Arsenic in ground-water under oxidizing conditions, south-west United States

Abstract

Concentrations of dissolved arsenic in ground-water in alluvial basins of Arizona commonly exceed 50 μg L⁻¹ and reach values as large as 1,300 μg L⁻¹. Arsenic speciation analyses show that arsenic occurs in the fully oxidized state of plus 5 (As+5), most likely in the form of HAsO4^∼2, under existing oxidizing and pH conditions. Arsenic in source areas presumably is oxidized to soluble As before transport into the basin or, if after transport, before burial. Probable sources of arsenic are the sulphide and arsenide deposits in the mineralized areas of the mountains surrounding the basins. Arsenic content of alluvial material ranged from 2 to 88 ppm. Occurrence and removal of arsenic in ground-water are related to the pH and the redox condition of the ground-water, the oxidation state of arsenic, and sorption or exchange. Within basins, dissolved arsenic correlates (P∼2 on active ferric oxyhydroxide surfaces.