Attenuation of Potential Pollutants in Landfill Leachate By Lower Greensand

Abstract

Unsaturated zone attenuation of contaminants common to landfill leachate, in particular heavy metals and carboxylic acids, has been studied in an in situ lysimeter constructed in calcareous clayey-sandstone (Lower Greensand), for a period of nearly 6 years. Considerable attenuation of heavy metals was found to be attributable to a combination of processes including adsorption and precipitation. Leaching with a blank carboxylic acid solution (volatile fatty acids containing no heavy metals) resulted in limited remobilization and the order of mobility of the metals (Ni > Cd > Zn > Cu > Cr > Pb) was found to be unchanged, with only nickel showing extensive remobilization. High heavy metal concentrations at the surface of the lysimeter were found to cause inhibition of carboxylic acid degradation. However at depths in the lysimeter, below the limit of heavy metal penetration, anaerobic decomposition rates of approximately 100 mg 1^-1 day^-1 for acetate, propionate and butyrate were obtained by extrapolation to input concentrations of 5000 mg 1^-1. The degradation of the carboxylic acids was found to be dependent upon the establishment of an adapted population of micro-organisms and a single pulse of carboxylic acids was released before leachate-degrading micro-organisms could colonize the sediment.