Although environmental modelling is increasingly performed within a GIS framework, analysis of the associated error is far from routine, and rarely presented with the results. An important benefit of performing error analysis is its value in determining which elements of a vulnerability assessment framework need improving. With this in mind, it was decided to examine the extent to which error might propagate through a model of groundwater vulnerability to pesticide contamination. A pesticide leaching model was developed and incorporated into an assessment of groundwater contamination risk from normal agricultural use of the herbicide isoproturon, in a 30 km x 37 km area of river catchment to the north-west of London, England. The model, which comprised two main components accounting for (i) degradation and (ii) attenuation of the pesticide, was based on conventional contaminant transport calculations, combined with existing soil, rainfall, hydrogeological and depth to water table data. The results of an error analysis on the model were used to assign confidence limits to the resulting risk maps. In this instance, correlation of model variables led to a reduction of error in the final output. However, the results of the analysis showed how inclusion of low quality input data can lead to a large increase in output uncertainty. It is suggested that error propagation analysis should be routinely included in groundwater vulnerability assessment.