Private wells are unregulated and often at risk for arsenic contamination. Research objectives included distribution of groundwater arsenic concentrations, identification of arsenic sources, and establishment of best practices for well construction to minimize risk for wells in Cerro Gordo County, Iowa. We sampled 68 wells over 3 years with 393 water samples and 79 rock samples. Geochemical modeling was used to better understand arsenic mobilization. Arsenic in groundwater ranged from 1.0 to less than 10.0 μg/L for 75 water samples and 31 water samples had arsenic concentrations greater than or equal to 10 μg/L. The arsenic source is naturally occurring sulfide minerals (typically pyrite) in the bedrock aquifers. The shallow (100–150 feet) Lime Creek Aquifer was most at risk for arsenic. Arsenic is likely mobilized from the rock into the water in the shallow aquifer under more oxidizing conditions, subject to water level changes. The study resulted in a policy change for arsenic testing and well completion in Cerro Gordo County to better protect domestic well users.