Submarine Groundwater Discharge Research Articles

Evidence for subsea permafrost in subarctic Canada linked to submarine groundwater discharge

Evidence for subsea permafrost in subarctic Canada linked to submarine groundwater discharge

Effects of groundwater pumping on pore water flow and salt transport in tide‐controlled unconfined coastal aquifers

AbstractUnconfined coastal aquifers are a main pathway for land‐sourced solutes to enter the oceans. The migration of these solutes in aquifers is highly affected by the groundwater flow and salt transport processes, which are, to a great extent, controlled by tides. While many studies have examined how tidal oscillations would influence the subsurface hydrodynamics in coastal aquifers, most of them ignored the potential impact of groundwater pumping, a common practice in coastal areas to satisfy the demand for freshwater. This study, by means of laboratory experiments and numerical simulations, explored the combined effects of tides and groundwater pumping on the pore water flow and salinity distributions in an unconfined coastal aquifer. The results show that, in a tide‐controlled aquifer, the addition of groundwater pumping would exacerbate the degree of seawater intrusion and lead to wider spreading and deeper penetration of the upper saline plume. Moreover, groundwater pumping would enhance the tide‐driven circulation in the upper saline plume and weaken the density‐driven circulation in the saltwater wedge, ultimately leading to the reduction in total submarine groundwater discharge. These findings may promote a deep insight into the complex coastal groundwater systems experiencing human activities, and provide guidance for better evaluating the environmental impact of groundwater pumping.

Proxies of hypoxia and submarine groundwater discharge in the coastal ocean: Foraminiferal shell chemical perspectives

Coastal marine settings are important in terms of geochemical cycles and biological productivity. Climate change is predicted to affect coastal environment via hypoxia and Submarine Groundwater Discharge (SGD). Appropriate proxies could help to better understand oxygenation history and the role of SGD in regulating hypoxia. This would also benefit prediction of potential outcomes of future environmental changes. The sensitivity of benthic foraminiferal shell chemistry to environmental conditions opens the possibility to use them as proxies of coastal hypoxia and SGD. We report here that the average Mn/Ca ratios in tests of living benthic foraminiferal shells in the Changjiang River Estuary (CJE) is 2.3 times higher during hypoxia periods than under well-mixed conditions. In addition, Ba/Ca ratios in living benthic foraminiferal shells co-varied well with radon-inferred SGD signals. Fluctuations of Mn/Ca and Ba/Ca ratios in tests of a single foraminiferal shell along successive chambers corresponds well with seasonal-scale variations of hypoxia and SGD. We suggest that Mn/Ca and Ba/Ca ratios within intra-tests of benthic foraminifer can provide a reliable proxy for past hypoxia and SGD trends.

Unveiling the dominance of submarine groundwater discharge on nutrient sources in the Eastern China Marginal Seas

River and atmosphere are traditionally recognized as the primary nutrient sources impacting coastal ecosystems. Despite the increasing attention towards the often-neglected submarine groundwater discharge (SGD), its understanding and significance in highly human-impacted marginal seas remain limited. This study utilizes unprecedented high-resolution data (561 seawater and 282 groundwater radium samples) to provide precise estimates of 226Ra and 228Ra sources and sinks in the Eastern China Marginal Seas. A coupled 226Ra and 228Ra mass balance model enable an integrated SGD flux of (3.7 ± 2.4) × 1012 m3 yr−1, surpassing rivers by 3.4 times. Furthermore, nutrient delivery from SGD exceeds riverine and atmospheric inputs, potentially inducing substantial changes in coastal nutrient cycles. These alterations have profound implications for primary production and biological communities, deviating significantly from the Redfield ratio. Therefore, comprehending the significance of SGD in nutrient budgets is vital for a comprehensive understanding of biogeochemical dynamics and functionality of marginal sea ecosystems.

A unique warm–water oasis in the Siberian Arctic’s Chaun Bay sustained by hydrothermal groundwater discharge

Chaun Bay, located on the fringe of the East Siberian Sea, has been described since the mid-20th century to support a unique marine ecosystem that is atypical for the local Siberian Arctic. Here we use ship-board physical, biogeochemical and geological measurements taken in October 2020, along with hydrographic observations taken from land-fast ice in April 2023, to demonstrate that these warm-water biological communities are supported by hydrothermal submarine groundwater discharge that delivers heat, salinity, nutrients, and trace elements to the bay. We identify a cyclonic eddy that mixes the warm nutrient-rich groundwater with oxygen-rich surface water, resulting in a water mass within Chaun Bay that has similar physical and chemical properties to the highly productive waters of the North Pacific and Southern Chukchi Sea. The bay showed elevated concentrations of chlorophyll-a and zooplankton, and the abundance and species diversity of epibenthos significantly exceeded values observed elsewhere in the East Siberian Sea. The benthic communities contained a number of boreal species that are not typically found in the Arctic Ocean. We also observed Thysanoessa krill populations, a pelagic species generally considered an expatriate in Arctic waters.

