Natural attenuation of leachate - letting nature take its course (Reprinted from Trans. Instn Min. Metall., vol 108, 1999)

Filling quarries with waste materials is a common method used to restore la nd to productive use. Whereas modern landfill techniques are designed to mi nimize groundwater pollution, older landfills that were not engineered for containment and have received domestic or industrial wastes create a risk o f pollution. Landfill gas from domestic waste is composed of methane and ca rbon dioxide, which are, respectively, explosive and toxic, whereas leachat es produced by water percolating through domestic waste may be more polluti ng than raw sewage. Once polluted, an aquifer may be technically difficult and extremely expensive to decontaminate, if not impossible. As leachate migrates through an aquifer the concentration of solutes is red uced as a result of natural physical, chemical or biological processes. Los s of contaminant mass, development of redox zones, appearance of the breakd own products of organic decay and development of an adapted microbial conso rtium are all evidence of in-situ biodegradation. This natural attenuation can often reduce the concentration of contaminants before they have a harmf ul effect on streams, springs or water supplies. If these reactions can be quantified and included in models to predict long-term leachate migration, decisions on whether the contamination must be treated or can be left to re duce naturally can be made with greater confidence. Although natural attenuation processes are relatively easy to identify, mor e quantitative information and models are needed-in particular, on the pote ntial for natural attenuation to occur and on rate-limiting factors. Such i nformation could be obtained by detailed investigations of representative s ites where long-term monitoring data are available. This information could be used to produce guidelines for the identification and quantification of natural attenuation that could be applied in the management of groundwater pollution in similar settings. Two case studies are presented to illustrate the concept of natural attenuation and the means by which it can be identi fied and quantified.