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Combining a geoelectrical survey with integrated groundwater quality data to map the spatial distribution and temporal variations of a leachate plume in a closed landfill (Southern Ontario, Canada)

Abstract:

Geoelectrical surveys are not used as often as borehole water quality (WQ) analyses in standard practices for characterizing and monitoring contaminated sites. This work puts forward a methodological approach combining current results of these two techniques to study a closed landfill in southern Ontario. The data examined are the ground-WQ record for this site and a pseudo-3D visualization of a subsoil low resistivity anomaly. A Gaussian Mixture Model (GMM) classifier and a data stacking algorithm are applied to group and map the WQ information. This is the first time these two numerical methods are employed to study a landfill, bounding the site in zones with distinct contamination levels. In addition, the geoelectrical data in the pseudo-3D visualization were validated by overlapping the maps of integrated WQ information on a slice that cuts the resistivity cube through the boreholes' screening depth. The result of this comparison defines a continuous area of maximum conductivity and pollution levels that can be overextended to an isoresistivity 3D surface enclosing the approximate volume of the leachate plume. Besides, the historical records of rainfall and integrated WQ data are used to track down the landfill's impact on an unconfined aquifer. The interannual and interseasonal variations of the contamination zoning might be related to the rainfall-mediated spreading of the leachate plume and/or the dissolution or concentration of pollutants in the aquifer during wet and dry periods respectively. The landfill's final phase of waste maturation and stabilization also seems to have an influence on these changes.