The migration and degradation of waste organic compounds in groundwater

The disposal of liquid wastes and sludges directly into a lagoon in hydraulic continuity with the saturated zone of a shallow sand aquifer at Villa Farm in Warwickshire, U.K., has produced an extensive plume of groundwater pollution. Oil/water mixtures, effluent treatment sludges containing heavy metals, acids, alkalis, organic solvents, phenols, and paint wastes, have been deposited over the years, and many boreholes have been drilled to determine the extent of the pollution and to investigate geochemical and hydrogeological controls on contaminant migration. A redox zonation occurs in the transition from a heavily polluted and highly reducing zone near the lagoons and at the base of the aquifer, to oxidising conditions in the natural uncontaminated groundwater above. A series of redox reactions predicted by thermodynamics, have been identified, namely, aerobic respiration, denitrification, manganese reduction, nitrate reduction, iron reduction, sulphate reduction, and finally methanogenesis. Heavy metals are strongly attenuated in the anaerobic zone as carbonates and sulphides, but nickel persists at concentrations of 0.1–0.5 mg/l within the plume in association with relatively high molecular weight organic compounds. The character of the dissolved organic carbon in groundwater changes with increasing travel time from the lagoons and metabolites of phenol degradation have been identified. Because methanogenic conditions occur throughout the plume, a soil gas survey above the water table was expected to reveal methane which could be used as indirect evidence of groundwater pollution below. However, methane was only found in trace quantities, and carbon dioxide was found to be a better indicator of groundwater pollution at depth. Profiles of dissolved methane and carbon dioxide through the methanogenic and sulphate reduction zones suggest that methane is being oxidised anaerobically to carbon dioxide within the zone of sulphate reduction.