Fresh Submarine Groundwater Discharge Research Articles

Spatial and seasonal fluctuations in fresh submarine groundwater discharge revealed by marine continuous resistivity profiling

Submarine Groundwater Discharge (SGD) is a major pathway for the discharge of fresh and saline groundwater and associated dissolved compounds into marine environments. However, assessing SGD processes in coastal aquifers is challenging due to inaccessibility, dynamic conditions, complex subsurface geology, and the need for long-term monitoring to capture temporal and spatial variations in SGD rates accurately. This study employs marine continuous resistivity profiling (MCRP) as a main method to assess the presence of freshwater or brackish SGD offshore and to examine its potential seasonal variations. The method has been applied in the coastal alluvial aquifer of Maresme (Spain) and validated with other methods to trace SGD, including salinity profiles, Ra isotopes, and piezometric levels. Several MCRP transects of 700 m long, perpendicular to the coastline, were performed in a coastal marine area to obtain electrical resistivity data of the seabed covering an area of 3 km2. The data was acquired in two field campaigns with contrasting hydrological conditions (dry and wet seasons). The MCRP results allow the identification of areas of fresh SGD in marine sediments, with a clear seasonal variability that indicates a higher discharge of fresh groundwater in the wet season.

Potential Linkages Between Submarine Groundwater (Fresh and Saline) Nutrient Inputs and Eutrophication in a Coastal Aquaculture Bay

AbstractSubmarine groundwater discharge (SGD) plays a crucial role in nutrient budgets of coastal systems, encompassing both submarine fresh groundwater discharge (SFGD) and recirculated saline groundwater discharge (RSGD). Despite its significance, the specific importance of these components in mariculture bays has not been thoroughly assessed. Here, utilizing Ra isotopes and water‐salt mass balance model, we show that SFGD flux (1.1 ± 0.4 cm d−1) represented only 17% of the SGD in the Zhenzhu Bay, a typical mariculture bay along the South China Sea. Interestingly, the nutrient contribution from SFGD surpassed that from RSGD, accounting for 82% of the dissolved inorganic nitrogen (DIN) flux within the SGD. Analysis of the monthly satellite Chlorophyll‐a (Chl‐a) data confirmed that the decline in phytoplankton biomass can be linked to the limited dissolved silicate (DSi) transported by SFGD. Additionally, the elevated nitrogen to phosphorus ratio (241:1) and reduced silicon to nitrogen ratio (0.5:1) in SFGD compared to the Redfield ratio suggested that SFGD characterized by nitrogen excess and silica deficient, which likely played a role in transitioning from biogenic element constraints in coastal water. This shift may impact the proportions and functionality of the phytoplankton community, potentially mitigating water eutrophication. These findings underscore the significant influence of SGD on nutrient dynamics and the ecological environment in the Zhenzhu Bay.

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).

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 > 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.

Tracing the contributions of different nitrate sources associated with submarine groundwater discharge in coastal seawaters off Jeju Island, Korea

We measured the concentrations of dissolved inorganic nutrients and the dual isotopic composition of nitrate (δ15N-NO3− and δ18O-NO3−) in coastal waters off Jeju, a volcanic island in Korea, to trace its main sources. Sampling of seawater and fresh groundwater was conducted in four different coastal areas of Jeju Island: Haengwon (HW), Pyoseon (PS), Ilgwa (IG), and Sagye (SG) in May 2020 and 2021. The significant negative correlations between NO3− and salinity in the four areas indicate that the main source of NO3− is fresh submarine groundwater discharge (FSGD), with the extrapolated fresh groundwater endmember values ranging from 170 to 300 μM (δ15N-NO3−: 4.1–10.8 and δ18O-NO3−: 1.7–6.4). The actual sources of SGD-driven NO3− in these coastal waters were determined using a bi-plot diagram (δ15N-NO3− vs. δ18O-NO3−) and a Bayesian stable isotope mixing model (MixSIAR). The results showed that, besides the background contribution from open-ocean waters, the main sources of NO3− in HW were fertilizer (69 ± 5%) and manure and sewage (24 ± 7%) and those in PS, IG, and SG were manure and sewage (PS: 53 ± 11%, IG: 57 ± 12%, SG: 63 ± 13%) and fertilizer (PS: 27 ± 8%, IG: 24 ± 5%, SG: 22 ± 5%). Our extrapolation approach for NO3− dual isotopes provides a better way to evaluate the main sources of NO3− in coastal waters off volcanic islands where SGD-driven NO3− is significant, but its actual source groundwater cannot be located.

