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The number of studies concerning Submarine Groundwater Discharge (SGD) grew quickly as we entered the 21st century. Many hydrological and oceanographic processes that drive and influence SGD were identified and characterized during this period. These processes included tidal effects on SGD, water and solute fluxes, biogeochemical transformations through the subterranean estuary, and material transport via SGD from land to sea. Here we compile and summarize the significant progress in SGD assessment methodologies, considering both the terrestrial and marine driving forces, and local as well as global evaluations of groundwater discharge with an emphasis on investigations published over the past decade. Our treatment presents the state-of-the-art progress of SGD studies from geophysical, geochemical, bio-ecological, economic, and cultural perspectives. We identify and summarize remaining research questions, make recommendations for future research directions, and discuss potential future challenges, including impacts of climate change on SGD and improved estimates of the global magnitude of SGD.

Groundwater is a crucial freshwater source for coastal communities. However, population growth, urbanization, industrial activities, and the discharge of polluted sewage water have led to the contamination of coastal groundwater with nutrients, metals, and organic compounds. This contaminated groundwater and terrestrial groundwater discharges into the ocean through a process known as Submarine Groundwater Discharge (SGD). This study aims to review (i) the driving forces behind SGD across coastal barriers, (ii) methods for identifying and quantifying SGD sites, and (iii) the status of SGD in Indian coastal aquifers and groundwater resource availability. The study indicates that groundwater discharge is higher on the east coast of India than on the west coast. Data on groundwater resources in India’s coastal states show an increase in annual groundwater extractions for irrigation, industry, and domestic use, with a decreasing trend in net groundwater availability for future use between 2011, 2013, and 2017. Despite this, there is limited evidence on the quantity of SGD flux along the Indian coastline. However, preliminary studies by the Mission SGD project have made some progress in understanding this phenomenon. This research aims to improve the estimation of water resources in India and highlight the volume of SGD entering the ocean. A comprehensive understanding of hydrogeological settings, computational methods, coastal aquifer geometries, and other factors is essential for accurately estimating SGD along the Indian coastline.

Submarine Groundwater Discharge (SGD) represents a significant mode of chemical transport to water bodies, making it an important flux to understand. Small Unmanned Aircraft Systems-deployed thermal infrared sensors (sUAS-TIR) provide a financially and logistically inexpensive means of identifying SGD source zones and quantifying SGD thermal infrared (TIR) plume areas over regional scales at high spatial resolutions. sUAS-TIR additionally offers the unique capability of high temporal resolution measurements of SGD. As a developing science application, the use of sUAS-TIR to image SGD requires substantial background knowledge. We present a proposed methodological construct for implementing a sUAS-TIR program for SGD-TIR data gathering, with applications extending to other research fields that can benefit from airborne TIR. Several studies have used airborne TIR in combination with empirical SGD flux measurements to quantify SGD, reporting a consistently strong regression between SGD flux and SGD TIR plume area. We additionally discuss novel research opportunities for sUAS-TIR technologies, as applied to SGD flux. The combination of high spatial and temporal resolution capabilities, at relatively low costs, make sUAS-TIR a promising new technology to overcome the scaling challenges presented by empirical studies and modeling of SGD fluxes, and advance our understanding of the controls on SGD fluxes.

