Urban areas have a multiplicity of pollution sources, often unknown. What is the risk of a new well being polluted? We have developed a probabilistic analysis of this problem which has two data elements, and two model components. The objectives of the analysis are to value urban groundwater, and to decide if it suitable for potable, industrial or environmental uses. The data elements are (a) a groundwater flow model of the region, and (b) a GIS database of the time history of industrial land and associated contaminants in the region. For any well position, the analysis starts with the well’s capture zone; the groundwater flow model is used stochastically to zone the probability that parcels of land fall within the capture zone. These zones identify the probability that potential pollution sources (i.e. industrial land) will affect the well. A simplified transport model estimates the probability distribution of pollutant concentrations at the well. The analysis can be repeated for any location, building up a map of the risk of pollution across the city. The analysis has been ‘validated’ with two wells in Nottingham (UK), and used to map risks for the city.