Radon (222Rn) as a geochemical tracer for submarine groundwater discharge (SGD): Measuring techniques, source to sink mass balance model and practical constraints.

Mangrove-derived dissolved organic carbon and allochthonous refractory subsidies via submarine groundwater discharge at regional and global scales.

222Rn in Argentina: A comprehensive review of environmental and hydrological applications.

Abstract. Subterranean estuaries (STEs) are biogeochemical reactors modifying the chemistry of salt- and freshwater as they flow through the subsurface sediments. Boundary conditions such as tides, waves, beach morphology, seasonal meteoric groundwater recharge and storm events control endmember mixing and residence time distributions within STEs. These in turn affect biogeochemical reactions and thus elemental fluxes discharging to the ocean via submarine groundwater discharge (SGD). Especially at high-energy beaches exposed to high tidal ranges and high wave energy, boundary conditions are very dynamic and likely imprint on groundwater flow and reactive transport within the STEs. A quantitative understanding of mixing processes and residence time distributions is necessary in order to adequately describe biogeochemical processes and can be achieved with the help of numerical modelling. Yet, transient field-scale modelling approaches calibrated to comprehensive observational data sets are still lacking, in particular for real-world high-energy STEs. In the present study, for the first time a density-dependent groundwater flow and transport model was developed and calibrated for a high-energy beach. The north beach of the barrier island Spiekeroog, northern Germany, thereby served as an example field site exposed to high-energy characteristic boundary conditions. The model was calibrated to a 1.5-year extensive dataset of groundwater heads, salinities, temperatures and 3H / He groundwater ages at various shore-perpendicular locations along the beach at depths down to 24 m below ground surface. The calibrated model is able to replicate the principal behaviour of the highly transient system and enabled the identification of hot spots of high temporal variability in the investigated state-variables. The dynamics in salinity are most intense at the in- and exfiltration locations of the tide-induced recirculating seawater. The groundwater age variability was largest seawards of the low tide mark as well as below the deep recirculating seawater cell at around 20–30 m depth near the dunes, where very old freshwater from the islands' freshwater lens mixes with young brackish water from the upper beach. Temperature variations were seasonal and confined to the upper 5–10 m below the beach. Computed saline SGD water fluxes varied considerable on daily and spring-neap time scales, as well as on the longer term, i.e., monthly to yearly time scales. The rather gradual, longer-term changes in flux appear to be mainly controlled by changes in spatial variability of the beach slope. The simulated groundwater age of the fresh SGD component varied between 4 and 25 years, and predominantly depended on the magnitude of saline SGD flux. Overall, the model provided important insights into the dynamics of the flow and transport processes.

Distribution of tritium in the Changjiang estuary area, China: Transport flux and water age.

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.

Declines in tourism during COVID-19 reduces nitrogen loading of groundwater input to a coral reef in West Maui, Hawai'i.

Submarine groundwater discharge along India’s shorelines: a systematic review of recent studies and sustainability management approach

Submarine groundwater discharge and gas hydrate dissociation fuel organic matter formation in Arctic fjord sediments

Submarine groundwater discharge derived metal elements fluxes in a seawater intrusion influenced area

ABSTRACTSubmarine groundwater discharge (SGD) supplies large quantities of nutrients and other terrestrial elements to coastal ecosystems, impacting marine biota and ecosystem functioning. Despite the relevance of prokaryotes for marine biogeochemistry, little is known about their responses to groundwater inputs. Here we explored the impact of SGD on the spatiotemporal patterns of prokaryotic communities from the Mar Menor (Murcia, Spain), a highly anthropized hypersaline coastal lagoon that receives large amounts of nutrient‐polluted SGD. Using 16S rRNA amplicon sequencing, activity assays, and flow cytometry, we investigated the dynamics of prokaryotic communities across the lagoon and its connected environments (freshwater streams, groundwater, soils, the sea) on two occasions. We found that the lagoon areas most influenced by SGD (i.e., sites closest to the shore) presented on average three‐fold higher heterotrophic prokaryotic protein production than the inner lagoon samples, and 2.7‐fold higher taxonomic richness. This spatial pattern was likely influenced by solutes supplied by SGD, as higher concentrations of dissolved nitrogen, silicate and SGD tracers (radium [Ra] isotopes) were found in nearshore waters. Increases in these elements also influenced the relative abundance of dominant bacterial groups (e.g., Flavobacteriales, Rhodobacterales). In autumn, increases in lagoon 224Ra were strongly linked to higher taxonomic richness in microbial assemblages and the influx of allochthonous taxa from the catchment, pointing to seasonally variable transport of coastal groundwater microorganisms via SGD. Our study highlights SGD as an overlooked driver of prokaryotic dynamics in coastal ecosystems, and suggests that changes in this process may significantly impact microbial community structure and functioning.

