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Journal of Hydrology: Regional Studies





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Study region: Eastern shore of Ringkøbing Fjord, a coastal lagoon at the west coast of Denmark

Study focus: A dual tracer approach based on salinity and δ18O is used to assess seasonal dynamics at the saltwater-freshwater interface of a coastal lagoon. At the site, salinity is prone to vary on a sub-seasonal or daily frequency due to riverine freshwater inputs to the lagoon. In contrast, δ18O compositions of end-members only vary seasonally.

New hydrological insights: The dual tracer approach shows to be valuable in coastal settings where end-member concentrations vary substantially over the seasons and hence, an unambiguous end-member definition does not exist. Calculated mixing fractions using only salinity, deviated from the dual tracer approach on average by 18%, but were as high as 97%. Although, these differences decrease to 6% on average when using only δ18O, our study strongly suggests their simultaneous application.

Moreover, we found that seawater intrusion occurs during the summer when salinity in the lagoon is high and fresh submarine groundwater discharge (SGD) is low. This process reverses during the winter (wet season) when SGD increases by a factor of 2–3, due to the recession of the saltwater wedge from land. Our findings show that in absence of waves and tides, density-driven dynamics, and particularly the terrestrial freshwater fluxes, create a major impact on saltwater wedge dynamics.