Study of Abandoned Uranium Mining Impacts
On Private Land Surrounding US Forest Service Land In Western South Dakota
North Cave Hills Soil & Sediment Figures
Map analysis according to the watershed approach led to the identification of 14 drainages that leave USFS-administered land and are potentially impacted by sediment transport from the mapped mine sites. A stratigraphic relationship between the sediments of the various drainages could not be established. The transition from drainage sediment to weathered in-situ formation in the cores was based on the presence of lag deposits like gravel or coarse sand within the section that indicate the base of the channel. In some cores, a paleosol horizon was chosen to represent the transition zone. Nevertheless, this transition could not be determined in several cores because a clear lag deposit or paleosol horizon was missing, and the underlying in-situ sediments of the Ludlow or Hell Creek Formations were similar in grain size and appearance to the drainage sediments.
Drainages potentially impacted by mining activities generally had uranium concentrations within the background limits at the USFS-administered/private land interface. Background limits were exceeded only minimally by 0.1 mg/kg in Schleichart Draw and 0.4 mg/kg in the drainage below confluence of Devils and Sawmill Canyons. Uranium concentrations also varied between background and 2x background. These variations may be attributed to either the scouring and random redistribution of more contaminated sediments within the stream channel or to sediment contributions from coal seam outcrops that may contain low-level uranium concentrations in the lower Pete’s and Crooked Creek drainages. The upper Pete's Creek drainage displayed the highest uranium concentrations.
Sediment sampling sites and distribution around the abandoned uranium mines.
Uranium concentrations of sediment samples in the Pete’s Creek drainage. Samples plotted in red are the highest concentrations and naturally attenuate within about 7 km of stream length.
Uranium concentrations of sediment samples plotted against their distance to potential sources in the Pete’s Creek drainage. Samples in the lower part of the drainage might plot several times at the same concentration, but at different distances to various source areas. Side drainages are plotted arbitrarily at 0.5 km distance because they are not related to any of the sources.
Sediment samples consisting of in-situ formation generally exhibited arsenic concentrations within the background range except for below the confluence of Devils and Sawmill Canyons where the concentration was 2x background. Arsenic concentrations from in-situ formation samples in the upper Pete’s Creek drainage were also within the background or 2x background range, even within the coal seams that contained elevated uranium concentrations.
Arsenic concentrations varied in the lower Pete’s Creek and Crooked Creek drainages between the background and 2x background. Site #31 is located at the lower end of and . Sediments at the base of a side drainage of Pete’s Creek, connected to the west side of an abandoned uranium mine on private land near the town of Ludlow (Flat Top mine) exhibited arsenic concentrations of 2x background which could indicate historic releases of contaminated sediments from the Flat Top mine on private property. Nevertheless, the uranium concentration was within the background limit, and the sediment flux in this drainage is well checked by a series of large stock dams. It is more likely that that the varying arsenic concentrations at this site and in the lower Pete’s Creek and Crooked Creek drainages are due to the same reasons as for the varying uranium concentrations. Furthermore, crop farming increases in the lower part of these drainages, and contributions from non-point sources of arsenic related to the use of arsenic-containing pesticides could also be possible.
Sediment sampling sites for arsenic.
Arsenic concentration in stream sediments potentially affected by abandoned mine sites.
Arsenic concentrations of sediment samples plotted against their distance to potential sources in the Pete’s Creek drainage.