GNGTS 2014 - Atti del 33° Convegno Nazionale

electrical behaviour of the sand. The main objective of this study is to analyse in time-lapse the contamination phenomenon with integration of electromagnetic data in presence of a constant water flow and evaluate the transport occurring in porous media in saturated and unsaturated conditions. Then the results obtained are compared with those obtained with a simulated model built with COMSOL multiphysics software to understand the results and consequently to optimize them. Cross-borehole ERT. Electrical Resistivity Tomography based on cross-borehole configurations (Binley et al. , 2002; Kemna et al. , 2002; Cassiani et al. , 2006; Deiana et al. , 2007) is widely used to study and monitor fluid-dynamics and contaminant flow in the subsurface in the vadose and saturated zone. In cross-borehole ERT, quadripoles resistance measurements are made using electrodes in two or more boreholes. Maps of resistivity are obtained after inversion of the resistance data adopting finite element methods that allow computing the show resistivity values which satisfies both the measured dataset and some a priori constraints, in order to stabilize the inversion and constrain the final image (deGroot-Hedlin and Constable, 1990). The approach with cross borehole resistivity imaging provide a great advantage compared to more conventional surface electrical resistivity tomography, due to the high resolution at high depth (obviously depending on the depth of the well instrumented for the acquisition). Furthermore there are two critical issues; the first one is due to borehole electrodes characteristics that usually cause a high data noise levels, the second one is the reciprocal distance of the boreholes that reduce the sensitivity of the analysis. Using the fundamental Archie’s law (1942) that describes with an empirical relationship the complex electrical behaviour of the soil considering the formation factor F, porosity ϕ , matrix cementation m, pore geometry a, degree of saturation (s) and n saturation exponent, it’s possible write: (1) The equation may be rearranged considering the formation factor F defined as: (2) to obtain in the saturated zone: (3) TDS of natural waters can be measured by standard gravimetric techniques or by the use of conductivity/ TDS meters. The specific conductance (electrical conductivity normalized to 25°C) of groundwater is directly related to the TDS based on the assumption that TDS in the water consist mainly of ionic constituents that conduct electricity (e.g., Wood, 1976; Hem, 1985; Lloyd and Heathcote, 1985). The relationship between electrical conductivity σ (or resistivity) and salt concentration in the water expressed as TDS (total dissolved solute, is a measure of the total ions in solution) is: (4) After the evaluation of formation factor and resistivity values with the Eq. (4) is possible calculate the concentration of saline contaminant present in the subsoil. For a correct assessment of the TDS, is very important assess the formation factor. Resistivity maps in uncontaminated conditions can provide formation factor maps to be used subsequently to assess TDS concentrations in the water. For the case investigated the resistivity of formation water was equal to 0.3 mS/cm and the resistivity values of the soil are calculated according each maps acquired for the different sequences before the injection of NaCl solution. Experiment set-up. At the CNR-IMAA “Hydrogeosite” laboratory in Marsico Nuovo a controlled system, equipped with an automatic system for the simulation of the water level fluctuations, was built to study the spatial and temporal dynamics of a phreatic aquifer through hydrogeophysical techniques. The simulated aquifer consists of a sand box filled with about 1 mc of homogeneous silica 216 GNGTS 2014 S essione 3.3