A permanent automated geoelectrical imaging system was installed at a contaminated land site to monitor\ud resistivity changes associated with groundwater quality after the completion of a remediation programme.\ud The former gasworks site had been designated statutory contaminated land due to the risks of pollution of\ud an underlying minor aquifer. The system collected data at regular scheduled intervals from a network of\ud electrodes arranged in vertical borehole arrays on the boundaries of the site. The incoming data from the\ud system were automatically inverted to produce 4D resistivity images. Analysis was carried out over one\ud year, revealing resistivity fluctuations in the infilled ground beneath the tarmac that were strongly\ud temperature dependent. But in the underlying sand and gravel aquifer there was a steady increase in\ud resistivity that was suggestive of a reduction in groundwater contamination after the removal of the\ud contaminant source zones. A tracer test was also undertaken to investigate the groundwater flow velocity\ud and demonstrate rapid 4D geoelectrical monitoring of natural attenuation processes. The motion and\ud evolution of the tracer were visualised directly in high-resolution volumetric images in near realtime.\ud Seepage velocities measured from the images agreed closely with estimates based on the piezometric\ud gradient and assumed material parameters
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