An assessment of miniaturised electrical imaging equipment to monitor pollution plume evolution in scaled centrifuge modelling

Abstract This paper demonstrates the capabilities and limitations of a miniaturised electrical imaging technique (resistivity tomography) developed at Cardiff University to image contaminant plumes in scaled centrifuge models of the vadose zone. For this purpose a generic model of contaminant infiltration into unsaturated sand was designed. The imaging technique produces two-dimensional contoured plots of the resistivity distribution before and during contaminant infiltration experiments. During the experiments, dyed NaCl solution was released into the model and the change in resistivity associated with the contaminant plume evolution was imaged as a function of time and g -level. Capillary pressure was monitored constantly by matrix potential probes (tensiometers) in order to investigate the effect of capillary forces on plume evolution. Tests at 1 g (static conditions) and 10 g are described in this paper. Comparison of resulting two-dimensional tomography with observed plume geometry at the end of the 1 g test showed this imaging technique to be highly effective. Contaminant plume evolution in the unsaturated sand model was observed to be mainly gravity-driven, with plume migration and geometry being strongly affected by a tenfold increase in gravity in the centrifuge experiment. It is concluded that miniaturised electrical imaging can be a useful tool for monitoring pollution plume evolution during centrifuge tests, but when plume evolution is rapid, the time taken to interrogate each array restricts the effectiveness of the technique in monitoring changes in plume geometry. However, in such cases, resistivity tomography does provide valuable information on residual levels of contaminant fluid retained within the soil after passage of the plume.