Abstract
ABSTRACTA major problem for the freshwater supply of coastal regions is the intrusion of saltwater into aquifers. Due to extensive extraction of freshwater to suffice increasing drinking water demands and/or in periods of reduced groundwater recharge, the equilibrium state may be disturbed. The result is an upconing or movement of the fresh–saline groundwater interface, which reduces the local drinking water resources at coastal regions or islands. The saltwater monitoring system (SAMOS) is a vertical electrode chain installed in a backfilled borehole. It provides a solution to observe the transition zone in detail, both temporally and spatially. We present monitoring data of the first year from three locations ‐ with different geological conditions that show disturbances in the resistivity distribution that result from the drilling processes. A clayey backfilling, for example, can lead to beam‐like artefacts, and a mixed fluid within the backfilling changes its bulk resistivity, both leading to misinterpretations. We performed data inversion under cylindrically symmetrical conditions in full‐space in order to separate these resistivity artefacts from the undisturbed background. Data inversion reveals that it is possible to separate drilling effects on the resistivity distribution from the undisturbed background. Thus, an interpretation of the natural transition zones can be made immediately after the installation.
Highlights
Seawater and freshwater are in an equilibrium state in coastal regions, forming the well-known saltwater wedge
After introducing the three test sites and the saltwater monitoring system (SAMOS) system in detail, we describe the numerical data analysis that inverts the data into 2D subsurface images, differentiating between the near-borehole and the undisturbed areas, for different time steps
We have presented a methodology for analysing the monitoring data from vertical electrode chains to observe freshwater– saltwater interfaces and their dynamics
Summary
Seawater and freshwater are in an equilibrium state in coastal regions, forming the well-known saltwater wedge. In case of a homogeneous subsurface, it takes the form of a concave plane towards inland, replacing freshwater due to its higher density. Increasing population, human overexploitation and climate change put pressure on coastal regions. The demographic change leads to faster growing cities in the coastal regions. That leads to an extensive extraction of freshwater to suffice increasing drinking water demands. Periods of reduced groundwater recharge may disturb the equilibrium state of the freshwater-saltwater interface. The result is saltwater upconing or a movement of the fresh-saline inter-
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