GEOPHYSICAL ASSESSMENT OF LANDFILL CAP INTEGRITY AND LEACHATE LEAKAGE AT A MIXED WASTE LANDFILL

Concerns have been expressed about a closed, mixed-waste landfill in Southwestern Ontario regarding precipitation infiltration, because of a non-contiguous cap and potential leachate leakage. This study aims to use two surface geophysical techniques (conductivity and resistivity profiling) to delineate the thickness of the landfill cap and locate any leachate plumes. The electrical imaging survey was conducted on 6 parallel E-W lines across the length of the landfill, using an ABEM Lund Imaging System. The surveys were made using a Wenner-α array with 2 meter electrode spacing. A conductivity survey, using a DualEM2S and DualEM4S, was conducted over the same lines, followed by a survey conducted on 15 N-S lines perpendicular to the original six lines. Several conductivity profiles were also made in a field just to the south of the landfill. ArcGIS geospatial analysis modeling software was used to produce conductivity maps for depths of 1, 2.2, 2.8 and 5.8 m (DualEM2S PRP, HCP and DualEM4S PRP and HCP, respectively). All showed similar patterns. The conductivity values varied considerably (2.2-461mS/m). Several N-S linear anomalies were evident extending outside of the landfill towards the adjacent field. On the resistivity profiles, a number of low resistivity anomalies, near the surface, suggested that the landfill cap was not intact. Deeper low resistivity anomalies (> 5 meters) suggested the presence of leachate and was consistent with the known position of leachate within the landfill. The conductivity data correlated well with the resistivity surveys to delineate inconsistencies in the landfill cover and indicated the presence of leachate which may affect the field adjacent to the landfill.