National Water-Quality Assessment (NAWQA) Program
Ryker, S.J., Nov. 2001, Mapping arsenic in groundwater: Geotimes v. 46 no. 11, p. 34-36.
PDF in red/yellow/green (0.4 Mb)
Original map (Welch and others, 2000)--
The May 2000 county map was based on 18,850 ground-water samples, and showed arsenic concentrations found in at least 10% of samples per county. The map below is based on 31,350 ground-water samples, and shows arsenic concentrations found in at least 25% of samples per county.