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

Submarine groundwater discharge (SGD) is an important source of nutrients and metals to the coastal ocean, affects coastal ecosystems, and is gaining recognition as a relevant water resource. SGD is usually quantified using geochemical tracers such as radon or radium. However, a few studies have also used dissolved silicon (DSi) as a tracer for SGD, as DSi is usually enriched in groundwater when compared to surface waters. In this study, we discuss the potential of DSi as a tracer in SGD studies based on a literature review and two case studies from contrasting environments. In the first case study, DSi is used to calculate SGD fluxes in a tropical volcanic-carbonate karstic region (southern Java, Indonesia), where SGD is dominated by terrestrial groundwater discharge. The second case study discusses DSi as a tracer for marine SGD (i.e. recirculated seawater) in the tidal flat area of Spiekeroog (southern North Sea), where SGD is dominantly driven by tidal pumping through beach sands. Our results indicate that DSi is a useful tracer for SGD in various lithologies (e.g. karstic, volcanic, complex) to quantify terrestrial and marine SGD fluxes. DSi can also be used to trace groundwater transport processes in the sediment and the coastal aquifer. Care has to be taken that all sources and sinks of DSi are known and can be quantified or neglected. One major limitation is that DSi is used by siliceous phytoplankton and therefore limits its applicability to times of the year when primary production of siliceous phytoplankton is low. In general, DSi is a powerful tracer for SGD in many environments. We recommend that DSi should be used to complement other conventionally used tracers, such as radon or radium, to help account for their own shortcomings.

ABSTRACT Despite the relevance of submarine groundwater discharge (SGD) for ocean biogeochemistry, the microbial dimension of SGD remains poorly understood. SGD can influence marine microbial communities through supplying chemical compounds and microorganisms, and in turn, microbes at the land–ocean transition zone determine the chemistry of the groundwater reaching the ocean. However, compared with inland groundwater, little is known about microbial communities in coastal aquifers. Here, we review the state of the art of the microbial dimension of SGD, with emphasis on prokaryotes, and identify current challenges and future directions. Main challenges include improving the diversity description of groundwater microbiota, characterized by ultrasmall, inactive and novel taxa, and by high ratios of sediment-attached versus free-living cells. Studies should explore microbial dynamics and their role in chemical cycles in coastal aquifers, the bidirectional dispersal of groundwater and seawater microorganisms, and marine bacterioplankton responses to SGD. This will require not only combining sequencing methods, visualization and linking taxonomy to activity but also considering the entire groundwater–marine continuum. Interactions between traditionally independent disciplines (e.g. hydrogeology, microbial ecology) are needed to frame the study of terrestrial and aquatic microorganisms beyond the limits of their presumed habitats, and to foster our understanding of SGD processes and their influence in coastal biogeochemical cycles. The authors review the available literature on the microbial aspects of submarine groundwater discharge, from the freshwater aquifers to the coastal ocean, and identify current challenges and future directions to foster knowledge on microbial ecology at the land–ocean interface.

Intrusion of seawater into the coastal aquifers is a major concern as it affects the quality of groundwater. The objective of this study is to delineate the extent of seawater intrusion in the Indian coast based on previous studies and estimate the area as well as locations of seawater intrusion and submarine groundwater discharge based on the groundwater level of the years 2007 and 2017. Several researchers have reported seawater intrusion in the coastal regions of India by different methods of investigation. These studies indicate that the east coast of India is affected greater than the west coast by seawater intrusion. The maximum extent (about 14 km) of seawater intrusion in India is reported in regions north of Chennai. It is estimated that around 7% of the total coastal area is affected by seawater intrusion, where groundwater is below mean sea level. Around 57% of the coastal area of India has groundwater level in the range from 0 to 10 m msl. Future research needs to focus on the areas where seawater intrusion and submarine groundwater discharge were identified based on this study.

