Arsenic is a contaminant of concern in groundwater at a large proportion of sites on the National Priority List (NPL, so called “Superfund”) sites in the US. Approximately 2.5 million private well owners in the US are at risk of the adverse health effects associated with chronic exposure to As. Across South and Southeast Asia, over 100 million villagers are chronically exposed to arsenic by drinking groundwater from their well. This Core supports biogeochemical field work and laboratory measurements to address these issues under the Columbia University Superfund Research Program.