A comprehensive analysis of submarine groundwater discharge and nutrient fluxes in the Bohai Sea, China

Groundwater discharge and associated nutrient fluxes in the Bohai Sea, China has attracted great attention, but most studies lacked high spatial resolution for the whole sea. As the largest semi-enclosed sea in China, the Bohai Sea is confronted with strong environmental pollution problems such as eutrophication induced by terrestrial nutrient inputs. However, the role of SGD has not been evaluated well for the whole Bohai Sea. In this study, stable isotopes (hydrogen and oxygen), radioactive isotope (228Ra), salinity, and temperature were combined to trace the diluted seawater. Mass balances of 228Ra, oxygen isotope, and salinity were used to quantify SGD and nutrient fluxes to the Bohai Sea. The estimated submarine fresh groundwater discharge (SFGD) and SGD to the Bohai Sea were (6.0 ± 0.5) × 109 and (2.7 ± 1.6) × 1011 m3 a−1, respectively. SFGD represents 10 % to 11 % of the total river discharge and SGD is about 2 to 8 folds of the total river discharge to the sea. Moreover, SGD derived dissolved nutrients to the Bohai Sea were (4.8 ± 4.0) × 1010 mol a−1 for dissolved inorganic nitrogen, (1.9 ± 1.7) × 1010 mol a−1 for dissolved inorganic phosphorus, and (6.7 ± 5.5) × 1010 mol a−1 for silicon. These nutrient inputs were about 10 to 20 folds of the total riverine inputs. Overall, this study underscores the importance of evaluating SGD to better understand the terrestrial imported nutrients in regional scale.

Fresh and saline groundwater nutrient inputs and their impacts on the nutrient budgets in a human-effected bay

Submarine groundwater discharge (SGD) can be highly enriched in nutrients, especially in bays with strong human activity, but has often been overlooked in coastal nutrient budgets. This study investigated the impact of both fresh and saline SGD on nutrient budgets in Sanmen Bay, China, a region heavily influenced by human activities. Based on the 224Ra mass balance model, the total SGD flux was estimated to be (1.1 ± 0.1) × 108 m3 d−1 (13.9 ± 0.5 cm d−1). Additionally, a water-salt mass balance model revealed that fresh SGD flux accounted for ~9.0 % of the total SGD flux. The results highlight the significance of fresh SGD as a freshwater source, contributing to 35.9 % of the total dissolved inorganic phosphorus (DIP) flux via SGD. Considering all nutrient sources and sinks in the Sanmen Bay, SGD was identified as the primary source of nutrients in Sanmen Bay, contributing 53.9 % and 11.9 % of the total dissolved inorganic nitrogen (DIN) and DIP input, respectively. Furthermore, the discharge of industrial/domestic sewage and mariculture wastewater also posed a potential threat to nutrient levels in the bay. Thus, initiatives such as reasonable control of culture species and scale, strengthening wastewater discharge and SGD management are crucial for maintaining the ecological environment of the Sanmen Bay.

