ReviewBiogeochemistry of landfill leachate plumes
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):
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aromatic hydrocarbons;
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halogenated hydrocarbons;
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phenols;
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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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