Arsenic and Uranium in Four Aquifer Settings: Occurrence, Distribution, and Mechanisms for Transport to Supply Wells

A U.S. Geological Survey (USGS) study of the transport of anthropogenic and natural contaminants to public-supply wells (PSWs) in four aquifer systems documents the occurrence and distribution of arsenic and uranium, and determines some of the mechanisms for their mobilization and movement. The study areas include a glacial aquifer system in Woodbury, Connecticut (referred to herein as CT); the Floridan aquifer system and overlying units in Tampa, Florida (referred to herein as FL); the High Plains aquifer in York, Nebraska (referred to herein as NE); and the Central Valley aquifer system in Modesto, California (referred to herein as CA). Chemical analyses of solid-phase material indicated geologic sources of arsenic and uranium, but did not always correspond to their relative concentrations in ground water. In CT, low extracted concentrations of arsenic and uranium in most glacial deposits corresponded to low concentrations in ground water. Arsenic and uranium concentrations in sediment were highest in NE and among the lowest in CA, yet dissolved concentrations at both sites were similar. The predominance of calcium-uranyl-carbonate complexes, which can inhibit uranium adsorption, probably contributes to uranium mobility in NE, CA, and FL. Shallow, high-bicarbonate water is drawn downward by pumping and irrigation recharge at both sites. Arsenic correlates well with orthophosphate in shallow ground water in both areas, possibly indicating a tendency for competitive desorption of arsenic by phosphate. In FL, the highest concentrations of arsenic and uranium in depth-dependent samples from the public supply well were found in a highly transmissive zone in the Suwannee Limestone. The arsenic likely results from reactions between oxic water from the surficial aquifer system and arsenic-rich pyrite in parts of the limestone. Evidence of short-circuit pathways were observed in all four aquifer systems, and those in FL, NE, and CA result in sporadic exceedances of drinkingwater standards for arsenic and (or) uranium in PSWs. At the FL, NE, and CA sites, these pathways resulted from human-induced changes in the flow system.