The Attenuation of Heavy Metals from Landfill Leachates by Bunter Sandstone and Chalk

Abstract

Extensive quarrying in the Chalk and Triassic Sandstones, which are the two largest aquifers in the U.K., has produced a need for reclamation. Infilling these quarries with waste materials will create a potential pollution hazard to underground water supplies. Laboratory experiments were set up to investigate the natural chemical processes by which the heavy metals and alkaline earth metals in landfill leachates may be attenuated by movement through these rocks. Results obtained indicate that carbonate precipitation is the most important mechanism in heavy metal uptake by the Bunter Sandstone, although cation exchange on clays and a possible two stage uptake by iron sesquioxides also play significant roles. Precipitation of the metals as carbonates was found to govern heavy metal mobility in Chalk groundwater. Further experiments to compare heavy metal uptake by, both rock types in the absence and presence of the other common constituents of leachate and under varying pH conditions were made. The leachate constituents tested were the alkaline earth metals, ammonium, chloride and short chain carboxylic acids. The results of those experiments showed that the initial solution pH has little importance due to the buffering capacity of the Bunter Sandstone. Neither alkaline earth metals nor chloride had any adverse effects on heavy metal uptake. Metal uptake is decreased in the presence of ammonium ions and by increasing concentrations of the carboxylic acids, with acetic acid having greatest effect. The conclusions reached imply that heavy metals are unlikely to contribute seriously to groundwater contamination in either the Bunter Sandstone or Chalk provided that an adequate unsaturated zone is left beneath the fill to maintain aerobic conditions.