MAPPING THE DISTRIBUTION OF ARSENIC IN SOILS OF THE CONTERMINOUS UNITED STATES

Understanding the abundance and spatial variation of arsenic in soils at scales ranging from local to global is critical for environmental regulators, public health specialists, and the agricultural/food safety community. The U.S. Geological Survey recently completed a soil geochemical survey of the conterminous United States based on 4,857 sample sites (1 site per approximately 1,600 km²). At each site, a sample from a depth of 0 to 5 cm was collected along with samples from the soil A and C horizons. Each sample was sieved to <2 mm and then ground to <150 μm prior to chemical analysis for 45 major and trace elements by methods that yielded the total elemental content. Arsenic was analyzed by hydride-generation atomic absorption spectrometry following fusion in a mixture of sodium peroxide and sodium hydroxide. For all three sample types, arsenic varied by approximately three orders of magnitude from less than 0.6 mg/kg (the method detection limit) to about 1,000 mg/kg, with a median of 5.2 mg/kg for the 0-5-cm and A-horizon soils and 5.7 mg/kg for the soil C horizon. The dominant process controlling the spatial distribution of arsenic is weathering of soil parent material of various compositions. The highest arsenic concentrations were found in soils formed on mineralized bedrock (e.g., western Montana and parts of Nevada) and marine shales (e.g., portions of Ohio, Pennsylvania, and South Dakota). Soils collected from the rice-growing region along the Mississippi River in Arkansas, Mississippi, and Louisiana were also elevated in arsenic. These higher arsenic concentrations are likely caused by a combination of factors including elevated concentrations of iron and clay minerals in these alluvial soils that lead to adsorption of arsenic and other elements, input from arsenical pesticides, and contributions from upstream natural sources and industrial activities. The lowest arsenic concentrations were found in soils formed from quartz-rich sediments (e.g., Florida, South Carolina). Anthropogenic arsenic input is superimposed on this highly variable background distribution and, at the national scale of the USGS survey, is not easily recognized.