Analytical results for arsenic in water samples from 5,023 wells obtained during 1969-2007 across Pennsylvania were compiled and related to other associated groundwater- quality and environmental factors and used to predict the prob- ability of elevated arsenic concentrations, defined as greater than or equal to 4.0 micrograms per liter ( g/L), in ground- water. Arsenic concentrations of 4.0 g/L or greater (elevated concentrations) were detected in 18 percent of samples across Pennsylvania; 8 percent of samples had concentrations that equaled or exceeded the U.S. Environmental Protection Agen- cy's drinking-water maximum contaminant level of 10.0 g/L. The highest arsenic concentration was 490.0 g/L."