Saltwater Circulation Driven by Shoreline Curvature in Coastal Aquifers

<p>Submarine groundwater discharge (SGD) acts as a source of fresh water and dissolved substances for coastal ecosystems. Evaluation of the actual controls on SGD and corresponding chemical fluxes require a closer understanding of the processes that take place in the mixing zone between SGD and the coastal waters. It is hypothesized that artificial infrastructures, like sediment channeling, may ease the hydrological connection between coastal aquifer and coastal bottom water. The resultant, increase of SGD, changes the residence time in the mixing zone, and thereby, reduces the impact of early diagenesis. The present study focuses on the distribution of SGD, including the characterization of different mixing zones in the urbanized Wismar Bay (WB), southern Baltic Sea. Short sediment cores were retrieved for geochemical porewaters and sediment analyses. Surface sea water samples were collected along across-shore transects in the WB.  Besides major ions, Ba, Fe, and Mn, the water samples were analyzed for nutrients, dissolved inorganic carbon (DIC), stable isotopes (H, O, C, S), and Ra isotopes. Sediments were analyzed for C, N, S, Hg contents as well as reactive components (e.g. Fe, Mn, P) by HCl extractions. Organic matter mineralization rates, DIC, and SO<sub>4</sub> fluxes for the sediment-water interface were modeled from porewater profiles. Shallow seismic techniques were applied to identify potential litho-morphological controls on SGD. Geochemical porewater data allow identification of active SGD sites in the WB. In the central part, the freshening of porewaters in the top surface sediments indicates the upward flow of SGD originating from a coastal aquifer. The acoustic profiles show that the bottom sediments in the central bay are under local impact of excavation, reducing the sediment thickness above the coastal aquifer. Overall, the impact of SGD on the coastal water body of the WB is diffuse and promoted by local anthropogenic activity. The water isotope composition of porewaters at this site are close to the local meteoric water line at Warnemünde (located 50 km east of the WB), suggesting a discharge of relatively modern fresh waters. The (isotope) hydrochemical composition of the fresh water discharging is controlled by water-rock interactions in the aquifer and modulated by intense diagenesis in the brackish surface sediments. Furthermore, the SGD facilitates the upward migration of elements and enhances their fluxes across the sediment-water interface, e.g. DIC concentrations in the fresh groundwater are further enhanced in the mixing zone, indicating that SGD is a potential source of excess CO<sub>2</sub> in the investigated coastal waters.</p><p>The investigations are supported by the DAAD, DFG RTS Baltic TRANSCOAST, KiSnet project, BONUS SEAMOUNT, FP7 EU Marie Curie career integration grant, DAM-MFG, and IOW.</p>

Mercury loads from Submarine Groundwater Discharge (SGD) may represent an overlooked source of methylmercury (MeHg) to the ocean, affecting human and ecosystem health. The SGD process involves the flow of fresh, saline, or mixed groundwater from coastal aquifers into the oceans classified in different components. Existing studies rarely report the fluxes supplied by the different SGD components; therefore, the relevance of SGD as a source of mercury remains unclear. We aimed to quantify SGD-driven mercury/methylmercury fluxes to the coast, focusing on the largest coastal lagoon in the western Mediterranean. We measured total dissolved mercury and MeHg in surficial and porewaters during the summer and autumn. Porewaters were enriched in total dissolved Hg and MeHg compared to the lagoon. Lagoon shore waters and porewaters with a high concentration of labile dissolved organic matter were prone to MeHg formation and thus had higher MeHg concentrations. The mercury input through SGD to the Mar Menor lagoon (4300 mmol year-1) was similar to atmospheric deposition and 1 order of magnitude greater than the stream input. Among different components of the SGD, long-scale lagoon water recirculation dominated. These findings have substantial implications for the regional Hg budget and raise awareness of the importance of considering the different SGD components for an accurate estimation of SGD-based Hg input to the coastal ocean.

