Hydrogeophysical characterisation of a complex aquifer in calcareous and gypsum formations

Borehole and surface geophysical methods can assist hydrogeological site characterization by providing efficiently and cost-effectively information on the continuity of permeable or less permeable zones, and by delineating the geometry of an aquifer. The use of time lapse geophysical surveying offers an effective tool for monitoring dynamic processes within the aquifer (e.g., effect of pumping or recharge of the aquifer), mapping 2D/3D evolution of water infiltration, and estimating the hydraulic parameters of the aquifer. Coupled hydrogeological and geophysical investigations were performed to characterize at small-scale an aquifer at shallow depth (less than 30 m). Highly heterogeneous aquifers consisting of discontinuous sequences of gypsum-marls and carbonates represent a challenging environment for the remediation of a diffuse contamination of chlorinated solvents. The clean-up design by means of injection of a suitable reagent requires an accurate prediction of the aquifer geometry to detect the main productive levels and to estimate their hydrodynamic parameters. The hydro-geophysical characterization was aimed at i) detecting the most permeable and porous level within the gypsum-calcareous formation in a depth range between 10 and 20 m from the surface, ii) observing the hydraulic connection of the main productive levels, and iii) estimating the hydrodynamic properties of the aquifer. The hydro-geophysical characterization involved several approaches. Geophysics aimed at estimating the lateral continuity of the most permeable layers and also estimating the water content of the main productive layers. Ground penetrating radar (GPR) deployed in cross-hole mode between three boreholes and in single-hole configuration allowed the detection of the main productive levels in the gypsum and calcareous formation. Radar signals did not transmit through the high electrical conductivity of marl and clay levels. Electrical resistivity tomography (ERT) in cross-hole configuration confirmed the presence of low resistivity layers (in the order of 10-20 ohm m), responsible for the low penetrating distance of the GPR survey. ERT surveys identified the presence of three main productive levels at depths between 13 m to 19 m. Specifically, the ERT in time lapse configuration showed the sensitivity of the uppermost permeable layer (13-14 m) to the effect of pumping of the aquifer; an abrupt increase of resistivity was noted after pumping, demonstrating the high transmissivity of that level. Single-well flowmeter logs and conventional pumping tests determined the different inflow-outflow zones along each borehole betweeen 15 and 22 m and the vertical profile of aquifer hydraulic conductivity. A characteristic very high vertical ambient flow into the boreholes (up to 3 l/min), due to a strong head difference between the productive levels, was observed. Cross-well flowmeter logs under stressed conditions provided a check of the connections between the productive levels. Moreover, the flowmeter log was also essential for planning and evaluating a dye tracer test. The tracer test involved the injection of dye tracers and the use of a downhole fluorometer, and allowed us to estimate the local aquifer flow velocity of 148 m/d in the most permeable layer. This information is critical to correctly plan the subsequent site remediation activity by means of reagent injection. We demonstrated that in the specific case, the complete hydrogeophysical characterization (hydraulic and geophysics) detects with good accuracy the spatial heterogeneity of the aquifer, permitting to improve the reliability of the design of the remediation activity.