Application of Stable Isotopes of Water to Study Coupled Submarine Groundwater Discharge and Nutrient Delivery

Abstract

Submarine groundwater discharge (SGD)—including terrestrial freshwater, density-driven flow at the saltwater–freshwater interface, and benthic exchange—can deliver nutrients to coastal areas, generating a negative effect in the quality of marine water bodies. It is recognized that water stable isotopes (18O and 2H) can be helpful tracers to identify different flow paths and origins of water. Here, we show that they can be also applied when assessing sources of nutrients to coastal areas. A field site near a lagoon (Ringkøbing Fjord, Denmark) has been monitored at a metric scale to test if stable isotopes of water can be used to achieve a better understanding of the hydrochemical processes taking place in coastal aquifers, where there is a transition from freshwater to saltwater. Results show that 18O and 2H differentiate the coastal aquifer into three zones: Freshwater, shallow, and deep saline zones, which corresponded well with zones having distinct concentrations of inorganic phosphorous. The explanation is associated with three mechanisms: (1) Differences in sediment composition, (2) chemical reactions triggered by mixing of different type of fluxes, and (3) biochemical and diffusive processes in the lagoon bed. The different behaviors of nutrients in Ringkøbing Fjord need to be considered in water quality management. PO4 underneath the lagoon exceeds the groundwater concentration inland, thus demonstrating an intra-lagoon origin, while NO3, higher inland due to anthropogenic activity, is denitrified in the study area before reaching the lagoon.

Groundwater–Surface Water Interactions: Recent Advances and Interdisciplinary Challenges

“…The majority of the contributions [3,5,[8][9][10][11][15][16][17][19][20][21][22][23] focus on the interactions between rivers or streams and groundwater. Three studies [6,13,14] investigate the interactions between lakes and groundwater, while two studies [4,18] deal with the exchange between groundwater and oceanic water. Two additional contributions [7,12] show the relevance of groundwater exchange processes in wetlands.…”

“…Tirado-Conde et al [18] compare the results of traditional seepage meter measurements with several analytical approaches of vertical sediment temperature profiles in a lagoon in Denmark. Duque et al [4] applied stable isotopes of water to delineate flow paths and the origin of water in a coastal aquifer. This enabled them to draw valuable conclusions on nutrient transport and their fate along the vector from freshwater to saline environments.…”

A State-Of-The-Art Perspective on the Characterization of Subterranean Estuaries at the Regional Scale

“…Terrestrial groundwater is substantially more depleted in the heavier isotopes than seawater (Gat, 1996;Hoefs, 2018) and contains further information about the integrated meteorological conditions in the recharge area. Therefore, water isotopes are widely used in studies of SGD or STE (e.g., Burnett et al, 2006;Povinec et al, 2011;Rocha et al, 2016;Oehler et al, 2017;Duque et al, 2019). Time series can reveal the variability of terrestrial groundwater inputs on different time scales, particularly in dynamic karst STEs.…”

Simulation of Density and Flow Dynamics in a Lagoon Aquifer Environment and Implications for Nutrient Delivery From Land to Sea

“…As part of coastal environments, lagoons are always located in lowland areas where nutrient inputs from the hinterland are often high, and thus lagoons often suffer from eutrophication (Coluccio et al, 2021). Besides riverine inputs, it has been shown that lagoons and coastal water bodies receive large amounts of nutrients through submarine groundwater discharge (SGD) (Corbett et al, 1999;Li et al, 1999;Ji et al, 2013;Rodellas et al, 2015;Duque et al, 2019). The groundwater component discharging into lagoons can vary from minor contributions (Menció et al, 2017) to contributions up to 90% (Sadat-Noori et al, 2016) with seasonal variations (Menció et al, 2017).…”

“…Density-driven hydrodynamic conditions in the subsurface of lagoons can be generally expected to be quite similar to coastal systems connected with the adjacent sea, yet the reduction or absence of diurnal tides potentially make the subsurface flow, and its linked subsurface nutrient cycling, different for coastal lagoon aquifers. Coastal subterranean groundwater flow dynamics and SGD are influenced by multiple factors, varying in temporal and spatial scales and hence, creating a complex subsurface flow system (Robinson et al, 2006;Heiss and Michael, 2014;Duque et al, 2019;Meyer et al, 2019). The spatial extension of the saltwater wedge and exchange fluxes between salt and freshwater depend on diffusion and dispersion processes linked to hydrogeological and chemical settings.…”