Long-term monitoring of coastal saltwater intrusion using the geoelectrical monitoring system SAMOS

Efficient groundwater management is the key to a sustainable use of freshwater aquifers. In the coastal areas worldwide, saltwater intrusions caused by sea-level rise, overuse of freshwater resources and changing groundwater recharge is a major threat to the availability of freshwater. A reduced groundwater recharge combined with an increased extraction can lead to vertical upconing or lateral movement of the freshwater-saltwater transition zones, therefore reducing the local freshwater resources. Long-term and continuous observation of the freshwater-saltwater transition zones is crucial to implement early warning procedures, yields more detailed insight into the groundwater system and therefore enables early adjustment and adaptation of extraction rates if needed.

The SAltwater MOnitoring System (SAMOS) consists of two main parts: a vertical electrode chain of steel ring electrodes permanently installed in a backfilled borehole and a measuring system at the surface. The number of electrodes (commonly about 80) and distance between adjacent electrodes (commonly about 25 cm) is generally flexible. The chain of electrodes is connected to a lightweight and small resistivity meter (LGM, 4-Point light 10W). Thanks to the maximum output current of 100 mA and a voltage of 380 V a low power consumption is achieved and long-term and autonomous monitoring is enabled by solar panel based power supply. Furthermore, the system is designed to run predefined measurement protocols and transfers the data to a remote server immediately after a measurement is performed. SAMOS is commonly installed in the transition zone between fresh- and saltwater allowing the detection of very slight resistivity changes (less than 1 Ohmmeter). While first systems were completely manufactured by LIAG, the latest subsurface systems were built by Solexperts which allowed us to include temperature sensors.

We present data from four SAMOS systems currently running at different locations. Two of them are installed in the central part of the freshwater lense of the North Sea island Borkum (in cooperation with Stadtwerke Borkum) in depths between 44 m and 65 m below the surface, close to freshwater wells of the local water supplier, thus monitoring the overall groundwater system and delivering data since 2009. Even though measurements immediately after the installation are disturbed by the drilling process and an adjustment to undisturbed natural conditions is observed, adapted inversion schemes allow to use all data. While in most cases only slight resistivity changes are observed up to now, at some depths larger seasonal resistivity changes occur at one Borkum site that can mostly be explained by changes of the groundwater recharge rate and changing pumping activities in a water catchment area. Two further systems have been installed in 2018 and 2020. One is located behind the dune line at the edge of the freshwater lense on the North Sea island Spiekeroog. In cooperation with the local water supply company OOWV (Oldenburg-Ostfriesischer Wasserverband) another system for their groundwater extraction fields is installed near Jever several kilometers from the coast-line used for early warning.