Factors driving varying salinity of a fresh water lens underneath a coastal barrier island

Groundwater is often the only drinking water resource of many small marine islands. The fresh water lenses underneath these islands are highly restricted in their extent and vulnerable to salt water intrusion caused by groundwater extraction or other factors. An example is the barrier island of Langeoog located off the North German coast. The drinking water supply of this island is exclusively provided by groundwater extracted from its fresh water lens. Since several of the drinking water wells exhibit strong salinity variations, a long-term sustainable drinking water management requires knowledge of the factors driving these variations.

To identify the reasons for the salinity variations in the drinking water wells on Langeoog, long-term data series on chloride concentrations, groundwater and sea water levels, pumping activity, climate data, soil moisture and dune locations were analyzed for the period of 1993-2023.  Measured and from measurements derived data were investigated using time series analysis, multivariant regression and artificial intelligence approaches to determine the relevance of different driving factors for the observed salinity variations and the ability to predict such variations.

The results of the study show that individual drinking water wells differ not only in the magnitude of the salinity variations but also regarding the reasons for these variations. In general, the salinity in individual wells is not driven by the present conditions and their short term variability but reflects the response of the groundwater system to factors integrated over periods of several years. Relevant factors include the water balance of the well field (groundwater recharge vs. extraction) as well as the storm flood frequency and the associated variations in the location of barrier dunes. Without detectable influence on salinity are groundwater levels, sea water levels and the operation intensity of individual wells. A limited prediction of the salinity based on the entire set of collected data is possible for selected wells.