Effect of submarine groundwater discharge on nutrient distribution and eutrophication in Liaodong Bay, China

Driven by the anthropogenic activities associated with coastal settlements, eutrophication has become a global issue. Submarine groundwater discharge (SGD) is a significant continuous pathway for transporting nutrients from land to coastal waters, but its influence on eutrophication in Liaodong Bay (LDB) has received limited attention. In this study, radium isotopes and nutrient data from coastal waters were analyzed to evaluate the SGD flux and its implications for potential eutrophication in LDB. We found that the mean concentrations of dissolved inorganic nitrogen (DIN), phosphorous (DIP), and silicate (DSi) in groundwater were higher than those of seawater and river water. Based on 223Ra and 228Ra mass balance models, the SGD fluxes were estimated to be (0.53–2.03) × 109 m3/d, of which the fresh SGD accounted for 4 %–15 %. SGD is a vital invisible source of nutrients, contributing more than 79 % of the total inputs of DIN, DIP, and DSi into LDB. With high DIN/DIP ratios (average=85.8) and large nutrient inputs, SGD may significantly drive the phosphorus limitation and eutrophication in LDB. This study shows that SGD-derived nutrient fluxes should be considered in the assessment of water eutrophication for the formulation of future environmental management protocols in coastal systems.

Decreasing groundwater temperature relieves seawater intrusion in coastal aquifers

Fresh submarine groundwater discharge (FSGD), as an important natural and renewable water resource, occurs along much of the global coast but finally gets lost into the sea. Exploiting FSGD, however, could induce seawater intrusion in coastal aquifers. Here, we show that decreasing groundwater temperature (by extracting fresh groundwater from and recharging cooled and more viscous water back into coastal aquifers) can relieve seawater intrusion and lead to safe exploitation of FSGD, based on laboratory experiments and numerical simulations. The performance of groundwater cooling can be approximately quantified by an analytical solution which shows its dependence on the FSGD extraction ratio and temperature contrast. The seawater retreat ratio (ratio of the seawater retreat distance to the length of seawater intrusion prior to water extraction) can reach 47.4% with groundwater cooling and without fresh groundwater exploitation. Furthermore, without incurring further seawater intrusion, groundwater cooling can lead to 35.7% FSGD exploitation.

Footprint of fresh submarine groundwater discharge in the Belgian coastal zone: An overview study

The presence of fresh groundwater is not limited to land; it also extends offshore and discharges as submarine groundwater discharge (SGD). The freshwater component of SGD (fresh submarine groundwater discharge [FSGD]) can be detected using geophysical techniques that are sensitive to salinity such as resistivity measurements. However, these measurements are often limited to either the land or marine realm, neglecting the land-marine interface. In this study, we focus on this gap by combining onshore and offshore techniques to assess variability in the FSGD footprint near the Belgian coastline through electrical resistivity tomography and continuous resistivity profiling. The difficult working conditions of the highly dynamic North Sea make this offshore survey one of the first of its kind. The footprint varies from limited outflow on the upper beach (e.g., Wenduine) to discharge around and below the low water line (e.g., De Panne, Oostduinkerke, and Knokke-Heist) in the studied areas. The occurrence, footprint, and quantity of SGD seem to be controlled by the presence and size of dune formations that constitute freshwater resources along the shore. Heterogeneity can also play a determining factor in FSGD location.

Fresh submarine groundwater discharge offshore Wellington (New Zealand): hydroacoustic characteristics and its influence on seafloor geomorphology

Fresh submarine groundwater discharge (FSGD) influences the biogeochemistry of coastal areas and can be a proxy for potential untapped resources of offshore freshened groundwater (OFG). In most areas however, the onshore-offshore connection and the recharge characteristics of offshore aquifers are poorly constrained, making a potential exploitation of this resource challenging. Offshore Wellington (New Zealand), a well-defined onshore aquifer system extends beneath the harbour, where substantial amounts of freshwater seep out from the ocean floor. The aquifer system has been studied in detail and recently the first attempts worldwide have been made here to use the offshore groundwater as a future source of drinking water. However, the locations and extent of FSGD as well as its influence on seafloor morphology are still poorly understood. Exact localisation of FSGD sites is essential to sample and quantify discharging waters but remains challenging due to a lack of robust and appropriate measurement procedures. Novel sensing strategies, such as the influence of seeping groundwater on hydroacoustic water column reflectivity could greatly improve the identification of groundwater discharge locations worldwide. Therefore, we use a multidisciplinary dataset and evaluate different methodologies to map the spatial extent of FSGD sites and determine their geomorphologic expressions on the seafloor of Wellington Harbour. In this study, single and multibeam hydroacoustics and towfish (temperature, salinity and turbidity) transects were combined with remotely operated vehicle (ROV) dives and sediment cores to better characterise FSGD sites. We observed several hundred seafloor depressions (pockmarks) that we attribute to continuous seepage of gas and groundwater from the seafloor. Different pockmark morphologies indicate different fluid flow regimes and the persistent flow allows even small pockmarks to remain unchanged over time, while the geomorphologic expressions of anchor scours on the seafloor diminish in the same region. Enhanced hydroacoustic reflections in the water column within and above the pockmarks indicate suspended sediment particles, which are likely kept in suspension by discharging groundwater and density boundaries.

