Redox processes in groundwater impacted by landfill leachate

Groundwater downgradient from Trandum municipal landfill was investigated in summer 1996, with a view to assessing the distribution of redox-sensitive species. The water table at Trandum is 10-26 meters below the surface. Monitoring well filters have been installed at depths of up to 15 m below water table, covering an area of 400 m by 200 m. Groundwater influenced by the landfill seems to be confined to this area. Pristine groundwater has a chemical signature indicative of calcite and pyrite weathering. Groundwater influenced by leachate from the landfill exhibits an excess of alkalinity relative to calcium, which is likely to be derived from degradation of organic matter in the landfill. Groundwater immediately below and downstream of the landfill contains elevated concentrations of Fe and Mn, largely mobilized under reducing conditions from the aquifer matrix and reflected in depleted HNO3-extractable Fe and Mn in sediment samples from the same area. Groundwater samples allow the tentative identification of redox zones based on oxidized and reduced forms of Fe, Mn, N and S. A methanogenic zone is not observed.