Previous research suggested a possible link between breast cancer and groundwater plumes in upper Cape Cod, Massachusetts. However, the geographic relationship between breast cancer clusters and groundwater plumes alone does not establish exposure. To test the hypothesis that drinking water contaminated by municipal wastewater effluent from a specific source is associated with breast cancer, Gallagher et al integrated residential histories, public water distribution systems, and groundwater modeling within geographic information systems to examine the association between exposure to drinking water contaminated by wastewater effluent and breast cancer. Exposure was assessed from 1947 to 1993 for breast cancer cases diagnosed from 1983 to 1993. The authors report that wastewater effluent affected the drinking water wells of study participants as early as 1966. Using detailed exposure assessment, the authors report an association with breast cancer that increased with longer latency and greater exposure duration.