Assessing seasonal flow dynamics at a lagoon saltwater-freshwater interface using a dual tracer approach

δ 2 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 δ 18 O 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, δ 18 O 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 de ﬁ nition 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 di ﬀ er- ences decrease to 6% on average when using only δ 18 O , 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 ﬁ ndings show that in absence of waves and tides, density-driven dynamics, and particularly the terrestrial freshwater ﬂ uxes, create a ﬂ ow dynamics and the movement of a saltwater-freshwater interface near Ringkøbing Fjord, a coastal lagoon system in Denmark. The site provides natural conditions to isolate the e ﬀ ects of density-driven subsurface ﬂ ow and freshwater ﬂ uxes on the seasonal development of a saltwater wedge. The lagoon stage is very stable and wedge dynamics are mainly introduced by the seasonal variation of salinities in the lagoon and seasonal varying inland hydraulic gradients. Our study demonstrated that this leads to seawater intrusion during summer conditions (dry period), when salinity is high and hydraulic gradients are low, and a retreating saltwater wedge in the aquifer during winter conditions (wet period), when surface water salinities are low and hydraulic gradients high. The total submarine groundwater discharge to the lagoon varies roughly by a factor of 2 – 3 between seasons. Flow dynamics and changes in salinity are almost entirely below the lagoon, however, our observations do not provide certain conclusions regarding the seasonal dynamics of the toe location of the saltwater wedge. a more robust than and end-member lagoon more stable and improve the estimations of mixing fractions. We that with the use of EC in a single tracer end-member fractions in wells di ﬀ er 18% on average, with individual wells di ﬀ ering as much as 97%. have the introduce biased conclusions about the lagoon system. By only using δ 18 O , di ﬀ erences be minimized to 6%, but single wells di ﬀ erences up to 30% compared to the dual-tracer approach. performance of a single tracer mixing estimate depends on the right choice of end-member concentration. the MIX-model estimates to be most reliable as the model as true