Migration of contaminants in groundwater at a landfill: A case study 6. Hydrogeochemistry

Abstract

Multilevel point-samplers located on a longitudinal cross-section through the plume of contaminated groundwater in the sand aquifer at the abandoned Borden landfill were used to obtain a suite of small-volume samples for analysis of major ions and of minor and trace-level inorganic constituents. Calcium is the dominant cation and sulphate and bicarbonate are the dominant anions in the contaminant plume, with maximum concentrations of 400, 2000 and 1200 mg l −1, respectively. Beneath the landfill the most highly contaminated water in the aquifer has total dissolved solids of ∼4000 mg l −1. Sulphate and iron are the only inorganic constituents that exceed the recommended limits for drinking water. The plume contains above-background concentrations of dissolved zinc and manganese. No heavy metals or other hazardous inorganic trace elements occur at levels above the maximum limits for drinking water. The plume contains levels of total dissolved organic carbon that are above background levels, ranging from 30 mg l −1 beneath the landfill to 5 mg l −1 near the front of the plume. The landfill, which is almost entirely above the water table, contains high partial pressures of carbon dioxide and methane. The high carbon dioxide levels induce substantial dissolution of calcite, which occurs in sand layers in the landfill, and in the aquifer immediately below the landfill. Calcite dissolution is the origin of much of the dissolved inorganic carbon and some of the calcium in the plume. Most of the sulphate and much of the calcium in the plume appears to be derived from gypsum in construction debris in the landfill. Groundwater beneath the landfill is saturated with respect to gypsum and slightly supersaturated with respect to calcite. Some of the dissolved iron in the plume results from the release under reducing conditions of iron from ferric iron coatings on sand grains located in the aquifer beneath the landfill and in the layers in the refuse. Dissolved iron concentrations appear to be limited by the solubility of siderite. Dissolved sulphide is present at only very low concentrations and is probably controlled by ferrous sulphide solubility. Exchange of cations between the aquifer material and the leachate-contaminated groundwater is an apparent cause of calcium release and subsequent precipitation of calcite in the plume. As much of 80% of the K +, 20% of the Mg 2+ and 15% of the Na + in the plume exists on the exchange sites. The effects of dispersive and geochemical attenuation were evaluated from the ratios of chloride to other species along a central flow path from the landfill to the front of the plume. Platinum electrode measurements and electrochemical potentials calculated from equilibrium speciation of analysed constituents varied over a wide range of values and provided poor agreement among methods for the same sample. Only potentials derived from the iron redox couples and Pt-electrode measurements exhibited fair agreement for any one sample. The rather narrow range of Pt-electrode potential values along the central flow line through the plume suggest that the electrochemical potentials are buffered, apparently as a result of Fe(OH) 3(s) /ag FeCO 3(s) equilibria.