GNGTS 2014 - Atti del 33° Convegno Nazionale

of NaCl, were injected in ten days into the vat through the injection point located at ten cm from the surface with a system without pressure. In order to create a water flux longitudinal compared to boreholes a peristaltic pump pumped water at a velocity of 0.035 l/min for all the experiment. The test was monitored continuously with phreatimeter moved in the four boreholes instrumented for cross-borehole ERT. At the same time conductivity measurements were taken with the two probes. A cross-borehole electrical resistivity survey, based on time-lapse geoelectrical prospecting methods, has been carried out in the investigated area during the tracer test. During the tracer injection, resistivity measurements were made every 6 hours. From the dataset collected, the data acquired every 3 days were inverted to study contaminant flow in the sand. A cross- borehole azimuthal dipole–dipole array, with reciprocal measurements, was adopted and apparent resistivity values were inverted using the R2 code (Binley, 2007). The data error was, for all the inversions, less than 10% and the RMS < 5%. The laboratory test supported and at the same time was supported by a simulation performed with Comsol Multiphysics Version 4.4, a multiphysics software tool for the solution of partial differential equations based on the finite element method. Hydrogeophysical properties used in the model are listed in Tab. 2 and were assumed to be constant throughout the entire solution domain. Tab. 2 - Hydrogeophysical parameter of simulation. Hydraulic permeability K 4e-5[m/s] Porosity ϑ 0.45[dimensionless] α van Genuchten (1980) 0.22 [1/m] n van Genuchten (1980) 1.81 [dimensionless] Bear coefficient 9.2e-7[1/Pa] Dispersivity 0.1[m] Diffusion coefficient 1.5e-9[m^2/s] Residual porosity 0.1[dimensionless] Tortuosity factor 0.7 [dimensionless] The model uses the Richards’ Equation interface to define nonlinear relationships with retention and permeability properties according to van Genuchten as well as Richards’ equation governs the saturated-unsaturated flow of water in the soil and accounts for changes in the fluid volume fraction with time, and also for changes in the storage related to variations in the pressure head according to Bear. The fluid flow is described by the equation: (5) where C denotes specific moisture capacity (m−1); Se is the effective saturation of the soil (dimensionless); S is a storage coefficient (m−1); Hp is the pressure head (m), which is proportional to the dependent variable, p (Pa); t is time; K equals the hydraulic conductivity (m/s); D is the direction (typically, the z direction) that represents vertical elevation (m). Results and discussions. A dataset of about 840 ERTs was acquired to investigate the distribution of the tracer in the subsoil and several radargrams were recorded at different frequencies to analyse the variations in the e-m field due to the presence of a high conductivity substance. Fig. 2 shows the results obtained for the scenario Sc-2 after 30 days from the tracer injection according the illustrated sequences A-B, C-D, E-F. The TDS values are more higher GNGTS 2014 S essione 3.3 219