Attenuation of landfill leachate pollutants in aquifers

Abstract

Landfill leachate contains a variety of pollutants that may potentially contaminate the ground water and affect the quality of surface waters and well waters. The literature has been critically reviewed in order to assess the attenuation processes governing the contaminants in leachate‐affected aquifers. After an introductory section on leachate composition, the physical and chemical frameworks for the attenuation processes are discussed in terms of dilution/dispersion and redox zones in the plume, respectively. A separate section focuses on the microbiology in terms of the occurrence of bacteria in plumes, the fate of pathogens, and microbial mediation of redox processes. In individual sections, the attenuation of dissolved organic matter, anthropogenic‐specific organic compounds, inorganic macrocomponents as anions and cations, and heavy metals are discussed. The focus is on laboratory experiences and field investigations. The review shows that most leachate contamination plumes are relatively narrow and do not, in terms of width, exceed the width of the landfill. The concept of redox zones being present in the plume has been confirmed by the reported composition of the leachate‐contaminated groundwater at several landfills and seems to constitute an important framework for understanding the compositional changes in the plume as the leachate migrates away from the landfill. Diverse microbial communities have been identified in leachate plumes and are believed to be responsible for the redox processes. Dissolved organic carbon in the leachate, although it appears to be only slowly degradable when the volatile organic acids are gone, apparently acts as substrate for the microbial redox processes. Several anthropogenic‐specific organic compounds have been found to be degradable in leachate‐contaminated groundwater, but much remains to be learned about degradation under anaerobic redox conditions. Apparently, observations in actual plumes indicate more extensive degradation than has been documented in the laboratory. The behavior of cations in leachate plumes is strongly influenced by exchange with the sediment, although the sediment often is very coarse and sandy, and exchange reactions and dilution create a very complicated migration pattern for cations in the plume. Heavy metals do not seem to constitute a significant pollution problem at landfills, partly because the heavy metal concentrations in the leachate often are low, and partly because of strong attenuation by sorption and precipitation. The information available on attenuation processes has increased dramatically during the last 10 years, but much remains to be learned. Apparently, the attenuation processes in leachate plumes may for many contaminants provide significant natural remediation, limiting the effects of the leachate on the groundwater to an area usually not exceeding 1000 m from the landfill. This review deals exclusively with dissolved contaminants from mixed‐waste landfills in aquifers, and most of the information retrieved was on unconsolidated sandy/gravel aquifers.