TDEM Soundings as a Tool to Determine Seasonal Variations of Groundwater Salinity (Villafafila Lakes, Spain)

Interaction between groundwaters with different salinities and lakes show seasonal variations driven by changes in precipitation and evapotranspiration. In the vicinity of Villafafila lakes, local fresh and brackish regional groundwaters feeds the lakes, forming a brine in the lake sediments aquitard. Two TDEM surveys (summer 2019 and winter 2020) were carried out. Five TDEM soundings were acquired at the same location for each survey, forming a profile from the hills to the lake-shore. Simultaneously to the TDEM surveys, electric conductivity of lake water and groundwater was measured. The resistivity boundary between the local fresh (10-35 Ohm/m) and regional brackish groundwater (2-5 Ohm/m) is well marked at 600 m above sea level (masl) below the hills, and at 650 masl below the lowlands surrounding the lakes. During the summer, fresh-brackish groundwater interphase rises due to evaporative pumping occurring in the lowlands. This increases groundwater salinity close to the terrain surface favoring precipitation of halite efflorescences. Annual record of EC in a piezometer confirms the summer ascendant of the brine contained in the lake aquitard. TDEM sounding is fast and simple technique to monitor seasonal variations in fresh-brackish groundwater interphase and to detect possible salinization of consumption wells and environmental changes.