ATTENUATION OF LANDFILL LEACHATE POLLUTANTS IN AQUIFERS

Landfill leachate contains a variety of pollutants that may potentiall y contaminate the groundwater 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 lea chate-affected aquifers. After an introductory section on leachate com position, the physical and chemical frameworks for the attenuation pro cesses are discussed in terms of dilution/dispersion and redox zones i n the plume, respectively. A separate section focuses on the microbiol ogy in terms of the occurrence of bacteria in plumes, the fate of path ogens, and microbial mediation of redox processes. In individual secti ons, the attenuation of dissolved organic matter, anthropogenic-specif ic organic compounds, inorganic macrocomponents as anions and cations, and heavy metals are discussed. The focus is on laboratory experience s and field investigations. The review shows that most leachate contam ination plumes are relatively narrow and do not, in terms of width, ex ceed the width of the landfill. The concept of redox zones being prese nt in the plume has been confirmed by the reported composition of the leachate-contaminated groundwater at several landfills and seems to co nstitute an important framework for understanding the compositional ch anges in the plume as the leachate migrates away from the landfill. Di verse microbial communities have been identified in leachate plumes an d are believed to be responsible for the redox processes. Dissolved or ganic carbon in the leachate, although it appears to be only slowly de gradable when the volatile organic acids are gone, apparently acts as substrate for the microbial redox processes. Several anthropogenic-spe cific organic compounds have been found to be degradable in leachate-c ontaminated groundwater, but much remains to be learned about degradat ion under anaerobic redox conditions. Apparently, observations in actu al plumes indicate more extensive degradation than has been documented in the laboratory. The behavior of cations in leachate plumes is stro ngly influenced by exchange with the sediment, although the sediment o ften is very coarse and sandy, and exchange reactions and dilution cre ate a very complicated migration pattern for cations in the plume. Hea vy metals do not seem to constitute a significant pollution problem at landfills, partly because the heavy metal concentrations in the leach ate often are low, and partly because of strong attenuation by sorptio n and precipitation. The information available on attenuation processe s has increased dramatically during the last 10 years, but much remain s to be learned. Apparently, the attenuation processes in leachate plu mes may for many contaminants provide significant natural remediation, limiting the effects of the leachate on the groundwater to an area us ually not exceeding 1000 m from the landfill. This review deals exclus ively with dissolved contaminants from mixed-waste landfills in aquife rs, and most of the information retrieved was on unconsolidated sandy/ gravel aquifers.