Investigating spatio-temporal salinity dynamics in coastal aquifers

Coastal aquifers build the transition zone of freshwater and saltwater. Hence, large salinity gradients are encountered in the subsurface below beaches and it is important to assess the salinity in a high resolution in order to understand coastal groundwater flow dynamics and consequently geochemical and microbial processes in subterranean estuaries. Within the project DynaDeep, we used both geophysical and hydrogeological methods to determine the bulk and fluid electrical conductivities (bulk/fluid EC) with the aim to convert the EC to salinity to monitor its temporal and spatial changes. This was done at a high-energy beach on the North Sea Island of Spiekeroog.

Numerous EC techniques have been used to acquire a unique dataset since 2022, covering a 2D transect from the dune base to the low water line. The site was subject to strong topographic changes over the seasons. Among the methods applied, we used electrical resistivity tomography (ERT) to get access to 2D distributions every six weeks. Additionally, continuous monitoring was carried out using a saltwater monitoring system (SAMOS) with a vertical electrode chain down to a depth of 20 meters located at the high water line. Direct push (DP) data at various locations as well as fluid EC values from water samples gathered via DP give access to high resolution information. In three multilevel wells (four levels each at 6, 12, 18, and 24 meter depth below ground) we logged the fluid EC and temperature and took water samples on a regular basis.

For an especially dense dataset between January and March 2023 we compared in detail the applied EC methods and found a general agreement in all of the gathered data after suitable calibration and temperature correction. We furthermore derived a formation factor model for the conversion to salinity.

Finally, we a combined inversion of the ERT data with the additional data aiming for fluid EC directly under the assumption of this temporally fixed formation factor model. In contrast to standard inversion techniques, this allowed for a naturally occurring smooth transition of salinities over the different geological units, which was critical when analyzing the spatial and temporal changes.