Subterranean estuaries (STEs), where groundwater interacts with seawater, influence surface and subsurface coastal ecology and biogeochemistry. In Arctic‐STEs overlying permafrost, groundwater flow and heat transport determine the fate of organic matter. Yet, direct observations of groundwater flow and heat and solute transport processes in Arctic STEs remain limited. This study characterized groundwater flow paths and fluxes and heat transport within an Arctic‐STE along Alaska's Beaufort Sea coast during thawing, summer, and freeze‐up. Intertidal seabed temperature‐depth profiles collected along a 10‐m transect captured the active groundwater flow period, from thaw and flow onset in mid‐June to freeze‐up in late‐September. During this period, aquifer geometry evolved non‐uniformly due to spatially varying thaw rates across the STE (mean (m) thaw depths–beach: 0.25 to 0.55–0.6 m on 20 June, 25 July, 1 October; seabed: 0.6–0.9 m from 25 July to 1 October). Groundwater and surface water levels, salinity, and subsurface temperature profiles measured over tidal time scales were interpreted alongside groundwater flow‐heat transport numerical simulations. Fresh groundwater discharge was sporadic during thawing (m: 0.32 m3/day/m), abundant in summer (m: 0.45 m3/day/m), and was largely absent during freeze‐up. During freeze‐up, groundwater flow was driven exclusively by seawater recirculation via tidal pumping (from thawing to summer to freeze‐up: 0.00025–0.15–0.5 m3/day/m) and convection. Heat advection dominated near aquatic interfaces (shaping intertidal ice), and conduction controlled vertical temperature gradients in low‐flow and unsaturated sediments. These findings will help predict how prolonged summers will alter Arctic‐STE cryo‐hydrology and biogeochemistry.

Coastal ocean acidification is a worldwide problem associated with anthropogenic activity and climate change. In this study, a close relationship between submarine groundwater discharge (SGD) and coastal ocean acidification in Hong Kong's coastal waters is discovered. We for the first time evaluated the direct influence of SGD on seawater pH decline. Results show that SGD can contribute to up to 48% of seawater pH variation among terrestrial water input, air‐sea CO2 exchange, photosynthesis/respiration, and bay‐open water exchange in semi‐closed bays through direct input of carbonate species. Local air‐sea CO2 exchange has negligible influences on the seawater pH. In the semi‐closed bay areas, SGD and aerobic respiration are major contributors to seawater pH variation but in areas with open space, the exchange process with open waters is more important. In addition to the direct input of carbonate species (total alkalinity and dissolved inorganic carbon), SGD also influences the seawater pH via considerable nutrient loadings. The findings highlight the importance of the investigation and management of groundwater to alleviate the fast coastal ocean acidification. Plain Language Summary Seawater becoming more acidic is a big problem in coastal areas around the world due to climate change. The main causes of this acidification are excessive nutrient release and human‐generated carbon dioxide. When groundwater flows into coastal areas, it carries chemicals from the land that can affect the pH of the seawater. In our research, we discovered that groundwater releases more dissolved inorganic carbon compounds, which lower the seawater's pH, compared to the total alkalinity that normally helps maintain a stable pH. Groundwater is a major factor in causing acidification in semi‐closed bays along the coast. We want to emphasize the importance of understanding the impact of coastal processes and managing groundwater in order to slow down the acidification of coastal waters. Key Points Submarine groundwater discharge (SGD) strongly correlates with long‐term ocean acidification rates, unveiling their significant relationship SGD accounts for ∼50% of seawater pH decline in semi‐closed bays, primarily through direct carbonate species input SGD indirectly impacts seawater pH decline by influencing aerobic respiration processes, amplifying its role in pH dynamics

Previous studies have revealed the groundwater flow and salt transport in unconfined coastal aquifers in response to numerous factors. However, previous studies assumed a flat aquifer base, and it is unclear how a sloping aquifer base would affect the subsurface flow and salinity distributions. In reality, the bases of many coastal aquifers are inclined toward either the sea or the inland. This study presents numerical simulations that examine the subsurface hydrodynamics in sloping unconfined coastal aquifers. The results show that, when the aquifer base is seaward sloping, the salt‐freshwater mixing zone is wider and the seawater intrusion distance is longer. Also, a seaward base slope reduces/increases the percentagewise contribution of tide‐/density‐driven recirculation to the total submarine groundwater discharge (SGD), extends the transport pathway and transit times of particles. While a landward aquifer base reverses these trends. These findings may assist more accurate estimation of total SGD and strategies for mitigating marine contamination.