Electrical methods (VES and ERT) for identifying, mapping and monitoring different saline domains in a coastal plain region (Alt Empordà, Northern Spain)

The spatial variability through time of the degree of salinity of soils and groundwater is a major issue concerning the sustainable management of water resources. The problem is accentuated in coastal plain areas where saline bodies of different origin coexist. We present an extensive geoelectrical resistivity dataset collected in the Alt Empordà clastic coastal aquifer system (NE Spain) integrating the results of field campaigns undertaken within a period of more than two decades. The dataset is unique in the literature and comprises highly valuable information for the investigation of saline domains. The joint application of Vertical Electrical Sounding (VES) and Electrical Resistivity Tomography (ERT) methods allowed the identification of saline domains and monitoring their evolution through time. Geophysical data gathered during field campaigns in 1982, 2002 and 2010 were calibrated and interpreted on the basis of soil and groundwater physico-chemical analyses and hydrogeological, stratigraphic and geomorphological data. The results illustrate the potential of electrical resistivity methods at differentiating saline domains in coastal aquifer areas and at modelling their configuration and evolution. Although the VES data processing allows solving the one-dimensional resistivity structure of the subsurface, the inversion imposes a 1D layered model, thus interpretation of two-dimensional structures is subject to the interpolation between discrete measures. In contrast, ERT data offer a continuous 2D-image of the resistivity distribution, both laterally and in depth. Given that the bulk resistivity of the medium is extremely sensitive to salt content, the ERT profiles permitted identifying and determining the geometry of hyper-saline areas (characterized by resistivities of less than 5 Ω m). The study demonstrates the efficiency of the electrical methods to map subsurface conductive zones and illustrates how the accurate knowledge of the hydrogeological pattern results critical to meet this objective.