Submarine Groundwater Discharge (SGD) is recognized as a major source of water and solutes to the coastal ocean, and it is particularly relevant in arid or semi-arid zones. SGD is generally defined as the flow of groundwater from continental margins to the coastal ocean, including thus both fresh groundwater from aquifer recharge and seawater recirculation through the coastal aquifer. Due to its high heterogeneity both in space and time, SGD is difficult to detect and quantify. As a consequence, numerous methods to study SGD have been developed over the last decades. These approaches mainly include hydrogeological approaches, geophysical techniques, direct seepage measurements, and the use of geochemical tracers. Each method presents its challenges, limitations, and advantages and each one works on different spatial and temporal scales, thus targeting different components of SGD. Therefore, comparing SGD studies with estimates derived from different methods is often complex and misleading if the characteristics and assumptions of each quantification technique are not taken into account. This highlights the need to conduct studies comparing SGD derived from different methods, not only to obtain more accurate SGD estimates but also to obtain instrumental information on the characteristics of the estimated fluxes. To this aim, a combined use of different approaches to estimate SGD was applied in a Mediterranean coastal lagoon (Mar Menor, Spain), including direct measurements with seepage meters, radium isotopes, and radon mass balance, 224Ra/228Th disequilibrium in coastal sediments, radon vertical profiles in porewater sediments, and hydrologic modeling. Mar Menor is Europe's biggest saline coastal lagoon, and it is connected to a highly anthropized quaternary aquifer. In this coastal system, SGD is likely playing a major role in the eutrophication of the lagoon. However, despite the economic and biological importance of this lagoon, data about this system is still incomplete, and mostly only hydrological modeling has been performed. Keywords: Submarine Groundwater Discharge, radioactive tracers, seepage meters, porewater exchange, hydrological modeling.

Submarine groundwater discharge: An important source of new inorganic nitrogen to coral reef ecosystems

Identification and quantification of chemical reactions in a coastal aquifer to assess submarine groundwater discharge composition.

Submarine Groundwater Discharge (SGD), which represents subsurface exchange of water between land and ocean, is a major component of the hydrological cycle. Until the mid-1990s, it was generally believed that SGD rates were not large enough to influence ocean water budgets. This thought might be due to the difficulty in quantifying rates of SGD, because most SGD occurs as diffusive flow, rather than discrete spring flow. However, there is a growing recognition that the submarine discharge of fresh groundwater into coastal oceans is just as important as river discharge in some areas of the coastal ocean. Due to growing ecological concerns about SGD, there is considerable progress on research about SGD with particular emphasis on how to quantify and trace the SGD, and to develop some forecasting or predictive capability of SGD rates based on climatic and seasonal effects. This chapter presents a comprehensive overview of the methods used to quantify SGD to coastal areas and summarizes the previous studies on SGD. In addition, this chapter also discusses driving forces of groundwater flow through coastal aquifers, mechanism of groundwater seawater interaction and some other important issues that are necessary to understand the methods for quantifying SGD in coastal areas. The main goal of this chapter is to provide an overview of the applied methodologies to quantify SGD in coastal areas, which in turn will allow researchers, coastal zone managers, and others to choose appropriate methods that meet their specific project requirements.

There are many factors affecting submarine groundwater discharge (SGD). However, systematic study of the influences of these factors is still limited. In this study, numerical modeling is performed to quantitatively explore the influences of various factors on SGD in a coastal aquifer. In such locations, tidal and terrestrial hydraulic gradients are the primary forces driving fresh and salt water movement. Unlike steady-state flow, dynamic fresh and salt water mixing at the near-shore seafloor may form an intertidal mixing zone (IMZ) near the surface. By constructing a general SGD model, the effects of various model components such as boundary conditions, model geometry and hydraulic parameters are systematically studied. Several important findings are obtained from the study results: (1) Previous studies have indicated there will be a freshwater discharge tube between the classic transition zone and the IMZ. However, this phenomenon may become unclear with the increase of heterogeneity and anisotropy of the medium’s conductivity field. (2) SGD and IMZ are both more sensitive to the vertical anisotropy ratio of hydraulic conductivity (Kx/Kz) than to the horizontal ratio (Kx/Ky). (3) Heterogeneity of effective porosity significantly affects SGD and IMZ. (4) Increase of the storage coefficient decreases fresh water discharge but increases mixing salt water discharge and total SGD. The increase will also change the shape of the IMZ. (5) Variation of dispersivities does not affect SGD, but significantly changes the distributions of the IMZ and the whole mixing zone. These findings will be helpful to the sampling design of field studies of SGD and to the application of dynamic SGD models to field sites for model development and calibration.