Fresh Groundwater Discharge as a Major Source of 90Sr into the Coastal Ocean.

The behavior and source of 90Sr in the coastal ocean remain uncertain. Here, we investigated the distributions of 90Sr in coastal fresh groundwater, river water, pore water, and seawater in three bays along the southeastern coast of China between 2019 and 2021 and evaluated the potential of submarine groundwater discharge (SGD) as a source of coastal 90Sr. The 90Sr activity in coastal fresh groundwater was higher than that in river water and seawater, while the 90Sr activity in pore water was comparable to that in adjacent seawater. In addition, nonconservative mixing behavior of 90Sr along the salinity gradient between river water and seawater was observed. These observations indicated that fresh SGD may serve as an additional source of 90Sr in coastal seawater. Combining our groundwater 90Sr data with the reported fresh SGD flux data, the estimated fresh SGD-derived 90Sr fluxes into the three bays were comparable to or even higher than those supplied by riverine sources. These results revealed that fresh SGD is a major but overlooked source of 90Sr in the coastal ocean. This subterranean pathway for transport of 90Sr to the coastal ocean should be considered in the monitoring and risk assessment of coastal areas, especially those near nuclear facilities.

Contribution of Fresh Submarine Groundwater Discharge to the Gulf of Alaska

AbstractHigh latitude mountain environments are experiencing disproportionately adverse effects from climate change. The Gulf of Alaska (GoA) region is an embodiment of this change, particularly concerning a shifting hydrologic balance. Even so, the magnitude and contribution of fresh submarine groundwater discharge (fresh SGD) remains virtually unexplored within the region, though it has gained increasing attention globally due to its chemical significance and influence on coastal ecosystems. Here we provide the first regional estimates of fresh SGD to the GoA using two established water balance approaches. This is an effective way to distinguish the contribution of terrestrially derived fresh SGD, rather than the more commonly quantified total SGD which includes discharge that is driven by marine forces such as sea‐level oscillations and density gradients. We compare the approaches and assess their capabilities in computing the magnitude of fresh SGD over a large regional scale. Mean annual fresh SGD flux ranges between 26.5 and 86.8 km3 yr−1 to the GoA, equivalent to 3.5%–11.4% of the total freshwater discharge. Contributions are highest in the Southeastern panhandle and lowest in the Cook Inlet basin, with the highest area normalized contribution occurring in the Prince William Sound. Fresh SGD exhibits high spatial and temporal variability throughout the region. Although freshwater discharge to the GoA is investigated considerably, the importance of fresh SGD has, thus far, been overlooked.

Spatiotemporal estimation of fresh submarine groundwater discharge across the coastal shorelines of Oahu Island, Hawaii

Abstract The integrated hydrological model is a powerful tool that is used to assess the temporal distribution of fresh groundwater discharge especially in coastal areas. The coastal regions of Hawaii are examples of crucial natural resources for the Hawaiian economy and general ecological health. To fully comprehend the intricate interactions between coastal hydrology processes and ecosystems, it is necessary to evaluate the fresh submarine groundwater discharge (FSGD) at the Heeia shoreline using an integrated hydrological modeling technique. Under steady-state settings, the results showed that the present daily average of FSGD is around 0.43 m3/days across 1 m of the shoreline. However, we showed that the FSGD values were considerably impacted by climate change, groundwater head of the coastal aquifer, recharge rate, and sea level rise, particularly by the end of the 21st century. The post-development FSGD fluxes were 1.5–3.5 times greater than the freshwater transported by the Heeia stream, demonstrating the considerable contribution of the FSGD to the coastal zones of Heeia. The results also showed an exponential association between the FSGD and the groundwater level for the coastal unconfined aquifer.