TECTONIC AND NON-TECTONIC MESO-STRUCTURES IN THE LISAN FORMATION AND EQUIVALENT RECENT SEDIMENTS OF THE JORDAN RIFT VALLEY-CHARACTERIZATION AND FORMATION MECHANISMS

In this study, the structural elements portrayed in the recent sediments of the Dead Sea, which became exposed at its shores due to the drop in its level are described, and the mechanisms leading to their formation are elaborated. These structures include Plumes and density inversions, subaquatic gliding, seismites, undulations and wavy structures, flexures, and overpressure structures. The formation of these structures is found to be the result of different triggering factors such as Earthquakes, tectonic activity, instability of slopes, density inversion, pore fluids overpressure resulting from gas production by biochemical processes within the sediments (sulfur bacteria reduction of sulfates and oxidation of organic matter (petroleum residues)), submarine groundwater discharge, and volume changes associated with the transformation of gypsum to basinite and anhydrite, and the reversal of that transformation. Each of these triggering factors can produce more than one type of structure, depending on the prevailing composition of the sediments, its layering attitudes, grain-size distribution, porosities, and differential pressure situations within the sediment packages. The main finding of the study is the clarification of the role of gas production within the sediments as a result of sulfur bacteria activity and of the volume change processes associated with the transformation of gypsum to anhydrite and the reversal of that transformation in the formation of the mesostructures of the Dead Sea sediments.

Physical and chemical characterization of remote coastal aquifers and submarine groundwater discharge from a glacierized watershed

AbstractCoastal aquifers play an important role in marine ecosystems by providing high fluxes of nutrients and solutes via submarine groundwater discharge pathways. The physical and chemical characterization of these dynamic systems is foundational to understanding the extent and magnitude of hydrogeologic processes and their subsequent contributions to the marine environment. We describe a km‐scale experimental field site located in a glaciofluvial delta entering Kachemak Bay, Alaska. Our characterization applies geophysical (ERT and HVSR), hydrogeologic (grain size analyses, slug tests and tidal response analyses) and geochemical (major ions and stable water isotopes) methods to describe the complexity of coastal aquifers in proglacial environments currently experiencing rapid transformations. The hydrogeologic and geophysical techniques revealed thick (20–84 m) sediments dominated by sands and gravels and delineated zones of freshwater, brackish water and saltwater at both high and low tides within the subterranean estuary. Estimates of hydraulic conductivities via multiple approaches ranged from 2 to 250 m d−1, with means across the four methods within the same order of magnitude. Tidal response analyses highlighted a coastal aquifer in strong connection with the sea as evidenced by clear spring‐ and neap‐tidal signals within a proximal piezometric hydrograph. Geochemical sampling revealed coastal groundwaters as substantially enriched in solutes compared to proximal river samples with limited variability across seasons. A clear connection between the Wosnesenski River and the adjacent aquifer was also observed, with concentrated recharge from the river corridor during the meltwater season. This combination of approaches provides the basis for a conceptual model for coastal aquifer systems within the Gulf of Alaska and an upscaled mean daily yield of freshwater and solutes from the delta subsurface. Our findings are critical for subsequent numerical simulations of groundwater flow, tidal pumping and chemical reactions and transport in these understudied environments. This approach may be applied for low‐cost, large‐scale hydrogeologic investigations in coastal areas and may be particularly useful for remote sites where access and mobility are challenging.

Conceptualizing Controlling Factors for PFAS Salting Out in Groundwater Discharge Zones Along Sandy Beaches.

