Date of this Version
In cooperation with the National Park Service, the U.S. Geological Survey conducted a stream-sediment-based environmental geochemical study in and near Yellowstone National Park (the Park). The main goals of the study were to (1) determine background concentrations for as many as 49 elements in samples of rock and stream sediment, (2) establish a geochemical baseline during the 1990s for future reference, (3) identify the source(s) of anomalies for selected elements, and (4) identify potential chemical impacts on the Park environment, especially on wildlife.
Two areas of the Park containing identified environmental geochemical problems were selected for detailed study: (1) an area in the western part of the Park that includes the Gibbon, Firehole, and Madison River basins, and (2) an area in the northeastern part that includes the Soda Butte Creek and Lamar River basins and part of the Yellowstone River basin. The geology of the first area is characterized mainly by Quaternary felsic volcanic rocks. Localities with major geothermal activity are present in this area. The second area has more complex geology that consists primarily of Tertiary volcanic rocks of intermediate composition and Precambrian schists and gneisses. Also present are scattered exposures of Paleozoic clastic and carbonate rocks and Quaternary felsic volcanic rocks. Geothermal activity is very limited in this area. Both study areas contain extensive deposits of glacial, fluvial, or lacustrine origin.
Analyses for as many as 49 elements in 393 samples of stream sediment collected from throughout the Park were evaluated statistically, including factor analysis. A five-factor model classified the elements on the basis of two lithologic factors, one mineral-deposit-related factor, one geothermal-process-associated factor, and one “miscellaneous” factor.
Data from the factor analysis, when combined with the distributions of anomalies for the elements determined in this study, show that most (34) of the elements are best correlated with rock chemistry. Many of these elements can be used to discriminate between major lithologic units and can therefore be used to assist geologic mapping studies in the Park area.
Anomalies of As, Cs, F, Hg, Mo, S, Sb, Tl, and W were determined to be associated with areas of geothermal activity. Of these nine elements, cesium is the best discriminator between anomalies related to geothermal activity and those related to outcrops of mineralized rock and to past mining activity near the Park. The effects of mineralization and past mining activity in the Cooke City, Mont., area, outside but near the northeastern boundary of the Park, are defined by anomalies of Ag, As, Au, Cu, Fe, Hg, Mo, Pb, S, Sb, Se, Te, Tl, W, and Zn, and possibly F. The effects are delineated best by anomalies of Au, Cu, and Te. Relatively weak anomalies of some of these elements extend as far as 18 km inside the Park.
In the area of Slough Creek, in the northeastern area of the Park, the source for a high concentration of lead was determined to be anthropogenic because of the sample location and a lack of anomalies of other elements that commonly are associated with natural lead anomalies. This anomaly probably is related to past fishing activity.
In high concentrations, a number of the elements associated with geothermal activity in the Park are potentially toxic to animals. Currently, only one of the 49 elements determined (fluorine) is known to affect the health and longevity of wildlife in the Park. Additional studies are needed to determine whether other elements are impacting the Park environment, including wildlife.