| dc.description.abstract | Groundwater degradation by salinization is one of the key concerns groundwater companies have to deal with. In order to monitor saline groundwater transitions, drinking water company Vitens uses approximately 110 Permanent Electrode Cable Systems (PECS). These devices consist of seven or thirteen electrode couples separated over depth, whereby resistances measured between two electrodes correspond to the conductance of both groundwater- and soil. Consequently, by using this method, it is possible to obtain more knowledge about groundwater conductance variabilities and consecutively chloride concentration alterations (Helderman, 2016; Goes et al., 2009).
However, Vitens does not adhere a strict salinization monitoring policy yet. Current PECS datasets consist of errors and monitoring devices are not optimally used. For determining the reliability of current PECS, it is necessary to calibrate PECS with nearby chloride concentrations. Herewith, it is possible to evaluate whether PECS correctly detect alterations in groundwater conductance. In this study it is aimed to both verify current PECS and to evaluate future salinization monitoring methods.
From the data-analysis, it appeared that a significant part of the current PECS does not respond correctly on chloride concentration alterations. In general, trends between PECS resistances and chloride concentrations were absent in low chloride concentration environments ([Cl-] < 50 mg/l). More salinized environments alternately resulted in clear relationships between PECS resistances and chloride concentrations. However, prolonged periods with obvious trends between PECS resistances and chloride concentrations are absent. Current PECS are possibly influenced by local soil conductors, measured incorrectly or not sensitive enough.
Additionally, a modelling study is conducted in order to verify the abilities for salinization monitoring by using hydrological models. In combination with observation well samples, it was possible to evaluate both current salinization trends, potential salinization origins and future salinization predictions.
Another monitoring device consisting of large potentials is the EM-Slimflex logger. Past studies showed reliable relationships between EM-Slimflex measurements and chloride concentrations. Vitens owns a dense network of monitoring wells, in which EM-Slimflex loggers could be fitted. This device determines the soil resistivity by generating an electromagnetic field. Also, by conducting natural gamma radiation, it is possible to review local lithological profiles.
In conclusion, PECS are probably not the most optimal method for salinization monitoring in the entire Vitens extraction area. Groundwater salinization monitoring by using a combination between hydrological models and in-situ groundwater samples is probably more reliable and more efficient. | |