Understanding fate and transport processes for per- and poly-fluoroalkyl substances (PFAS) is critical for managing impacted sites. "PFAS Salting Out" in groundwater, defined herein, is an understudied process where PFAS in fresh groundwater mixes with saline groundwater near marine shorelines, which increases sorption of PFAS to aquifer solids. While sorption reduces PFAS mass discharge to marine surface water, the fraction that sorbs to beach sediments may be mobilized under future salinity changes. The objective of this study was to conceptually explore the potential for PFAS Salting Out in sandy beach environments and to perform a preliminary broad-scale characterization of sandy shoreline areas in the continental U.S. While no site-specific PFAS data were collected, our conceptual approach involved developing a multivariate regression model that assessed how tidal amplitude and freshwater submarine groundwater discharge affect the mixing of fresh and saline groundwater in sandy coastal aquifers. We then applied this model to 143 U.S. shoreline areas with sandy beaches (21% of total beaches in the USA), indirectly mapping potential salinity increases in shallow freshwater PFAS plumes as low (<10 ppt), medium (10-20 ppt), or high (>20 ppt) along groundwater flow paths before reaching the ocean. Higher potential salinity increases were observed in West Coast bays and the North Atlantic coastline, due to the combination of moderate to large tides and large fresh groundwater discharge rates, while lower increases occurred along the Gulf of Mexico and the southern Florida Atlantic coast. The salinity increases were used to estimate potential perfluorooctane sulfonic acid (PFOS) sorption in groundwater due to salting out processes. Low-category shorelines may see a 1- to 2.5-fold increase in sorption of PFOS, medium-category a 2.0- to 6.4-fold increase, and high-category a 3.8- to 25-fold increase in PFOS sorption. The analysis presented provides a first critical step in developing a large-scale approach to classify the PFAS Salting Out potential along shorelines and the limitations of the approach adopted highlights important areas for further research.

Unravelling the signatures of submarine groundwater discharge and seawater intrusion along the coastal plains of Odisha, India: a multi-proxy approach.

Submarine Groundwater Discharge (SGD) and Seawater Intrusion (SWI) are two contrary hydrological processes that occur across the land-sea continuum and understanding their nature is essential for management and development of coastal groundwater resource. Present study has attempted to demarcate probable zones of SGD and SWI along highly populated Odisha coastal plains which is water stressed due to indiscriminate-exploitation of groundwater leading to salinization and fresh groundwater loss from the alluvial aquifers. A multi-proxy investigation approach including decadal groundwater leveldynamics, LANDSAT derived sea surface temperature (SST) anomalies and in-situ physicochemical analysis (pH, EC, TDS, salinity and temperature) of porewater, groundwater and seawater were used to locate the SGD and SWI sites. A total of 340 samples for four seasons (85 samples i.e., 30 porewater, 30 seawater and 25 groundwater in each season) were collected and their in-situ parameters were measured at every 1-2km gap along ~ 145km coastline of central Odisha (excluding the estuarine region). Considering high groundwater EC values (> 3000 μS/cm), three probable SWI and low porewater salinities (< 32 ppt in pre- and < 25 ppt in post-monsoons), four probable SGD zones were identified. The identified zones were validated with observed high positive hydraulic gradient (> 10m) at SGD and negative hydraulic gradient (< 0m) at SWI sites along with anomalous SST (colder in pre- and warmer in post-monsoon) near probable SGD locations. This study is first of its kind along the Odisha coast and may act as initial basis for subsequent investigations on fresh-saline interaction along the coastal plains where environmental integrity supports the livelihood of coastal communities and the ecosystem.

Carbon export from submarine groundwater discharge in a semi-enclosed bay: Impact for the buffering capacity against coastal ocean acidification

Submarine groundwater discharge (SGD) serves as an important pathway for the transport of dissolved carbon from land to ocean, significantly affecting the coastal biogeochemical cycles. However, the impact of SGD-derived dissolved carbon on the coastal carbon budget remains poorly understood. This study first quantified SGD and associated dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total alkalinity (TA) fluxes in Daya Bay using mass balance models based on radium isotopes (223Ra, 224Ra, 226Ra and 228Ra). We then constructed carbon mass balance models to evaluate the impact of SGD-derived carbon on the buffering capacity against coastal ocean acidification. The estimated SGD fluxes ranged from 0.80 × 107 to 2.64 × 107 m3d−1. The DIC, DOC and TA fluxes from SGD were 17.90–36.44 mmol m−2d−1, 0.93–2.13 mmol m−2d−1, and 21.19–28.47 mmol m−2d−1, respectively. Based on carbon mass balances, the DIC flux from SGD was 19–39 times the riverine input, accounting for 27.16 % ∼ 37.64 % of the total carbon source. These results suggest that SGD is a major contributor to DIC, significantly affecting the coastal carbon budget. Furthermore, the average TA:DIC ratio of groundwater discharging into Daya Bay was approximately 1.13. High TA exports enhance the buffering capacity of the coastal ocean and contribute bicarbonate to the ocean, playing a significant role in the ocean carbon sequestration process. This study demonstrates the importance of SGD-derived dissolved carbon in the assessment of coastal carbon budgets.

