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An assessment tool is needed that will allow jurisdictions to identify communities where failing onsite sewage dispersal systems are likely to occur. By identifying risk factors associated with failing onsite sewage systems, a geographic map of high risk areas can be created. After reviewing records of existing onsite sewage dispersal systems, a logistic regression analysis was performed using system failure as the dependent variable and system age, parcel size, and soils as independent variables. System age and parcel size were both significant, 002 and .004 respectively, with older systems and smaller parcels being associated with failing systems. Soil was not significant, .216. By utilizing the regression analysis, a predicted probability of failure value was assigned to each of the 6254 parcels in the Catoctin Creek watershed. These probabilities were mapped which resulted in an effective assessment tool for predicting onsite wastewater system failure.