Discussion of ‘Transport of ammonium in aquifers: retardation and degradation’ by A. D. Erskine

Abstract

M. A. Jones, D. H. Hall, E. O. Davies & M. J. Jefferies write: We have two comments on ⇓Erskine (2000). The first discusses an improved model for chloride transport in the unsaturated zone applied to the Burntstump landfill site. The second clarifies the use of default parameters in performance assessment modelling. The site specific data presented by Erskine (and incorporated in our Figure 1) show that the ‘Ogata solution’ (⇓Ogata & Banks 1961) is not realistic for Burntstump landfill. The fact that constant source models may be unrealistic was recognized during the development of the LandSim model (⇓Environment Agency/Golder Associates 1996), and as a consequence, the authors of LandSim developed a ‘declining source’ model. This is a generalization of the Ogata solution that allows the source concentration to decrease as a function of time. The model is discussed in the LandSim manual (⇓Environment Agency/Golder Associates 1996) and its derivation is the subject of a future publication (Jones in prep.). The key aspect of the declining source model is that the leachate concentration decreases exponentially with time t …