INVESTIGATING SEASONAL IMPACTS ON SUBSURFACE SALINIZATION IN NORTHEASTERN THAILAND WITH ELECTRICAL CONDUCTIVITY TOMOGRAPHY

Salinization of groundwater resources can be characterized using Electrical Resistivity Tomography (ERT) but has not been fully evaluated in aquifers that are impacted by cyclic and long-term trends in monsoon rainfall. Groundwater and soil salinization in northeastern Thailand is associated with dissolution of rock salt layers in the Mahasarakham Formation, a Cretaceous sedimentary sequence usually containing three layers of rock salt. While parts of the overlying unconsolidated aquifer produce high-quality fresh water, others produce brackish to saline water. We evaluated two ERT profiles obtained from fresh and saline aquifers near Khon Kaen, Thailand to consider the importance of seasonal water levels and ground water salinity in ERT interpretations. Seasonal differences in water depths and specific conductance were monitored in existing irrigation and domestic supply wells where associated lithologic logs were available. Average groundwater depths differed by 50% and 47% at fresh and saline sites, respectively, during rainy and dry seasons, and differences of 34 µS/cm and 97 µS/cm were observed in specific conductance. The ERT profiles portray horizontal layers with higher resistivity in the uppermost layer and lower resistivity in the underlying layer. Water-level depths at both sites do not coincide with the positions of steep ERT resistivity gradients observed at 2.5 m and 5 m at the fresh and saline sites. Fine-grained soil textures at both locations are likely to promote capillary rise and the upward migration of saline pore water in unsaturated sediments, which can significantly influence ERT resistivity patterns. Modelled capillary rise estimates are consistent with the positions of steep ERT resistivity gradients at both locations. The results indicate that seasonal and long-term water-level trends should be considered when evaluating resistivity measurements in saline subsurface conditions, which are essential to successful salinity management.