Combined use of tracer and ERT tests as effective tool for the investigation of heterogeneous alluvial aquifers

The presence of preferential groundwater transport was investigated in the heterogeneous alluvial aquifer of the upper Alento River, located in Campania (southern Italy), in a fluvial sector characterised by braided geomorphology and sedimentary deposition. Groundwater tracing methods were applied by means of two-well tracer tests (natural and forced-gradient tracer tests) and the inflow of a fluorescent dyes detected by a mobile downhole fluorometer. Tracer concentrations monitored over the entire screened interval of the well (8 m long) and the breakthrough curves obtained at different depths highlight that the tracer appears only over a limited length of the screen interval. The results suggest that groundwater flow occurs trough alternative paths according to the complex geometrical and granulometric features of the underlying deposits, so that groundwater flow (direction and specific discharge) varies with depth. These results have been confirmed by using of electrical resistivity tomography (ERT) that indicates, within alluvial deposits, the presence of two distinct electrostratigraphic horizons. The topmost superficial one, characterised by greater resistivity values, can be related to more coarse alluvial sediments with little finegrained matrix and likely to more permeable materials. This evidence supports the thesis of a non-uniform groundwater flow within a high heterogeneous aquifer. The results emphasise the usefulness of tracer tests and ERT as effective tools for the investigation of aquifer heterogeneity in studies concerning the fate of dissolved pollutants.