Seawater intrusion occurs commonly in coastal aquifers around the world, threatening the availability and usability of fresh groundwater resources for vegetation and human uses. The rapid growth of the world population and urbanization requires sound strategies for protection and management of the freshwater resources, especially coastal groundwater. This goal can only be achieved through proper understanding of the processes that underlie seawater intrusion in coastal aquifers. Major insights have been gained over the past several decades, in particular, roles of the density-driven flow in driving and maintaining the invasive flow of saltwater. However, most studies have linked the seawater intrusion process merely to the level of salt content through the free convection induced by the salinity contrast between groundwater and seawater but ignored their temperature difference. In reality, the thermal contrast between coastal groundwater and marine seawater may range up to 15°C in absolute value with either warmer or colder seawater. Such thermal contrast can alter seawater circulation through the coastal aquifer, which in turn affects the biogeochemical reactions of land-sourced pollutants in the aquifer prior to discharge to the marine ecosystem.This research aimed to investigate the combined effects of salinity and temperature contrasts on the interactions between freshwater and seawater in unconfined coastal aquifers. Using laboratory experiments and numerical simulations, this research explored the coastal groundwater dynamics under various boundary settings with regards to thermal variations, tidal forcing and seasonal changes of seawater temperature. Findings from this research provided insights into the importance of temperature variations on various key processes in coastal aquifers under the condition of either static sea level or tidal oscillation.The effects of temperature contrast were first investigated for the seaward boundary of static level using physical experiments and numerical models in combination with tracer tracking. With the static sea level, the thermal contrast induced long-term impacts on the aquifer and altered background flow patterns and transport activities. The position of the saltwater wedge toe was modified significantly by the presence of temperature gradient either landward or seaward. Colder seawater enhanced the advancement of saltwater while warmer seawater hindered it. More importantly, the seawater circulation pattern changed dramatically in the latter case. A second circulation cell was discovered for the first time near the seaward boundary. The regular landward circulation cell was pushed to the vicinity of the interface where considerably larger velocity was observed. In-depth sensitivity analysis revealed the important role of spatial correlation between temperature-induced and salinity-induced density gradients, especially at the base of the aquifer, in driving the formation of the new cell.Both laboratory experiments and numerical simulation were carried out to investigate the thermal effects under the condition of tidal oscillation. The responses of the saltwater wedge were found to be similar to those under the static sea level, in particular, the retreat and advance of the wedge with warmer and colder seawater, respectively. The mixing zone widened as a result of the tidal fluctuation. Meanwhile, the upper saline plume and the freshwater discharge zone expanded in the warmer seawater case and contracted with colder seawater. The increased seawater temperature also intensified water exchange across aquifer-ocean interface, seawater circulation and the submarine groundwater discharge. Furthermore, tidally induced seawater circulation intensified with increased contribution to the submarine groundwater discharge compared with density-driven seawater circulation. All these characteristics were persistent over a range of tidal amplitudes. These results shed light on the importance of the thermal effects and have important implications for the assessment of the biogeochemical processes in coastal aquifers.The seasonal variations of the temperature contrast were then examined based on numerical simulations. The results showed clearly seasonality of the aquifer – ocean exchange and seawater circulation induced by the seasonal variation of seawater temperature in both cases with the static sea level and tidal conditions. Compared with the cases of the isothermal condition, all fluxes increased during colder months and decreased during warmer months. The periodic oscillation of the thermally induced density gradient resulted in a continuously changing mode of saltwater flow in the saltwater wedge. The flow path and transit time of circulating seawater shortened considerably in comparison with that in the isothermal case. This finding is particularly important for the evaluation of transport of land-sourced contaminants to the marine environment.The insights into the thermal effects on coastal unconfined aquifers gained from laboratory experiments and numerical simulations were applied to calculate a thermal impact factor and a thermal sensitivity index for aquifers along global coastlines based on local conditions of freshwater temperature and temperature contrast. The results suggested that the temperature effect is significant and would either amplify or reduce the impact of sea level rise on the vulnerability of coastal aquifers over a large proportion of the global coastlines.

