Hydrogeophysical imaging of an aquifer in the Talysh mountain area, Azerbaijan

Underground water resources face an increase stress due to human activities and global climate change. To ensure sustainable and effective management of water resources, it is important to identify and characterize hydrogeologic systems and associated processes. In Azerbaijan, the groundwater is unevenly distributed due to a wide variety of climate conditions and the lowland areas depend mainly on water supply from the mountain areas to subsist. Especially since the alluvial plain aquifers undergo over consumption and pollution due to industrial and agricultural activities.

Our study focusses on the Talysh Mountain area in the Lesser Caucasus basin, where aquifers are characterized by alluvial terrain over volcanic-sedimentary tuff. This implies a high flow rate and quick discharge to low land. Yet, this area is also characterized by higher precipitation than evaporation which makes it favorable to host consequent groundwater aquifer. Geophysics has proved many times that it can bring valuable information to hydrogeological issue in subsurface environment such as unconsolidated ground or weathered hard rock aquifers. Thus, this project aims to characterized the underground structure (lithology, weathering profiles, fault and fissures, etc.) of a mountain aquifer in the region with Electrical Resistivity Tomography (ERT). We realized three separate measurements along the Lenkaran river which should enable us to image the 2D heterogeneity at the catchment scale and identify preferential pathways which influence the hydrodynamic circulations.

All three tomographies display two main resistivity layers which seem to be linked to the location along the river. The first layer shows resistivities in between 100 Ω.m with 3 m thickness upstream to 600 Ω.m and 6 m thickness downstream. The second layer is less resistive as a whole and goes from 60 Ω.m upstream to 15 Ω.m downstream. The resistivity limit in-between these two layers is rather abrupt and appear more linear downstream whereas the two profiles up stream display a non-linear limit. We interpret the first layer as dry alluvial sediments over a sedimentary tuff with an irregular top limit. The difference of resistivity from upstream to downstream could be linked to small changes in the lithology as well as variations of water content.

To help interpret further the hydrodynamic circulations in the region, these geophysical images will need to be associated with two essential data: groundwater levels and rainfall. Hydrogeology and hydrogeophysics campaign are rarely applied in Azerbaijan, especially in mountain areas, and existing data are either not available or date back to the 1960’s. This study represents the first step in developing environmental geophysics research in Azerbaijan and help put light on key environmental issues in the country.