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.
Highlights
Coastal regions are exposed to anthropogenic pressures frequently associated with high-density populations that require water supply [1]
Submarine groundwater discharge (SGD) at the field site shows a clear distinction between the hydrodynamic characteristics and processes taking place in the freshwater-dominated zone and the saline zone
The drivers of fluxes, the origin of discharging water, the chemical reactions that can take place, and the type of mixing of water must be considered in the analysis and methods for studying nutrients borne by SGD
Summary
Coastal regions are exposed to anthropogenic pressures frequently associated with high-density populations that require water supply [1]. Excess nutrient loads cause eutrophication [3,4,5] and algal blooms in coastal waters [6,7], and may have a global scale impact by altering marine food webs [2,8]. Another consequence is the effect on touristic activities such as fishing and leisure, [9] which are important drivers of economic activity. Subsequent investigations on submarine groundwater discharge have used seepage meters, radioactive tracers, or hydraulic methods to estimate the flux of groundwater [13,14,15]
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