Deciphering Multi‐Scale Submarine Groundwater Discharge in a Typical Eutrophic Bay

AbstractAs a significant source of nutrients and other components into the coastal ocean, submarine groundwater discharge (SGD) is a combination of multiple spatial‐temporal scale processes, including fresh terrestrial groundwater (FSGD), recirculating seawater (RSGD), and porewater exchange (PEX). Quantifying the different types of SGD is extremely important for understanding the coastal biogeochemical cycles of various materials. The green tide bloom in the southern Yellow Sea (China) has attracted increasing attention during the past decade. Haizhou Bay is thought to be an important green tide proliferating area, but the source of an apparent high supply of nutrients has not been identified yet. We report here on our investigations of the distribution patterns of Ra and Rn isotopes in groundwater and seawater in Haizhou Bay. By solving a combined mass balance for 224Ra, 223Ra, 226Ra, and 222Rn, we estimated that the bay's water residence time is 28.8 days, FSGD is highest at 3.6 cm d−1, RSGD is 2.7 cm d−1, and PEX is lowest at 0.6 cm d−1. The total SGD into Haizhou Bay is estimated at 9.40 × 107 m3 d−1 (6.9 cm d−1), about 12 times that of the local river discharge into the bay. SGD‐derived nutrients are shown to play an important role when considered among all known nutrient sources. Our results suggest that dissolved inorganic nitrogen (DIN) and silicate (DSi) are transported by SGD mainly via FSGD. We also note that the phosphorus (DIP) budget is heavily influenced by RSGD.

Nitrification in a Subterranean Estuary: An Ex Situ and In Situ Method Comparison Determines Nitrate Is Available for Discharge

AbstractSubterranean estuaries (STEs) form in the subsurface where fresh groundwater and seawater meet and mix. Subterranean estuaries support a variety of biogeochemical processes including those transforming nitrogen (N). Groundwater is often enriched with dissolved inorganic nitrogen (DIN), and transformations in the STE determine the fate of that DIN, which may be discharged to coastal waters. Nitrification oxidizes ammonium (NH4+) to nitrate, making DIN available for N removal via denitrification. We measured nitrification at an STE, in Virginia, USA using in situ and ex situ methods including conservative mixing models informed by in situ geochemical profiles, an in situ experiment with 15NH4+ tracer injection, and ex situ sediment slurry incubations with 15NH4+ tracer addition. All methods indicated nitrification in the STE, but the ex situ sediment slurries revealed higher rates than both the in situ tracr experiment and mixing model estimations. Nitrification rates ranged 55.0–183.16 μmol N m−2 d−1 based on mixing models, 94.2–225 μmol N m−2 d−1 in the in situ tracer experiment, and 36.6–109 μmol N m−2 d−1 slurry incubations. The in situ tracer experiment revealed higher rates and spatial variation not captured by the other methods. The geochemical complexity of the STE makes it difficult to replicate in situ conditions with incubations and calculations based on chemical profiles integrate over longer timescales, therefore, in situ approaches may best quantify transformation rates. Our data suggest that STE nitrification produces NO3−, altering the DIN pool discharged to overlying water via submarine groundwater discharge.

Salinity Profiles in Coastal Aquifers: A Characterization Framework for Field Measurements

AbstractThe freshwater‐seawater mixing zone (“interface”) is affected by tides, seawater intrusion, submarine groundwater discharge and aquifer heterogeneities. Our current understanding is based primarily on theoretical studies involving numerical simulation and sand‐tank experimentation, with limited field analyses of interface characteristics in coastal aquifers that serve as important freshwater reservoirs. The most common field evidence for interface characteristics is derived from electrical conductivity (EC) profiles, often obtained from long‐screen wells. However, the EC profiles from long‐screen wells have been assessed previously using only simple statistics like the depth to 50% of seawater concentration. This study introduces a methodology for characterizing EC profiles using curve fitting and objective assessment of multiple interface attributes, with application to 170 EC profiles from the Lower Burdekin Delta, Australia. The adopted fitting function showed an excellent match (mean R2 = 0.99). Fitting parameters were linked to the curvatures of transitions from fresh to brackish water and from brackish water to seawater, the elevation and gradient of the mid‐salinity value, and the thickness of the mixing zone. Field EC profiles showed approximately linear relationships between head changes averaged over the previous 5 months and the elevation of 5 mS/cm isochlor for some wells. The slope of this relationship was approximately one, which is approximately 1/40th of the value arising from the Ghyben‐Herzberg relation. The framework for characterizing salinity profiles presented in this article offers a new, objective approach for investigating the interface, providing future studies with a technique for examining relationships between interface behavior and key controlling forces.

