In the assessment of the risks to groundwater posed by most landfills, the production and transport of ammonium is the key factor. Although a long list of toxic and dangerous substances can appear in the leachate from a landfill, the chemical component with the greatest impact both on the ecology and on groundwater abstractions is usually ammonium. It is therefore vital that information on the behaviour of ammonium in groundwater is compiled in order to reduce the uncertainties associated with predictions of groundwater impact.

The transport of ammonium is attenuated by two main processes: cation exchange and oxidation (often biologically enhanced). The effect of these can probably be approximated by a combination of linear retardation and exponential degradation. In principle, comparison of the breakthrough curves of ammonium and chloride at monitoring points should allow quantification of the contributions of these two processes in terms of retardation factors and decay rates.

Three case studies at different scales are compared. The results indicate that retardation factors are not higher than 3, and that degradation does contribute to reduction in concentration. While only three studies have been examined, the results seem to contrast with the typical assumptions made in risk assessments.

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