EFFECT OF HYDROGEOLOGICAL HETEROGENEITY ON SPATIAL DISTRIBUTION AND MOBILIZATION OF ARSENIC BENEATH A CLOSED LANDFILL

Arsenic has been shown to be mobilized in glacial sediments underlying a landfill by reducing, leachate-affected groundwaters. For the heterogeneous sub-landfill aquifer, relationships between grain size, tracer velocity, hydraulic conductivity (K) measured by heat pulse flowmeter/slug tests, and down hole electromagnetic (EM) conductivity were examined. Of these hydrogeologic parameters, only grain size and tracer velocity correlate with one another (p < 0.001) while K and EM conductivity do not correlate with any hydrogeologic parameter. Contaminant transport was examined with statistical correlations between aqueous and hydrogeologic parameters; elevated groundwater concentrations of contaminants (e.g. arsenic and landfill leachate tracers) were found predominantly associated with large grain sizes, high purposefully injected tracer velocities, and low EM conductivity. However, elevated contaminant concentrations were also found associated with low hydraulic conductivity (K) measured by heat pulse flowmeter (HPFM), in direct conflict with the tracer velocity and grain size data sets and possibly in conflict with the EM conductivity data set. Observations indicate that tracer velocity is the most robust predictor of the arsenic geochemistry of the relevant groundwaters. The data support the conceptual model that landfill leachate predominantly affected more highly conductive regions; this has led to greater mobilization of sediment arsenic in these regions.