ARSENIC DISTRIBUTION IN SOILS OF THE UNITED STATES AND CANADA

Understanding the abundance and spatial variation of arsenic in soils at scales ranging from local to global is critical for environmental regulators and public health specialists. Soil arsenic concentration was studied in five data sets at widely varying scales in the United States (US) and Canada. The data sets represent (1) 1,257 samples (1 site per 6,000 km²) distributed uniformly throughout the conterminous US; (2) 265 samples from a 4,000-km east-to-west transect following the 38^th parallel across the center of the US and a 3,000-km north-to-south transect from northern Manitoba to the US-Mexico border, samples collected at approximately 40-km intervals; (3) 1,300 samples from a 20,000 km² study area in northern California (1 site per 15 km²); (4) 960 samples distributed uniformly throughout the state of Colorado (1 site per 280 km²); and (5) 433 samples from the Denver, Colorado metropolitan area (1 site per 2.6 km²). The analytical methodology in each study measured the total arsenic content of soil.

The variation in arsenic concentration at the national scale (data set 1) is approximately 3 orders of magnitude (0.1–100 mg/kg). The distribution at this scale is dominated by natural variation. Samples with the highest arsenic concentrations come from areas with known mineralization, whereas samples with the lowest concentrations are found in the Atlantic and Gulf coastal plains where the parent material consists of coastal sediments dominated by quartz sand. The variation in the other data sets is about two orders of magnitude. Despite the different scales of these five data sets, the median arsenic concentration only varies from 5 to 7 mg/kg. The processes controlling arsenic concentration tend to be different at different scales. The lower-density studies tend to show the influence of soil parent material on arsenic concentration and spatial distribution. Only in the higher-density studies is the influence of anthropogenic activities clearly illustrated. All of these studies demonstrate that it is extremely difficult to find a soil that does not exceed an arsenic concentration of 0.39 mg/kg, which is the carcinogenic target risk level for increased cancer risk of one in a million (10^-6) established for residential soils by the U.S. Environmental Protection Agency.