A saline trace test monitored via time-lapse surface electrical resistivity tomography

A saline tracer test has been performed at an industrial site, located in the Po River delta region (North-Eastern Italy). This tracer test was aimed at identifying possible hydraulic connections between a shallow unconfined aquifer and a deeper confined aquifer. Hydraulic head difference between the two aquifers is maintained via an active hydraulic barrier (pumping wells) screened in the deeper formation. The movement of the tracer in the subsurface has been monitored via electrical resistivity tomography (ERT) from the surface, using a network of twelve 48-electrode lines, for a total time period of about 45 days. This non-invasive geophysical method offers several advantages over direct water sampling from monitoring boreholes: (a) a wide area is investigated in a relatively short time; (b) a 2D picture of the subsurface is produced, in this case to a depth of about 15-20 in; (c) because of the above, it is possible to follow the tracer motion even if the tracer does not reach the few boreholes where water can be sampled. The latter is a major advantage over traditional methods, where it is not uncommon to miss the tracer plume altogether, or sample it only partially. The site is characterized by fairly saline groundwater, and the presence of diffuse silt and clay, that make the system rather conductive. Under such unfavourable conditions, careful inversion of resistance ratios was needed to identify changes in resistivity of as much as 50% with respect to background. The migration of the tracer was successfully monitored and confirmed by direct sampling of water from few boreholes. The results of the time lapse survey confirm that the system is very heterogeneous, and that some downward migration of the tracer through the confining layer can take place over relatively short time periods, maybe through some manmade connections. (C) 2005 Elsevier B.V. All rights reserved.