dpmd−1) into the Jiulong River estuary was estimated to be 22.3±3.0 and 33.7±5.5 during the winter and summer surveys, respectively. In comparison, total SGD flux of 224Ra (in 1010dpmd−1) was 11.3±8.6 and 49.5±16.3 in the respective seasons. By multiplying the PEX fluxes of 224Ra by the ratio of the concentration gradients of component/224Ra at the sediment–water interface, we quantified the total PEX fluxes of dissolved inorganic carbon (DIC) and nutrients (NH4+, NO3−, and H4SiO4) into the Jiulong River estuary. In the meantime, net export of DIC and nutrients via SGD were estimated by multiplying the SGD fluxes of 224Ra by the DIC (nutrients)/224Ra ratios in the SGD end-members around this area. Our results revealed that PEX-driven fluxes of solutes rival net SGD input and river input in an estuary. An additional new finding is that water column NO3− in the surface estuary was effectively sequestered due to SGD, probably as a result of intense denitrification occurring in the anoxic subterranean estuary.

Transport and transformations of groundwater-borne carbon discharging through a sandy beach to a coastal ocean

Submarine groundwater discharge (SGD) is now recognized as an important coastal process affecting local and regional sources of solutes to coastal oceans. The objective of this study was to compare SGD estimates through Martinique Beach (Îles-de-la-Madeleine, QC, Canada) using different sampling techniques in order to quantify SGD fluxes of dissolved organic and inorganic carbon (DOC, DIC) exported to a coastal embayment. These fluxes were estimated using hydrogeological and geochemical methods, including direct measurement via seepage meters and hydrogeological estimation using Darcy’s law. Darcy estimates led to a fresh SGD flow ranging from 0.020 m3/s at the shoreline to 0.030 m3/s at the beach face. Direct measurements by seepage meters revealed higher SGD flows, with a mean value of 0.090 m3/s. These SGD flows were mainly composed of recirculating seawater flushing by the falling tide. The unconfined aquifer of Martinique Beach is a biogeochemically reactive zone, where DOC and DIC are nonconservative. The groundwater residence time (~32 days), low oxygen conditions (~20%) and high DOC concentrations (~ 2 mmol/L) represent a suitable environment for biogeochemical reactions to occur, and subsequently alter groundwater-borne carbon concentrations. We calculated a fresh-groundwater carbon load of 147 kg/d of DIC and 27 kg/d of DOC at the beach face. Even though recirculating seawater dominated the volume of total SGD, fresh SGD was an important carbon pathway, accounting for 12 and 20% of total DIC and DOC, respectively. This site-specific study is the first attempt to estimate volumetric and chemical groundwater fluxes to a coastal Canadian ocean, and demonstrates the strong interaction between fresh groundwater and coastal systems.

The influence of a metal-enriched mining waste deposit on submarine groundwater discharge to the coastal sea

Submarine groundwater discharge (SGD) was investigated at El Gorguel Bay (Cartagena-La Unión Pb–Zn mining district, Murcia, Spain), a Mediterranean region where mine tailings have been accumulated for decades at the shoreline. At this site, groundwater may become enriched in metals prior to discharge to the coastal sea and may lead to significant releases that can contaminate the coastal environment. The distribution of dissolved metals and Ra isotopes were studied in seawater and groundwater samples collected during the summer season in 2013. Cross-shore gradients of 224Ra in the bay allowed the calculation of a SGD flow of (39±14)103m3day−1. SGD-driven metal fluxes were 47–180molday−1km−1 for Zn, 0.20–0.60molday−1km−1 for Pb, 4–32molday−1km−1 for Fe, 0.8–2.3molday−1km−1 for Cu and 0.9–2.8molday−1km−1 for Ni. Compared to other coastal zones not affected by mining activities, SGD-driven metal fluxes were especially significant for dissolved Zn and Pb. The magnitude of these SGD-driven metal fluxes indicates that SGD is a relevant pathway in delivering metals to the coastal environment at El Gorguel Bay. This fact highlights the importance of investigating the role of SGD as a possible source of contamination to the sea in mining districts located in the proximity of the coast.

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.

