CONCENTRATIONS OF SELECTED METALS IN UNCONTAMINATED ALLUVIAL DEPOSITS OF THE CHEYENNE AND BELLE FOURCHE RIVERS, SOUTH DAKOTA

Sediments contaminated with various metals from mining activities in the Black Hills and potentially other anthropogenic sources have been deposited throughout downstream reaches of the Belle Fourche and Cheyenne Rivers in western South Dakota. The Water Resource Development Act of 1999 (amended in 2000) mandated a study of contaminated sediments within the Cheyenne River Basin, which is being conducted by the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers. Herein, we summarize the first part of this study, which is to determine the concentrations of selected metals in alluvial sediment deposited prior to the effects of mining and other anthropogenic activities that may have contributed to contamination in the basin. Such data provide estimates of concentrations of metals in “uncontaminated” sediment. Of particular interest are the concentrations of arsenic and mercury in sediment. Five sites identified as uncontaminated were selected for sampling: four along the Cheyenne River and one along the Belle Fourche River. Samples were collected from terraces that were well above historical maximum flood levels at each of these sites. Samples were also collected from the active channel and floodplain at two sites on the Cheyenne River above its confluence with the Belle Fourche River. The chronology of terraces on the lower Cheyenne River is largely unresolved, but previous work indicates that terraces as much as 75 meters above the channel may be as young as late Pleistocene to early Holocene. Ages of 10.7 thousand years ago (ka) to 16.5 ka, and 6.1 ka were obtained from Cheyenne River terraces at approximately 199 and 183 river kilometers upstream from the confluence with the Missouri River, respectively. Both terraces are approximately 67 meters above the channel. Other dates are forthcoming from terraces at similar heights above the channel. Arsenic concentrations from a total of 88 samples from terrace and channel sites had a geometric mean of 7.5 parts per million (ppm). The maximum arsenic concentration of the sample population was 34 ppm. An arsenic concentration of 35.6 ppm is three standard deviations greater than the geometric mean for arsenic concentrations from the 88 samples. In most samples, mercury was undetected or less than laboratory analytical reporting limits.