Kjeldsen*, P.; Barlaz, M.; Rooker, A.; Ledin, A.; Christensen, T.H.

*Corresponding author

Department of Environmental Science and Engineering

Technical University of Denmark

DK-2800 Lyngby

Denmark

Fax: +45 45932850

E-mail: pk@imt.dtu.dk

The high number of cases of groundwater pollution at landfills and the substantial resources spent in remediation suggest that landfill leachate is a significant source of groundwater pollution. Landfill leachate from municipal solid waste landfills contains a variety of different pollutants (dissolved organic carbon, inorganic salts, heavy metals, and xenobiotic organics). Many investigations of the composition of landfill leachates have been presented giving a picture of typical present composition of landfill leachate. However this data only represent the initial 10-30 years of the lifetime of landfills. Most of the investigated landfills are in the methanogenic phase, and almost no data exists for the following phases where the methane production is ceasing. It has been hypothesized that the pollutant release from landfills in the mature phases might increase substantially due to oxidation processes.

The objective of this paper is by review of the literature to give an overview on typical present leachate composition, and to present and discuss hypothesis for the evolution of landfill leachate composition through the future landfill phases based on model considerations and laboratory experiments.

A typical decreasing concentration trend with time is observed for most of the pollutants observed through the initial acidogenic and methanogenic phases. The dissolved organic matter (measured as COD or BOD) seems to reach more stable concentrations, and the ratio BOD/COD is typical around 0.10 for methanogenic phase leachate. Nitrogen typical present as ammonia is not decreasing as much with time, and typical values in the order of 200-800 mgN/l are observed. The heavy metal concentrations are very low, and landfill mass balances show that only a small fraction of the heavy metals originally contained in the landfilled waste has been leached out. A large variety of xenobiotic organics have been observed in landfill leachate with very broad concentration ranges.

The landfill is postulated to undergo several degradational phases following the methanogenic phase. When the methane production gets low, air starts to significantly intrude the uppermost part of the waste (Air intrusion phase). After a further decrease in methane production the oxygen is intruding the upper waste layers and is primary used for oxidation of the methane transported to the top of the waste layers (Methane oxidation phase). When methane production becomes insignificant the oxygen is used for oxidation of solid organic matter and reduced inorganic species (such as metal sulphides) follow by production of carbon dioxide (Carbon dioxide phase). Due to the oxidation of insoluble metal salts and the degradation of solid organic matter serving as a sorptive capacity for metals, the metals are potentially released to the environment. A few laboratory experiments have been carried out to study the behaviour of metals in old waste samples (taken from landfills) under oxidising conditions. Most of the studies indicates that other attenuating reactions take over when the sulphides and solid organic matter are oxidised, and enhanced releases of heavy metals in this phase seems not to take place. More research is, however, needed to back up these initial findings.