News release: “New USGS groundwater studies explain what, when, and how contaminants may reach public-supply wells. All wells are not equally vulnerable to contamination because of differences in three factors: the general chemistry of the aquifer, groundwater age, and direct paths within aquifer systems that allow water and contaminants to reach a well. More than 100 million people in the United States receive their drinking water from public groundwater systems, which can be vulnerable to naturally occurring contaminants such as radon, uranium, arsenic, and man-made compounds, including fertilizers, septic-tank leachate, solvents and gasoline hydrocarbons. The USGS tracked the movement of contaminants in groundwater and in public-supply wells in four aquifers in California, Connecticut, Nebraska and Florida. The importance of each factor differs among the various aquifer settings, depending upon natural geology and local aquifer conditions, as well as human activities related to land use and well construction and operation. Findings in the four different aquifer systems can be applied to similar aquifer settings and wells throughout the nation.