Large submarine groundwater discharges to the Arabian Sea from tropical southwestern Indian Coast: Measurements from seepage meters deployed during the low tide

Submarine Groundwater Discharge (SGD) is the flow of fresh groundwater and recirculated seawater to the sea. SGD plays a crucial role in the transport of solute-rich terrestrial groundwater as well as recirculated seawater. We conducted seasonal investigations for two years on a tropical, high-rainfall occurring coastline in southwestern India to quantify the SGD from the nearshore. A combination of subsurface seepage meters and porewater samplers was used for an entire tidal cycle to estimate the seepage rates and to understand the processes controlling the discharge. The estimated seepage rates from this region are one of the highest reported in the world. This could be due to the large span of the highly porous coastal aquifer and the high annual rainfall (over 450 cm) in this region. The seepage rates are 754 cm/day, 572 cm/day and 296 cm/day for the pre-monsoon (February 2020), post-monsoon (December 2020) and pre-monsoon (February 2021) seasons, respectively and showed high spatial and temporal variability. The end-member concentrations of groundwater and seawater were used to delineate the fresh and recirculated SGD from the total seepage. The recirculated SGD (rSGD) dominates during all the seasons taking up to 99 % of the total SGD during the pre-monsoon seasons (February) and 70 % during the post-monsoon (December) season. The fresh SGD (fSGD) is low (∼1 %) during the pre-monsoon season and increases up to 30 % during the post-monsoon. A considerable amount (0.1–10 % of the total SGD) of salinity enrichment was observed at the upper saline plume. We suspect this could be due to the evaporation of recirculated seawater due to ambient weather conditions. The fSGD in the study area is mainly controlled by the high inland hydraulic head, and the rSGD is regulated by the tides. The subsurface seepage meters used in this study eliminate the instabilities in seepage measurements and can be replicated in less-studied tropical coastal zones to quantify the volume of SGD entering the world oceans. The inferences reported can benefit the public and decision-makers in managing coastal groundwater resources and are interesting to the researchers working on delineating the hydrological and geochemical processes in the nearshore.

Evaluation of submarine groundwater discharge and associated beach groundwater dynamics

ABSTRACT This paper reports the dynamism of a submarine groundwater discharge (SGD) zone in SW India. Two electrical resistivity tomography (ERT) surveys were conducted in the project area to delineate the seawater-fresh water interface and characterize the subsurface aquifer layers. Beach groundwater dynamics were also studied by continuously monitoring salinity, temperature, and nutrient concentration in the groundwater from three shallow dug wells on the beach. The ERT surveys were planned in such a way as to fall between these dug wells. The flow domain was modeled using a numerical modeling approach, and the rate of fresh SGD flux and its spatial variation along the coastline has been estimated. The study proves that the numerical modeling approach combined with other field measurements and observations, such as ERT and continuous monitoring gives a comprehensive understanding of SGD and associated processes.

Fresh and saline submarine groundwater discharge as sources of carbon and nutrients to the Japan Sea