We report hydrogeochemical and isotopic observations across the Baltic Sea from two research expeditions: (1) a ~5000 km cruise-track onboard the R/V Skagerak in 2023 and (2) a land-based sampling for terrestrial endmembers in 2024. The ship-based observations include continuous monitoring of hydrographic parameters, pH, and 222Rn in surface water. In addition, we collected 542 discrete samples from the water column, vertical profiles (n = 69 stations), and meteorological data. Land observations include discrete samples from beach groundwater (n = 77), nearshore surface water (n = 47), and rivers close to the coastline (n = 46). Discrete samples were analyzed for short-lived radium isotopes, nutrients, dissolved organic and inorganic carbon, total dissolved nitrogen, total alkalinity, methane, and stable isotopes (δ18OH2O, δ2HH2O, δ13CDIC, δ13CCO2, δ13CCH4). Data products include seven open-access files. This dataset forms the deposit for upcoming original research publications. This dataset will also be valuable to researchers interested in the hydrogeochemistry of coastal seas, like the Baltic Sea, and more generally interested in submarine groundwater discharge and estuarine biogeochemistry.

Abstract Coastal peatlands are important carbon, nutrient, and trace metal stores that have been extensively drained and degraded. Rewetting may restore ecological and hydrological functions but could also enhance the export of solutes through the submarine groundwater discharge (SGD) pathway to adjacent marine environments. We directly measured SGD and its associated solute concentrations in front of a coastal peatland using seepage meters across two shore-perpendicular transects in summer and fall 2021. In addition to SGD, groundwater and porewater samples were analyzed for dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved nitrogen (TDN), nutrients, trace metals, and stable water and carbon isotopes to characterize the discharging groundwater. Results indicated a mean SGD rate of 1.3 ± 1.7 cm d⁻ 1 (max. 3.9 cm d⁻ 1 ) during the summer campaign while recharge conditions prevailed in fall, due to intense landwards winds. Notably, salinity of SGD exceeded that of ambient seawater, and isotope analyses revealed that most SGD originated from the recirculation of brackish seawater. Solute concentrations were significantly higher in SGD than in seawater, but lower than values encountered in porewater and groundwater. The flushed permeable coastal sediments thus serve as a source for carbon, nutrients and trace metals. Summer solute fluxes were substantial (mmol m −2 d −1 ) – DOC: 10; DIC: 44; TDN: 2.8; NH₄⁺: 1.5, PO₄ 3 ⁻: 0.13; Fe: 0.23; and Mn: 0.17. However, estimates of solute fluxes might be conservative as land-derived groundwater influence was minor during the sampling campaigns. Our findings suggest that SGD in front of coastal peatlands is significant and could influence local and regional biogeochemical budgets.

Submarine groundwater discharge governs dissolved carbon cycling: Multi-scale evidence from a large river-dominated groundwater-saltmarsh-estuary system.

Massive carbon inputs from fish farming reduce carbon sequestration capacity in a macroalgae mariculture area.

Seasonality of submarine groundwater discharge pathways in a coastal lagoon revealed by radium isotopes: the importance of porewater exchange in summer

Abstract Nutrient availability drives community structure and ecosystem processes, especially in tropical lagoons that are typically oligotrophic but often receive allochthonous inputs from land. Terrestrially derived nutrients are introduced to tropical lagoons by surface runoff and submarine groundwater discharge, which are influenced by seasonal precipitation. However, terrigenous inputs presumably diminish along the onshore–offshore gradients within lagoons. We characterized nutrient availability in the lagoons of a tropical high island, Moorea, French Polynesia, using spatially distributed measurements of nitrogen content in the tissues of a widespread macroalga during the rainy season over 4 yr. We used synoptic water column sampling to identify associations among macroalgal nutrient content and the composition of inorganic macronutrients, dissolved organic matter, and microbial communities. We paired these data with quantifications of land use in nearby watersheds to uncover links between terrestrial factors, aquatic chemistry, and microbial communities. Algal N content was highest near shore and near large, human‐impacted watersheds, and lower at offshore sites. Sites with high algal N had water columns with high nitrite + nitrate, silicate, and increased humic organic matter (based on a fluorescence Humification Index), especially following rain. Microbial communities were differentiated among nearshore habitats and covaried with algal N and water chemistry, supporting the hypothesis that terrigenous nutrient enrichment shapes microbial dynamics in otherwise oligotrophic tropical lagoons. This study reveals that land–sea connections create nutrient subsidies that are important for lagoon biogeochemistry and microbiology, indicating that future changes in land use or precipitation will modify ecosystem processes in tropical lagoons.

