Aerobic Oxidation of Municipal Solid Waste Leachate As It Cascades from a Perched Aquifer Through an Unsaturated Zone to a Regional Aquifer

A municipal solid waste landfill for a small city in British Columbia, Canada, is situated atop a perched aquifer that drains off the edge of the clay aquitard through a 27 m thick unsaturated zone to a regional aquifer.  This groundwater cascade is situated near an escarpment that promotes recharge of oxygen from the atmosphere.  The landfill leachate includes elevated concentrations of ammonia and bicarbonate, and the landfill produces an estimated 800 tonnes of methane per year.  Due to the high gas permeability of the soils within the cascade, there is very efficient mass transfer of gases into and out of the groundwater.  Biogeochemical calculations of the mass of ammonia (to nitrate) and methane oxidized by soil gas oxygen, and the corresponding production of nitrogen gas and carbon dioxide indicate that negative soil gas pressures would be expected.  This negative soil gas pressure is primarily due to the consumption of oxygen due to ammonia and methane oxidation.  This interpretation of negative soil gas pressure is supported by groundwater water level data which showed fluctuations in the barometric pressure corrected data that could not be attributed to groundwater pumping or temperature fluctuations.  The negative soil gas pressure generated by the biogeochemical reactions raises the water level by as much as 0.10 m.  This interpretation of dynamic gas flow near the groundwater cascade suggests that future site characterization efforts should focus on soil gas monitoring.  The resulting plume in the regional aquifer includes nitrate and selenium mobilized from the aquifer solids by the oxidation of the landfill leachate within the cascade.