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

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 v adose zone. For this purpose a generic model of contaminant infiltration in to unsaturated sand was designed. The imaging technique produces two-dimens ional contoured plots of the resistivity distribution before and during con taminant infiltration experiments. During the experiments, dyed NaCl soluti on was released into the model and the change in resistivity associated wit h the contaminant plume evolution was imaged as a function of time and g-le vel. Capillary pressure was monitored constantly by matrix potential probes (tensiometers) in order to investigate the effect of capillary forces on p lume evolution. Tests at Ig (static conditions) and 10g are described in th is paper. Comparison of resulting two-dimensional tomography with observed plume geometry at the end of the Ig test showed this imaging technique to b e highly effective. Contaminant plume evolution in the unsaturated sand model was observed to b e mainly gravity-driven, with plume migration and geometry being strongly a ffected by a tenfold increase in gravity in the centrifuge experiment. It i s concluded that miniaturised electrical imaging can be a useful tool for m onitoring 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. Ho wever, in such cases, resistivity tomography does provide valuable informat ion on residual levels of contaminant fluid retained within the soil after passage of the plume. (C) 2001 Elsevier Science B.V. All rights reserved.