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The pesticide impact rating index (PIRI) has been integrated with a Geographic Information System (GIS) to enable regional assessment of pesticide impact on groundwater and surface water resources. The GIS version of PIRI (PIRI-GIS) was used to assess the impact of pre-planting atrazine use in the pine plantations on the Gnangara Mound, Western Australia. The impact on groundwater was found to be spatially variable, mainly dependent on soil type and depth to groundwater, because land use variables were spatially constant. Areas with the greatest impact on groundwater were those where the soil had a low sorption capacity for atrazine. Knowledge of the spatial distribution of the sorption coefficient based on organic carbon (Koc) for atrazine was found to significantly improve the results from PIRI-GIS. Average values for Koc (i.e. based on overseas data) were too low for most of the local soil types, resulting in a general overestimation of pesticide impact on groundwater resources, but an underestimation of impact in areas that should be of greatest concern (i.e. where the soil has a low sorption capacity for atrazine).