Estimated county level domestic well population with arsenic greater than 10 micrograms per liter based on probability estimates for the conterminous U.S.

Arsenic concentrations from 20,450 domestic wells in the U.S. were used to develop a logistic regression model of the probability of having arsenic > 10 g/L (high arsenic). We use only domestic well arsenic data and a national-scale modeling approach. This approach expands our understanding of potential exposure to arsenic in drinking water to a national scale and allows inter-regional comparisons. Variables representing geologic sources, geochemical, hydrologic, and physical features were among the significant predictors of high arsenic. For U.S. census block groups, the mean probability of arsenic > 10 g/L was multiplied by the population using domestic wells to estimate the potential high-arsenic domestic-well population. Approximately 44.1 M people in the U.S. use water from domestic wells. The population in the conterminous U.S. using water from domestic wells with predicted arsenic concentration > 10 g/L is 2.1 M people (95% CI is 1.5 to 2.9 M). Although areas of the U.S. were underrepresented with arsenic data, predictive variables available in national datasets can help estimate high arsenic in unsampled areas. However, linking predictive modeling to private well use information nationally, despite the uncertainty, is potentially beneficial for broad screening of the population at risk to elevated arsenic in drinking water from private wells.