Coral reef mortality around the world is accelerating due to human activities and rising sea temperatures that cause bleaching, which is expected to become more frequent. Our ability to predict which corals will be most resilient, however, remains limited due to insufficient information characterizing nearshore temperature and habitat conditions. In this study, we examine how submarine groundwater discharge (SGD) reduces nearshore water temperatures and exposure of corals to heat stress, complementing the understanding that SGD can adversely affect coral when it contains elevated nutrient concentrations. Data from fixed nearshore sensors and vertical depth profiles along ~100 km of the western shoreline of the Island of Hawai’i from 2003 to 2014 demonstrate that submarine groundwater discharge (SGD) can reduce nearshore water temperatures by 1 °C–5°C and create estuarine-like conditions with salinities as low as 20 PSU, where the prevalent coral species, Pocillopora meandrina, Porites lobata, and Montipora capitata, thrive. Time-series temperature records reveal that exposure to high ambient ocean temperatures, which are known to initiate bleaching events, are reduced up to 5%–46% of the time. Coral health surveys indicated coral bleaching in response to moderately high annual temperatures in 2010 and 2011, with more colonies affected farther from cold, SGD-fed waters. Synthesis of these results, along with coral response data following the more extreme marine heat wave of 2014–2015, demonstrates lower coral loss and greater coral recovery near groundwater seeps, particularly those with higher flux and influence on reducing nearshore water temperatures. Our results demonstrate that SGD may therefore provide a beneficial ecosystem service and enhance coral reef resilience, particularly where human-related nutrient additions to groundwater can be mitigated. The implications of our findings are relevant across tropical coasts where groundwater inputs can be substantial, such as the Caribbean and Indo-Pacific, and contribute to improving our understanding of coral sensitivity to gradients in temperature and nutrient stress. Improved management of groundwater resources could thus be vital to local–regional strategies for mitigating future heat stress.

Submarine groundwater discharge (SGD) introduces nutrient-rich, low-salinity water into coastal ecosystems, significantly altering reef biogeochemistry. At Black Point, Oʻahu, we employed a novel, two-pronged approach that integrates a cost-effective small unmanned aerial system equipped with a thermal infrared (sUAS-TIR) sensor and high-resolution benthic salinity time series to resolve previously unobserved, fine-scale patterns of SGD delivery. sUAS-based sampling overcomes key limitations of prior SGD mapping methods, such as labor-intensive and spatially constrained in situ sampling, by enabling rapid deployment, real-time visualization of mixing dynamics, and high-resolution imagery. Unlike previous studies that relied on customized systems that required independent sensor integration, our methods used a cost-effective, fully integrated sUAS-TIR platform that required no additional modification. Our results show that localized hydrodynamics strongly modulate groundwater delivery, creating spatially heterogeneous patterns of delayed transport, recirculation and retention (i.e., pooling), and offshore dispersal. Thermal imagery revealed persistent surface plumes in regions of reduced circulation, while sequential orthomosaics and benthic salinity time series captured the temporal progression of groundwater movement across the reef. These findings highlight complex SGD delivery patterns that contribute to ecological vulnerability, particularly in areas experiencing prolonged exposure to nutrient-rich, low-salinity waters. Such dynamics underscore the need to consider both spatial and temporal variability when evaluating SGD’s ecological impacts. Our two-pronged approach offers a valuable tool to (1) identify reef zones that may be disproportionately affected by point-sources of land-based pollution, and (2) elucidate the mechanisms driving groundwater transport in coral reef environments. By offering a cost-effective, scalable, and operationally flexible framework, this methodology advances the management of groundwater-impacted ecosystems and enhances our ability to assess the ecological implications of dynamic SGD delivery.

Seawater intrusion angle controls colloidal chromium migration across coastal groundwater interfaces.