SOURCES AND MOBILITY OF ARSENIC IN THE SALT RIVER WATERSHED, ARIZONA

Sources and mobility of arsenic in the upper Salt River (Arizona) were examined using a B-year long mass balance for nine U.S. Geological Su rvey chemical monitoring stations, a more spatially intensive low-flow synoptic study, and laboratory leaching studies. An arsenic-rich lacu strine deposit (the Verde Formation) comprised only 4% of the watershe d but contributed a third of the total watershed arsenic loading. Expo rt from this part of the watershed was 10-20 times higher than export from most other parts of the watershed. Laboratory experiments confirm ed the high As leaching potential for this formation. Although elevate d levels of As were found in a geothermal spring and in several mine s eeps, these sources of arsenic contributed little to the overall As lo ading. About 15% of the As loading from the upper Verde River watershe d was retained by a two-reservoir system. Sedimentation and chemical p recipitation appear to be likely mechanisms of As retention in the res ervoirs.