More than 60% of the global population lives in coastal areas, especially within 100 km from the coastlines, relying mostly on shallow groundwater resources. Seawater intrusion and submarine groundwater discharge (SGD) occur in the coastal aquifer systems, threatening these critical freshwater resources. Salinity seawater and fresh groundwater complexly interact with each other via SGD and SI. The SGD drives the discharge of not only a large volume of freshwater, but also terrestrial geochemical substances into the ocean through a mixing zone between discharging freshwater and recirculating seawater. The flux of SGD may be even greater than that of surface water through rivers and estuaries. For example, the SGD was estimated to be ~40 % and 80 %~160 % of the river water discharging flux into the South Atlantic Bight and Atlantic Ocean, respectively, and as a major source of dissolved organic matter and nutrients to Arctic coastal waters and the Mediterranean Sea.A few hydrological models, including MARUN, SEAWAT, SUTRA, and PHT3D, are commonly used for SGD studies. The recently developed TOUGHREACT is robust in simulating coupled hydrodynamic, thermodynamic, and geochemical processes. From TOUGH2 (Transport Of Unsaturated Groundwater and Heat, version 2), a multi-dimensional numerical model for simulating coupled transport of water, vapor, non- condensable gas, and heat in porous and fractured media. However, TOUGHREACT is rarely used for SGD analysis, despite it being a well-rounded model with wide applications. Additionally, relevant studies on the iron (Fe) precipitation during SGD have focused predominantly on its spatial distribution and the adsorption of dissolved species, and studies on the genesis and geochemical evolution are scarce.Therefore, we developed a systematic method using TOUGHREACT to simulate the hydrological processes in STEs and benchmarked the estimations; and then we numerically explored the groundwater flow and salt transport dur SGD by considering the influencing factors of tidal amplitude, freshwater head, seawater diffusion coefficient, and beach slope ratio. Consequently, by employing TOUGHREACT simulation, we analyzed the formation and spatiotemporal distribution of the Fe precipitation in the shallow beach aquifer due to the mixing of freshwater and seawater, and identified the key influencing factors during SGD.The results show that, freshwater-derived Fe2+ is oxidized by O2(aq) in seawater during SGD, then precipitates as Fe (hydr)oxides (Fe(OH)3) to form an Fe precipitation zone. Fe(OH)3 tends to accumulate in the freshwater side of the mixing zone, whereas Fe(OH)3 precipitation in the seaward side of the mixing zone is inhibited by locally high H+ concentrations. The Fe(OH)3 first precipitates in the shallow aquifer, then extends to deeper layers over time, which is attributed to the increase in the residence time with the depth of both freshwater and seawater. The spatial distribution, and particularly, the extent of the iron curtain are influenced by the water flux and the concentration ratio of O2(aq) to Fe2+. These results are beneficial for better understanding the formation and distribution of iron curtains, and shed light on enhancing the understanding of the hydrogeochemical processes in subterranean estuaries.

Submarine groundwater discharge in a subsiding coastal lowland: A 226Ra and 222Rn investigation in the Southern Venice lagoon

Numerical models of variable‐density groundwater flow and salt transport are a primary tool for predicting salinity distributions in coastal aquifers and estimating submarine groundwater discharge (SGD). Models are particularly useful to estimate the saline component of SGD, which can occur far offshore and is difficult to measure directly. Depending on the system and application, the level of geologic detail represented can range from homogeneous or layered to fully heterogeneous hydraulic conductivity fields. These features strongly affect model results, limiting understanding of subsurface salinity distributions and associated density‐driven saltwater circulation along coasts worldwide. In this study, the impact of the scale of representation of heterogeneity on salinity distributions and SGD was investigated using numerical simulations. Upscaling hydraulic conductivity can significantly modify salinity distributions and flow paths, resulting in unpredictable variations in simulated SGD, though the values for homogeneous fields with equivalent hydraulic conductivity show consistent trends. Simulated density distributions control both the rate and direction of subsurface saltwater circulation. The length of the mixing zone perimeter, a measure of salinity distribution complexity, is shown to correlate with both the rate of subsurface saltwater circulation and the amount of groundwater circulating in the reverse direction from homogeneous cases. Overall, the results demonstrate a strong dependence of salinity distributions and saltwater circulation on the scale and distribution of geologic heterogeneity represented in numerical models. This suggests that numerical models with simplified geologic structure may substantially underestimate saltwater circulation, and attempts to calibrate them using salinity distributions or SGD measurements may be problematic.

