The use of 3D electrical resistivity tomography to characterise waste and leachate distribution within a closed landfill, Thriplow, UK

Electrical Resistivity Tomography (ERT) has been used to map the 3D spatial distribution of waste and leachate concentrations within a closed and unconfined landfill. Borehole sampling and a 2D ERT survey down-gradient of the landfill boundary had failed to detect a pollution plume. Accordingly, a 3D survey was undertaken to determine the pattern of leachate drainage within the waste so that a more refined contaminant transport model could be developed. A full 3D survey was undertaken by sub-dividing the landfill into a number of discrete rectangular blocks and acquiring data on multiple parallel lines. The line data were merged into a single x-y matrix file and then inverted using a 3D finite element algorithm. The results are presented as 3D volumetric tomograms to show the inferred waste distribution and leachate flow-paths. The resistivity models indicate that leachate has accumulated at several discrete localities within the landfill. The controlling mechanism appears to be the depth and geometry of the original Chalk quarries used for waste disposal. The deepest leachate infiltration of the underlying Chalk bedrock invariably occurs below these drainage sinks. A narrow leachate plume is indicated in one part of the landfill, migrating down-gradient to the north-west. The inferred leachate concentrations in the Chalk were subsequently confirmed by drilling and hydrochemical sampling which showed elevated solute levels coincident with distinct resistivity lows of <15 ohm-m