Submarine groundwater discharge as a source of nutrients and trace metals in a Mediterranean bay (Palma Beach, Balearic Islands)

Submarine Groundwater Discharge (SGD) from a detrital coastal aquifer into the adjacent marine environment was investigated in a Mediterranean bay (Palma Beach, Balearic Islands). In this region, agriculture and tourism are potential sources of groundwater contamination. A survey in the Palma Beach revealed N, Fe and chlorophyll a enhancement associated to areas of preferential groundwater discharge from the nearby coastal aquifer. Groundwater sampling from wells and coastal piezometers indicated high concentrations of dissolved inorganic nitrogen and Fe (up to 2800μmolL−1 and 8100nmolL−1, respectively). Other nutrients, such as DIP, and trace elements were not particularly elevated, which is attributed to the adsorptive characteristics of the carbonated composition of this detrital aquifer and/or the lack of major sources. Cross-shore gradients of 223,224,226Ra isotopes indicated a diffusive shore-based source of these radionuclides and allowed estimates of a SGD flow of 56,000±13,000m3d−1. Our results show that SGD is a major pathway for delivering DIN (1900mmolm−1d−1), dissolved Fe (4.1mmolm−1d−1) and, to a lesser extent, DIP (16mmolm−1d−1) into the nearshore waters. This allochthonous input may sustain a substantial phytoplankton biomass resulting in an onshore–offshore gradient (4.7–7.1mgm−3 in nearshore seawater as compared with <1mgm−3 in offshore stations). This work emphasizes the relevance of SGD-driven nutrient and trace metal inputs in the regulation of nearshore phytoplankton communities of oligotrophic areas.

Influence of sea level rise on iron diagenesis in an east Florida subterranean estuary

Subterranean estuary occupies the transition zone between hypoxic fresh groundwater and oxic seawater, and between terrestrial and marine sediment deposits. Consequently, we hypothesize, in a subterranean estuary, biogeochemical reactions of Fe respond to submarine groundwater discharge (SGD) and sea level rise. Porewater and sediment samples were collected across a 30-m wide freshwater discharge zone of the Indian River Lagoon (Florida, USA) subterranean estuary, and at a site 250 m offshore. Porewater Fe concentrations range from 0.5 μM at the shoreline and 250 m offshore to about 286 μM at the freshwater–saltwater boundary. Sediment sulfur and porewater sulfide maxima occur in near-surface OC-rich black sediments of marine origin, and dissolved Fe maxima occur in underlying OC-poor orange sediments of terrestrial origin. Freshwater SGD flow rates decrease offshore from around 1 to 0.1 cm/day, while bioirrigation exchange deepens with distance from about 10 cm at the shoreline to about 40 cm at the freshwater–saltwater boundary. DOC concentrations increase from around 75 μM at the shoreline to as much as 700 μM at the freshwater–saltwater boundary as a result of labile marine carbon inputs from marine SGD. This labile DOC reduces Fe-oxides, which in conjunction with slow discharge of SGD at the boundary, allows dissolved Fe to accumulate. Upward advection of fresh SGD carries dissolved Fe from the Fe-oxide reduction zone to the sulfate reduction zone, where dissolved Fe precipitates as Fe-sulfides. Saturation models of Fe-sulfides indicate some fractions of these Fe-sulfides get dissolved near the sediment–water interface, where bioirrigation exchanges oxic surface water. The estimated dissolved Fe flux is approximately 0.84 μM Fe/day per meter of shoreline to lagoon surface waters. Accelerated sea level rise predictions are thus likely to increase the Fe flux to surface waters and local primary productivity, particularly along coastlines where groundwater discharges through sediments.

Estimation of submarine groundwater discharge from bulk ground electrical conductivity measurements

The utility of bulk ground conductivity (BGC) measurements in the estimation of submarine groundwater discharge (SGD) was investigated at four sites covering a range of hydrogeological settings, namely Cockburn Sound (Australia); Shelter Island (USA); Ubatuba Bay (Brazil) and Flic‐en‐Flac Bay (Mauritius). At each of the sites, BGC was surveyed in the intertidal zone, and seepage meters were used for direct measurements of SGD flow rates. In the presence of detectable salinity gradients in the sediment, a negative correlation between SGD and BGC was recorded. The correlation is site‐specific and is dependent on both the type of sediment and the mixing processes. For example, at Shelter Island the maximum mean flow rates were 65 cm d−1 at a BGC of ∼0 mS cm−1 while at Mauritius maximum mean flow rates were 364 cm d−1 at a BGC of ∼0 mS cm−1. BGC measurements are used to estimate SGD over a large scale, and to separate its fresh and saline components. Extrapolating BGC measurements throughout the study sites yields a total discharge of 2.91, 1.59, 7.16, and 25.4 103 m3 d−1 km−1 of shoreline with a freshwater fraction of 41, 24, 29, and 63% at Cockburn Sound, Shelter Island, Ubatuba Bay, and Flic‐en‐Flac Bay respectively. The results demonstrate that ground conductivity is a useful tracer to survey and separate freshwater and recirculated seawater component of SGD. The presented investigation is a subset within a series of experiments designed to compare different methods to investigate SGD co‐organized and carried out by SCOR, LOICZ, IOC and IAEA.