The catabolic activity of incumbent microorganisms in soil samples of eleven dissimilar soil series was investigated, with respect to the herbicide isoproturon. Soils were collected from a 30 × 37 km area of river catchment to the north-west of London, England. Catabolic activity in each soil type during a 500 h assay was determined by 14C-radiorespirometry. Results showed four soils that exhibited high levels of catabolic activity (33–44% mineralisation) while the remaining seven soils showed lower levels of catabolic activity (12–16% mineralisation). There was evidence to suggest that soils exhibiting high catabolic activity had low (< 22%) clay content and tended towards lower organic carbon content (< 2.7%), but that these higher levels of catabolic activity were also related to pre-exposure to isoproturon. The 14C-radiorespirometric results were used to produce a GIS layer representing levels of catabolic activity for the dissimilar soils across the study area. This layer was combined with other GIS layers relating to pesticide attenuation, including soil organic carbon content, depth to groundwater and hydrogeology, to produce a map showing risk of groundwater contamination by isoproturon. The output from this approach was compared with output from an attenuation-only approach and differences appraised. Inclusion of the catabolism layer resulted in a lowering of risk in the model in 15% of the study area. Although there appears to be limited benefit in including pesticide catabolic activity in this regional-scale groundwater risk model, this type of addition could be useful in a site-specific risk assessment.