Assessing contaminant migration pathways and vertical gradients in a low-permeability aquifer using multilevel borehole systems

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A comparison of groundwater and gas monitoring data is provided between multilevel and conventional long-screened boreholes constructed around the perimeter of a closed landfill in the English Midlands. Multilevel boreholes were constructed with up to seven separate sampling ports. All boreholes were constructed into strata of the Triassic Mercia Mudstone Group, which proved to consist of a succession of well-bedded, weathered mudstone and clay with occasional thin beds of siltstone and sandstone to approximately 30 m depth. These are low-permeability strata which exhibit sufficient fracture permeability to allow groundwater movement. It is suspected that fractures, particularly along bedding planes, provide discrete pathways for both gas and leachate migration from the landfill. The importance of purging channels prior to sampling both aqueous and vapour phases is emphasised in order to minimise diffusion biases. Monitoring records for the multilevel boreholes demonstrate that there are significant vertical differences in groundwater level, and groundwater chemistry. A predominantly downward vertical hydraulic gradient is proven in all the multilevel boreholes, with a head difference of up to 5 metres over a vertical distance of 13 metres between uppermost and lowest sampling ports. Chloride concentration in groundwater varies from a background of below 50 mg/L to in excess of 700 mg/L within ports of the same multilevel borehole. Total organic carbon varies vertically from less than 10 mg/L to over 50 mg/L. None of this detail is apparent in data gathered from the adjacent long-screened monitoring boreholes, which in general recorded concentrations which were significantly below the maximum in the neighbouring multilevel.