Abstract A method is developed for measuring the vulnerability of the catchment of a borehole to groundwater pollution based on observation of contaminant in the borehole and the region. The method uses Bayesian statistics to compare the proportion of pesticide detections in the region with that found in the borehole as a measure of groundwater vulnerability of that borehole catchment. Using data from California the method is demonstrated for multi-annual borehole observations of a set of compounds. The method has distinct advantages over present vulnerability assessment methods. The method calculates a continuous measure of borehole vulnerability rather than an index or score. This measure of borehole vulnerability is based solely on observations and not on expert opinion or the application of models. The use of Bayesian methods means that results can be used within a probabilistic risk assessment and can be updated as more information becomes available or as new regional data are considered. The assumptions and disadvantages of the methodology are discussed, but such methods could form the basis of testing the importance of climatic, soil and geological factors in controlling pollutant movement leading to improved management and protection of groundwater resources.