Submarine groundwater discharge (SGD) is an important pathway for carbon and nutrients to the coastal ocean, sometimes exceeding river inputs. SGD fluxes can have implications for long-term carbon storage, ocean acidification and nutrient dynamics. Here, we used radium (223Ra and 226Ra) isotopes to quantify SGD-derived fluxes of dissolved inorganic (DIC) and organic (DOC) carbon, nitrate (NO3−), nitrite (NO2−), ammonium (NH4+) and phosphate (PO43−) in a spring-fed coastal bay in the Japan Sea. The average coastal water residence times using 223Ra/226Ra ratios was 32.5 ± 17.9 days. Fresh and saline SGD were estimated using a radium mixing model with short- and long-lived isotopes. The volume of fresh SGD entering the bay (4.6 ± 4.6 cm day−1) was more than twice that of the volume of saline SGD (1.9 ± 2.1 cm day−1). Fresh SGD (mmol m2 day−1) was the main source of DOC (2.7 ± 2.6), DIC (13.9 ± 13.7), PO43− (0.3 ± 0.3) and NO3− (6.6 ± 6.5) to the coastal ocean, whereas saline SGD was the main source of NH4+ (0.2 ± 0.2). Total SGD-derived carbon and nutrient fluxes were 4 – 7 and 2–16 times greater than local river inputs. Positive correlations between chlorophyll-a, 226Ra and δ13C-DIC indicate that SGD significantly (p < 0.05) enhances primary productivity nearshore. Overall, fresh SGD of nitrogen and carbon to seawater drove chlorophyll-a, decreased DIC/Alkalinity ratios, and modified the carbonate biogeochemistry of the coastal ocean.

Submarine groundwater discharge in response to the construction of subsurface physical barriers in coastal aquifers

Submarine groundwater discharge (SGD) is an important component of the local and regional hydrologic cycles, affecting coastal water quality and ecology. However, the influence of subsurface physical barriers widely built in coastal aquifers for seawater intrusion control has not yet been fully understood with respect to the downstream SGD. In this study, we used both laboratory experiments and numerical simulations to quantify the contribution of subsurface physical barriers to the fresh and saline SGD fluxes. Different types of physical barriers, including cutoff wall and subsurface dam, were examined with various structures (e.g., barrier location, height, or penetration depth). The results demonstrated that subsurface physical barriers with different structures resulted in various changes in the location of the freshwater discharge at the aquifer-sea interface, e.g., the discharge location shifted seaward as the cutoff wall moved inland. The water exchange across the aquifer-sea interface was closely related to the structure of subsurface physical barriers. With the increase in the height of subsurface dam or the increase in the depth of cutoff wall, the fresh and saline SGD fluxes revealed a decreasing trend. Generally, the presence of a physical barrier hindered the hydraulic connection between the upstream and downstream aquifers, and the SGD fluxes were reduced compared to those without any barrier. For cutoff walls, the change in wall depths led to decreases in tide-induced saltwater circulation of 25%–33%, density-driven saltwater circulation of 10%–30%, fresh groundwater discharge of 17%–60%, and the total efflux of 18%–36%. Within the effective height range of the subsurface dam, fresh and saline SGD fluxes peaked at the minimum effective height. These findings provide a general framework for the prediction and monitoring of SGD behavior in coastal aquifers where subsurface physical barriers are intended or have been constructed.

Analytical approach using a chemical equilibrium formula and geochemical modeling for alkalinity measurements of small natural water samples

Alkalinity measurements have used spectrophotometric methods to measure concentrations from small samples, especially for HCO3− concentrations in terrestrial water. Nevertheless, the methods still pose challenges such as experimental and calculation complexity, along with the necessity of considering atmospheric CO2 when preparing standard solutions. This study improved theoretical aspects of the spectrophotometric method based on chemical equilibrium equations combined with PHREEQC, yielding a new approach to alkalinity measurement. The differences between the experimentally obtained HCO3− concentration and the calculated value based on the updated chemical equilibrium equation were 0.038–5.4 × 10−6 mg/L. PHREEQC quantified the leaching of atmospheric CO2 into the samples during experimentation. The effect was negligible at 0.01–0.02 mg/L HCO3−. The effect of increased atmospheric CO2 because of human respiration in the laboratory was estimated as 0.05 mg/L HCO3−. These results suggest that the laboratory's experimental and computational steps for atmospheric CO2 correction can be omitted when natural terrestrial water is targeted. We applied our new approach to various terrestrially derived natural waters (limestone cave drip water, hot spring water, fresh submarine groundwater discharge, groundwater, and river water) for simulations at different titration endpoints (pH 4.8 and 4.3). Differences between the values obtained by hydrochloric acid titration of HCO3− concentrations and those calculated using PHREEQC were smaller: less than 1%–7% different concentration for most samples. This study established a method for analyzing small samples of various natural terrestrial waters.