Table 3.

Estimated half-lives for NH4+ biodegradation (nitrification) in different subsurface lithologies under aerobic and anaerobic conditions.

Lithology1NH4+ half-life under aerobic conditions (years)2NH4+ half-life under anaerobic conditions (years)3Comments
Sands and gravels1–6Based on range of literature-derived values (<1–6 years) in unsaturated subsoil and aquifers
Unfissured Chalk and other strata with mean pore size of <1 μmNo degradation; pore size excludes entry of bacteria
Strata with mean pore size of >1 μm or showing a significant degree of fissure flow45–10No kinetic data exist but attenuation has been demonstrated to take place. Suggested range (5–10 years) is considered reasonably conservative
Landfill linersNo data, but pore size may exclude entry of bacteria. Assume no degradation to ensure liner design suitably conservative
  • Estimated half-lives are taken from literature and extrapolated to representative systems, which must be consistent with the conceptual model to which the data are applied during risk assessments.

  • 1Values are considered to apply equally to both the unsaturated and saturated zones.

  • 2Where a range is given a uniform distribution is recommended for probabilistic modelling.

  • 3It is assumed that no anaerobic NH4+ oxidation takes place unless site-specific data indicate otherwise.

  • 4Where mean pore sizes in the matrix of dual porosity media are less than 1 μm (e.g. fissured Chalk) care should be taken that only the fraction of contaminant flowing in the fractures is degraded by the model used.