Biogeochemistry of landfill leachate plumes

The literature has been critically reviewed in order to assess the attenuat ion processes governing contaminants in leachate affected aquifers. Attenua tion here refers to dilution, sorption, ion exchange, precipitation, redox reactions and degradation processes. With respect to contaminants, focus is on dissolved organic matter, xenobiotic organic compounds, inorganic macro components as anions and cations, and heavy metals. Laboratory as well as f ield investigations are included. This review is an up-date of an earlier c omprehensive review. The review shows that most leachate contamination plum es are relatively narrow and do not in terms of width exceed the width of t he landfill. The concept of redox zones being present in the plume has been confirmed by the reported composition of the leachate contaminated groundw ater at several landfills and constitutes an important framework for unders tanding the behavior of the contaminants in the plume as the leachate migra tes away from the landfill. Diverse microbial communities have been identif ied in leachate plumes and are believed to be responsible for the redox pro cesses. Dissolved organic C in the leachate, although it appears to be only slowly degradable when the volatile organic acids are gone, apparently act s as substrate for the microbial. redox processes. Several xenobiotic organ ic compounds have been found to be degradable in leachate contaminated grou ndwater, but degradation rates under anaerobic redox conditions have only b een determined in a few cases. Apparently, observations in actual plumes in dicate more extensive degradation than has been documented in the laborator y. The behavior of cations in leachate plumes is strongly influenced by exc hange with the sediment, although the sediment often is very coarse and san dy. Ammonium seems to be subject to anaerobic oxidation, but the mechanisms are not yet understood. Heavy metals do not seem to constitute a significa nt pollution problem at landfills, partly because the heavy metal concentra tions in the leachate often are low, and partly because of strong attenuati on by sorption and precipitation. Although complexation of heavy metals wit h dissolved organic matter is significant, the heavy metals are in most cas es still strongly attenuated in leachate-polluted aquifers. The information available on attenuation processes has increased dramatically during the l ast 15 a, but the number of well-documented full scale leachate plumes are still few and primarily from sandy aquifers. Thus, the diversity of attenua tion processes in leachate plumes is probably not yet fully understood. App arently, the attenuation processes in leachate plumes may for many contamin ants provide significant natural remediation, limiting the effects of the l eachate on the groundwater to an area usually not exceeding 1000 m from the landfill. (C) 2001 Elsevier Science Ltd. All rights reserved.