Mining, milling, and processing wastes containing quantities of arsenopyrite were produced around Lead, South Dokata, from 1875 to 1977. Much of this material was discharged into Whitewood Creek, and from there portions of the waste were transported to the Belle Fourche River, thence to the Cheyenne River, and finally to the Missouri River. In 1958, the Missouri River was dammed at Pierre, forming Lake Oahe. Analyses of cores collected from the lake bottom showed the presence of arsenic-rich layers in the bed sediments; substantial portions of the arsenic are due to arsenopyrite in the 8-16 and 16-32 ??m size fractions of the sediments. In addition, suspended-sediment samples collected from the Cheyenne River above Lake Oahe contain detectable quantities of arsenopyrite in the 8-16 and 16-32 ??m fractions. Solid material collected from the banks and floodplains of the Belle Fourche River and Whitewood Creek contains reduced and oxidized phases. The reduced phases have arsenic maxima in the 16-32 and 32-63 ??m size ranges. These fractions also contribute the most arsenic to the samples; the major source being arsenopyrite. The oxidized segments have arsenic maxima in the < 2 and > 63 ??m size ranges. The < 2 ??m maxima are associated with widely disseminated, arsenic-bearing iron oxide coatings. However, the > 63 ??m fractions contribute the most arsenic to the oxidized samples. This arsenic, despite the oxidized nature of the samples, is associated with arsenopyrite coated with thin iron oxide rinds. It has been calculated that 80% of the arsenic in these deposits is associated with sulfides (in the form of arsenopyrite), while 20% is associated with iron oxides. The arsenopyrite found in the banks and floodplains of Whitewood Creek and the Belle Fourche River are the likely source of the arsenopyrite found in the suspended sediments of the Cheyenne River and in the bed sediment of Lake Oahe.