Septic tanks are the most frequently reported causes of contamination in ground-water disease outbreaks associated with the consumption of untreated ground water in the United States. The placement of septic tanks is generally controlled by country-wide or state-wide regulations, with little consideration given to the local hydrogeologic, climatic, and land-use conditions. Using the travel time necessary to achieve a seven-order-of-magnitude reduction in virus number as the criterion, a wide range of septic tank setback distances (from less than 15 m to greater than 300 m) were calculated for a part of the Tucson Basin. The study makes use of disjunctive kriging to calculate the conditional probabilities associated with the setback distance estimates. The results are presented in two different ways: given a setback distance (e.g., prescribed by law) the probabilities that the level of viruses will be within acceptable limits are calculated; and the desired probability level is specified (e.g., 90%) and the setback distances required to achieve that level of confidence that the water will be free of virus contamination are calculated. The methods have potential for use by local government officials for land-use planning purposes.