Review
Biogeochemistry of landfill leachate plumes

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Abstract

The literature has been critically reviewed in order to assess the attenuation processes governing contaminants in leachate affected aquifers. Attenuation 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 macrocomponents as anions and cations, and heavy metals. Laboratory as well as field investigations are included. This review is an up-date of an earlier comprehensive review. 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 constitutes an important framework for understanding the behavior of the contaminants 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 C 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 xenobiotic organic compounds have been found to be degradable in leachate contaminated groundwater, but degradation rates under anaerobic redox conditions have only been determined in a few cases. 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. Ammonium seems to be subject to anaerobic oxidation, but the mechanisms are not yet understood. 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. Although complexation of heavy metals with dissolved organic matter is significant, the heavy metals are in most cases still strongly attenuated in leachate-polluted aquifers. The information available on attenuation processes has increased dramatically during the last 15 a, but the number of well-documented full scale leachate plumes are still few and primarily from sandy aquifers. Thus, the diversity of attenuation processes in leachate plumes is probably not yet fully understood. 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.

Introduction

The large number of cases of groundwater pollution at landfills (Arneth et al., 1989) and the substantial resources spent in remediation suggest that landfill leachate is a significant source of groundwater pollution.

Understanding the natural attenuation of leachate pollutants in aquifers is mandatory for evaluation of environmental risks associated with leachate entering the groundwater, for coherent interpretation of groundwater samples from monitoring wells, and for determining appropriate remedial action, including monitored natural attenuation. Attenuation is here used as a common term for processes reducing the concentrations of dissolved contaminants in the leachate plume, including physical (e.g. dilution), physico-chemical (e.g. sorption, ion exchange), chemical (e.g. precipitation) and microbial (e.g. degradation) processes.

Christensen et al. (1994) reviewed the information available before 1993 about processes important in attenuation of landfill leachate plumes. The purpose of this review is to provide an update of the 1994-review with emphasis on literature published since 1993. Most of the classical landfill leachate studies reviewed by Christensen et al. (1994) are also included in this current review, but general literature about attenuation processes have been excluded. This should make the current review fully up-dated and comprehensive with respect to landfill leachate plumes, but full appreciation of the information reviewed may require that the reader has some understanding of contaminant geochemistry in general.

As an introduction to this complex area of biogeochemistry, a summary of leachate composition and characteristics is presented. This is followed by a description of the physical (dilution), geochemical (redox zones) and microbial frameworks for the attenuation processes in leachate plumes. The biogeochemical processes are discussed in detail for the various contaminants under the terms: dissolved organic matter, inorganic macrocomponents (cations and anions), heavy metals and xenobiotic organic compounds (XOCs).

The review focuses on aquifers contaminated by leachate. Leachate is described as a water based solution of compounds from the waste. Only where large amounts of chemicals are disposed of will organic compounds possibly appear as free phases (e.g. non-aqueous phase liquids as solvents). However, free phase transport in aquifers and diffusive pollutant migration in clay and aquitards are not included in this review.

Section snippets

Landfill leachate

Landfill leachate is generated by excess rainwater percolating through the waste layers. Combined physical, chemical and microbial processes in the waste transfer pollutants from the waste material to the percolating water (e.g. Christensen and Kjeldsen, 1989). Focusing on the common type of landfill receiving a mixture of municipal, commercial and mixed industrial waste, but excluding significant amounts of concentrated specific chemical waste, landfill leachate may be characterized as a water

Dilution

All compounds in leachate entering an aquifer will be subject to dilution as the leachate mixes with the groundwater. For non-reactive components, of which Cl is the dominant component, dilution is the only attenuation mechanism.

Redox environments and processes

The entry of strongly reduced landfill leachate into a pristine, often oxidized, aquifer, leads to the creation of very complex redox environments. Important processes include organic matter biodegradation, abiotic redox processes, dissolution/precipitation of minerals, complexation, ion exchange, and sorption. The resulting redox environments strongly influence both the inorganic and organic biogeochemistry of the aquifer, and create the chemical framework for understanding the attenuation

Microbiology

Most uncontaminated aerobic aquifers are oligotrophic and entry of landfill leachate, reduced and rich in dissolved organic matter, will dramatically change the composition of the original microbial population. A general review of the microbial ecology of the terrestrial subsurface, contaminated as well as uncontaminated, has been presented by e.g. Ghiorse and Wilson, 1988, Dobbins et al., 1992, Chapelle, 1993, and Lovley and Chapelle (1995); but these authors paid only little attention to

Dissolved organic matter

Attenuation of dissolved organic matter (e.g. expressed as Total Organic C, Dissolved Organic C, Non-Volatile Organic C or Chemical O2 Demand) in the leachate plume, besides dilution, is caused by sorption and degradation.

Inorganic macrocomponents

The term macrocomponents refers here to inorganic constituents present at elevated concentrations (several mg per liter in leachate and contaminated groundwater) and includes the anions, Cl, HCO3 and SO42−, and the cations, Ca2+, Mg2+, K+, Na+, NH4+, Fe and Mn. Aluminum is a significant cation only at very low pH-values (<pH 5, Appelo and Postma, 1996) and will not be further addressed in this context.

These inorganic macrocomponents usually do not constitute a severe groundwater pollution

Heavy metals

Heavy metals, or trace elements, do not constitute a frequent groundwater pollution problem at landfills, partly because landfill leachates usually contain only modest concentrations of heavy metals (see Section 2: Landfill leachate) and partly because the heavy metals are subject to strong attenuation by sorption and precipitation in the plume. A survey in Germany (Arneth et al., 1989) including 92 abandoned waste disposal sites, revealed that for Cd 78% and for Pb 85% of downgradient

Xenobiotic organic compounds (XOCs)

Xenobiotic organic compounds (XOCs) are the organic chemicals identified as known, individual pollutants in the leachate. The number of known XOCs increases with time as better methods for analysis are developed. To date, more than 1000 organic chemicals have been identified in groundwater contaminated by landfills, most of which fall into the categories (see also Table 9.1):

  • aromatic hydrocarbons;

  • halogenated hydrocarbons;

  • phenols;

  • pesticides.

Other compounds such as anilines and sulfonated

Conclusion

Leachate from landfills contains a wide range of contaminants: dissolved organic matter, inorganic cations and anions, heavy metals, and xenobiotic organic compounds. Where leachate enters the groundwater, significant changes in water quality are observed and complicated biogeochemical patterns develop in the leachate pollution plume.

The general shape of the leachate plume is governed by the flow field of the aquifer, the leachate mound often observed below landfills, the increased density of

Acknowledgements

This literature review has been developed as a part of a major research program focusing on the effects of waste disposal on groundwater. The program is funded by the Danish Technical Research Council and the Technical University of Denmark. The assistance of Grete Hansen, Birte Brejl and Torben Dohlin is gratefully acknowledged.

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