A deep multiaquifer system: an integration of geophysical and hydrogeological data.

The need for good quality fresh water is becoming a global emergency, for both growing exploitation and pollution of water resources. As far as groundwater resources are concerned, they can be threatened both for pollution by human activities, especially in the recharge areas, and, in many cases of overexploitation, by the rising up of deep brackish/salt water. Saving this precious resource demands detailed knowledge of the subsurface, of the potential sources of pollution and of the equilibrium between exploitation and recharge processes. At the same time information about the whole sequence of aquifer bodies at the greatest depths becomes a strategic priority. Careful control and management activities should then be undertaken by public Authorities. In this spirit, in the western sector of the Friuli-Venezia Giulia region (northern Italy) analysis of a large amount of lithological, geomorphological, hydrological and hydrogeological data allowed us to define a) the principal aquifer system of the area down to a depth of about 500 m; b) the geometrical characteristics of the aquifers (thickness, lateral extension,..); c) the hydraulic parameters (permeability, transmissivity); and d) the vulnerability to pollution of each aquifer system. The studied area was chosen since it is characterized by a rich but vulnerable groundwater reservoir, generally composed of several aquifer bodies, each of them giving abundant and good-quality fresh water. In a selected portion of the area, where many holes were drilled for fresh water supply, the conceptual hydrogeological model was integrated by a detailed and repeated three-dimensional (3D) seismic and resistivity survey. The 3D resistivity models, which are described in this paper, were obtained using combined Electrical Resistivity Tomography (ERT) and Time-Domain ElectroMagnetic soundings (TDEM). Two 2D ERT’s and two 3D ERT’s were carried out, mainly aimed at furnishing detailed information about geometry and porosity of the overburden and to calibrate the shallowest TDEM information. The TDEM survey was composed of 15 soundings, located at a mean distance of 200 m, which were repeated in two presumably different hydraulic recharge-exploitation regimes. TDEM data were inverted using a 1D smooth inversion scheme and the 1D models were stacked together to get a local 3D model of the resistivity distribution. Reliable results about the geometry of deepest aquifer were obtained, which were confirmed by a purposedly drilled deep bore-hole. The repeated TDEM surveys show resistivity variations in the exploited aquifers which are in good agreement with direct hydrogeological information.