Rising saltwater, caused by either anthropogenic or natural effects, can threaten freshwater aquifers that are used for water supply. The strong resistivity contrast between saltwater and freshwater due to ion concentration allows for detecting and monitoring changes of water quality early by means of electric methods. The project SaMoLEG uses steel-cased boreholes for large-scale ERT monitoring.
Anthropogenic or natural influences can lead to saltwater contaminations of freshwater aquifers, which are vital for the regional supply of drinking water. The different electrical conductivities of salt- and freshwater allow for a detection and monitoring of changes in freshwater aquifers by electric or electromagnetic methods.
The main objective of SaMoLEG is the monitoring of the electrical conductivity of the subsurface to locate and observe the saltwater-freshwater interface. The possibility of using steal casings as long electrodes for geoelectric (ERT) measurements was investigated, to develop a cost efficient monitoring.
In the first step of the project extensive synthetic studies were conducted to model the underlying problematic using the software BERT (Boundless Electrical Resistivity Tomography). Comparisons of different approaches for implementing boreholes as long electrodes showed that the Shunt Electrode Model (SEM) provides the best trade-off between accuracy and computational effort (Ronczka et al., 2015). Figure 1 shows a distribution of the electric potential and current density for a pair of long electrodes. Homogeneous sensitivities in the depth range of the long electrodes lead to the assumption of a reduced vertical resolution. However, part of the vertical resolution can be regained by combining boreholes of different length or long electrodes with surface electrodes (see figure 2).
The second step involved surveys on two test sites to show the feasibility of the developed method on different scales and a monitoring. An optimisation algorithm based on the sensitivities increased the information content of the surveys. A monitoring was installed on a medium-scaled test site (500x300m) with support of BLM-Storkow. Inversion was done with BERT (Günther et al., 2006). A saltwater contamination was verified and results of the monitoring showed a decreasing salinity (figure 3). Finally, the developed method was successfully applied on a large scaled 4x4 km test site.