Most research on submarine groundwater discharge (SGD) focuses on sandy beaches. Less studies have investigated environments with low hydraulic conductivity (Ks) such as coastal peatlands, which are abundant along the southern Baltic Sea coast. Coastal peatlands, which have been drained for agricultural purposes, hold high quantities of carbon, nitrogen, and other compounds that could possibly be released to the sea upon rewetting of these sites. In this study, we simulated groundwater flow from a coastal rewetted fen with a peat layer extending out into the sea to understand the short– and long–term dynamics of SGD, quantify SGD water and matter fluxes, and assess the impact of a storm surge on SGD and seawater intrusion. Five-year (2016 – 2021) daily 2D numerical simulations of groundwater flow were based primarily on monitored groundwater and seawater level data and field-gathered soil hydraulic parameters. Hydraulic conductivities of geological layers were optimized against measured water levels. Manual seepage meter measurements were conducted and water samples were collected. The modeled seepage rates fitted the measured ones well. Our results reveal that SGD and seawater intrusion are highly dynamic and vary spatially and temporally. Two dominant submarine discharge areas were observed: 1) near the beach (up to ~30 m from shore) where mean seepage rates based on nodal water velocities reach up to 12.4 cm d-1 with waters originating from the dune dike and recirculated seawater; 2) seeps from the aquifer at about 60 m distance from the coast with discharge rates of 1.1 cm d-1 on average. Mean seepage rates from the discharge areas are comparable to other wetland and sandy environments. The low Ks of the peat layer limits water exchange between the peatland and the Baltic Sea to these regions. The groundwater-seawater interface below the dune moves between the beach and the central dune on an hourly to weekly basis. However, the extent of the interface changes at a seasonal scale. Higher SGD fluxes occur in spring and summer while seawater intrusion increases during fall and winter, as a consequence of the seasonal variations of the peatland’s water level and the resulting hydraulic gradient. During storm surges, higher seawater intrusion fluxes are expected, while low seawater would lead to higher SGD fluxes. The mean daily net flux which represents land-derived SGD from the peatland is 0.15 m2 d-1 (range: -6.12 m2 d-1 to 1.63 m2 d-1), with the highest intrusion occurring during the 2019 storm surge and the highest SGD occurring two days after the surge event. Our mean daily net flux compares well with previous studies but total SGD, which includes recirculated seawater, is likely underestimated. Nearshore carbon and nitrogen SGD concentrations are higher than ambient seawater concentrations demonstrating the potential impact of SGD on local biogeochemistry. Our findings show that SGD is an important coastal process even from low-lying and low Ks coastal peatlands. We emphasize the importance of conducting more interconnection studies between peatland hydrogeology and geochemistry disciplines to better understand SGD processes in these environments.

ABSTRACTThe inland extending length of the freshwater–saltwater interface toe is useful in studies of seawater intrusion in coastal areas. The submarine fresh groundwater discharge in coastal zones is affected not only by hydraulic conductivity and hydraulic gradient of the aquifer, but also by the position of the interface. Two observation wells at different distances from the coast are required to calculate the fresh groundwater flow rate in coastal unconfined aquifers. By considering that the submarine groundwater discharge is equal to the groundwater flow rate, the length of the interface toe extending inland can be estimated when the groundwater flow is at a steady-flow state. Aquifers with horizontal and sloping confined beds and without/with unique surface vertical infiltration are considered. Examples used to illustrate the application of these methods indicate that the inland extending lengths of the interface toe in aquifers with vertical surface infiltration are much shorter than those in aquifers without vertical surface infiltration, and the length of the interface in aquifers with a horizontal confining lower bed are smaller than those in aquifers with a confining lower bed sloping towards the sea. The extent of the interface on the northwestern coast near the city of Beihai in southern Guangxi, China, on 18 January 2013 was estimated as 471–478 m.Editor M.C. Acreman Associate editor not assigned

The flow of groundwater from land to the sea is a major component of the hydrological cycle. This flow, called submarine groundwater discharge (SGD), consists of continental meteoric water mixed with seawater that has infiltrated the aquifer. The composition of this mixture is usually different from that predicted by simple mixing because biogeochemical reactions in the aquifer modify the fluid chemistry. To emphasize the importance of mixing and chemical reaction in these coastal aquifers, they are often called subterranean estuaries. Geochemical studies within subterranean estuaries preceded studies that attempt to integrate the effect of these systems on the coastal ocean. This mixing zone has long been recognized as an important site of carbonate diagenesis and possibly dolomite formation. Biologists have likewise recognized that terrestrial inputs of nutrients to the coastal ocean may occur through subterranean processes. Further evidence of SGD comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifers and reach the ocean as surface and subterranean systems exchange fluids. Exchange between the subterranean estuary and the ocean may be quantified by measuring the tracer distribution in the coastal ocean. These studies have revealed that SGD provides important fluxes of nutrients, carbon, and metals to coastal waters.

Submarine groundwater discharge: An Asian overview