Lateral Chloride Migration from a Landfill in a Fractured Clay‐Rich Glacial Deposit

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Abstract
A plume of chloride (100 to > 1000 mg/L) was found to extend up to 40 m laterally from the edge of a landfill located on a thick sequence of clay‐rich glacial deposits. The plume is mainly confined to the seasonally saturated highly weathered and fractured zone (upper 3 to 4 m) of the deposits. In the high concentration region of the plume there was good agreement between chloride concentrations determined from piezometer water samples, which were expected to sample fracture pore water, and pore water extractions from continuous core samples, which were expected to be dominated by pore water from the fine‐grained matrix. The pore water extractions provided much better lateral and vertical resolution of the plume than was obtained from the piezometer water samples. The length of the plume based on the data from the piezometer water samples was 12 m less than the length determined from the core samples, apparently because the piezometers were located above or below the relatively thin plume, or were above the water table at the time of sampling. Chloride concentrations in many of the deeper piezometers were higher than measured in core samples. This was likely due to inflow from the shallow highly contaminated zone during drilling, as indicated by declining concentration values measured in many of these piezometers over a 10‐month period. The core samples were protected by a liner and were not significantly influenced by drilling‐induced contamination. Downhole electrical conductivity profiles measured with a Geonics EM‐39 correlated well with chloride concentration profiles determined from the core samples. Surface measurements of electrical conductivity with a Geonics EM‐31 were effective at detecting the edge of the waste (which was buried under 2 m of clay), but were less effective at detecting the lateral extent of the chloride plume. Based on this and previous investigations, it is expected that an equivalent porous medium (EPM) approach will be appropriate for simulating lateral contaminant transport in the highly weathered and fractured zone. However, predictive modeling of the behavior of the chloride plume will be complicated by the presence of multiple possible sources of chloride and uncertainty concerning the hydraulic conductivity distribution, as well as by difficulty in predicting the influence of large seasonal variations in the elevation and slope of the water table.