An impedance-computed tomography (ICT) algorithm and system, known as the Electroscan system, for impedance imaging with surface measurements is described. The algorithm determines the current flow paths iteratively and employs sparse-matrix techniques. The measurement system uses a grid of electrodes over the ground surface and introduces controlled currents into a subset of electrodes in a prescribed sequence. Voltage measurements (using high-impedance equipment) are taken at passive electrodes only. Tests are reported for controlled saline plumes injected into a porous media flow field in a laboratory plume, and an uncontrolled leachate plume coming from a sanitary landfill located in floodplain alluvium.