Temporal and spatial characteristics of radium isotope in phreatic water in the Pearl River Estuary and their influence on the calculation of submarine groundwater discharge fluxes

Temporal and spatial characteristics of radium isotope in phreatic water in the Pearl River Estuary and their influence on the calculation of submarine groundwater discharge fluxes

The Influence of Fresh Submarine Groundwater Discharge on Seawater Acidification Along the Northern Mediterranean Coast of Israel

AbstractIn the oligotrophic Southeastern Mediterranean Sea (SEMS), it has been shown that dissolved inorganic nutrient (DIN) from fresh submarine groundwater discharge (FSGD) enhance primary production in coastal waters. In this study pH, Total Alkalinity (TA) and DIN of seawater and fresh water in a sea‐cave in the northern part of the Israeli Mediterranean coast and a nearby contact spring, respectively, were measured during October 2018–March 2020. The results show gradients of measured salinity, TA, pH and DIN along the cave axis year‐round, suggesting that they are influenced by FSGD. The seawater near the back of the cave was supersaturated with respect to atmospheric CO2 nearly year‐round and there is a strong positive divergence from its regional open‐water thermal dependence, which suggests that FSGD is also a source of atmospheric CO2 in this region. Comparison of TA, salinity and pCO2 from the back of the sea cave to their corresponding values from an abrasion platform monitoring site, ca. 3 km south of the cave, suggests that FSGD is occurring along the entire shoreline in this region. Thus, despite the increased productivity due to FSGD mediated nutrient enrichment of adjacent coastal waters of the oligotrophic SEMS, they are still a source of atmospheric CO2 nearly year‐round. Finally, the apparent trends of seawater acidification (ΔpH/Δt = −0.006 yrs−1) and pCO2 increase (+8 ppmV yr−1) observed at the nearby monitoring site since 2013 are explained by increased groundwater recharge and resulting FSGD total alkalinity compared to dissolved inorganic carbon inputs (ΔTA/ΔDIC = 1:1.2).

Hydrographic proxies for submarine groundwater discharge in the Jiulong River estuary and global perspectives

Submarine groundwater discharge (SGD) significantly impacts most coastal waters. However, its quantification, depending on chemical tracers/proxies, limits its parameterization in numerical models. This study explored the hydrographic proxies of SGD in the Jiulong River estuary (JRE) using 226Ra and 228Ra as SGD tracers. Our results showed significant monthly fluctuations in the flux of SGD, with a peak in June and a minimum in April. On average, the flux of SGD was equivalent to 10 ± 1.67 % of the concurrent river discharge, with the area-normalized rate of 0.007 ± 0.017 to 0.13 ± 0.04 m/day. Positive SGD response to river discharge implies a connection with the surface runoff of the shallow aquifers. Furthermore, the flux of SGD presented a significant negative correlation with the return flow factor and flushing time of the estuary. The radium activities in the estuary were positively correlated with water depth, indicating that SGD was not driven by tidal pumping. Instead, physical mixing in low to middle salinity regions predominated such behavior of radium. Our results indicate that river discharge, flushing time and return flow factor may serve as hydrographic proxies of SGD in the JRE and potentially be applicable in parameterization of SGD in numerical models in similar coastal ecosystems. Globally, a positive correlation between SGD flux and river discharge emphasizes the latter as a general proxy in estuaries.

Correspondence on "Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef".

Correspondence on "Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef".

Rebuttal to Correspondence on "Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef".

Rebuttal to Correspondence on "Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef".

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

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

Global subterranean estuaries modify groundwater nutrient loading to the ocean

AbstractTerrestrial groundwater travels through subterranean estuaries before reaching the sea. Groundwater‐derived nutrients drive coastal water quality, primary production, and eutrophication. We determined how dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved organic nitrogen (DON) are transformed within subterranean estuaries and estimated submarine groundwater discharge (SGD) nutrient loads compiling &gt; 10,000 groundwater samples from 216 sites worldwide. Nutrients exhibited complex, nonconservative behavior in subterranean estuaries. Fresh groundwater DIN and DIP are usually produced, and DON is consumed during transport. Median total SGD (saline and fresh) fluxes globally were 5.4, 2.6, and 0.18 Tmol yr−1 for DIN, DON, and DIP, respectively. Despite large natural variability, total SGD fluxes likely exceed global riverine nutrient export. Fresh SGD is a small source of new nutrients, but saline SGD is an important source of mostly recycled nutrients. Nutrients exported via SGD via subterranean estuaries are critical to coastal biogeochemistry and a significant nutrient source to the oceans.