Electrical resistivity tomography of freshwater-saltwater mixing zone applied to the study of a highly anthropized detrital coastal aquifer (Carchuna-Calahonda, Betic Cordillera, Spain)

Abstract

The biggest greenhouse-crop area in Europe is located on the SE Iberian Peninsula. Because of the important agricultural activity in this area, it is necessary to extract groundwater as a part of the water supply. The Carchuna-Calahonda coastal aquifer, located along the Mediterranean coast of southeastern Andalusia, Spain, is a detrital aquifer almost totally covered by greenhouse crops. In this context, electrical resistivity tomography (ERT) was applied to characterize the freshwater-saltwater interface. Four ERT profiles were carried out, three of them parallel and close to the coastline, the fourth being orthogonal. Conductivity data from a well located close to the ERT profiles contributed to validating the results obtained in the four profiles. The saltwater-freshwater mixing zone reaches from 20 to 200 Ω·m and is located between 1 and 3 meters in depth near the coastline, reaching depths of 8 to 18 meters 450 m northwards along the coastline. This slightly dipping mixing zone evidences limited freshwater recharge in the Carchuna-Calahonda aquifer. The mentioned low recharge setting together with isolated aquifer water-extractions lead to an environment of extremely high aquifer salinization