Developing Site-Specific Total Suspended Solids Criteria for the Cheyenne River, South Dakota

The Cheyenne River Watershed below Angostura, in western South Dakota, is listed as impaired by the state of South Dakota for total suspended solids (TSS). The water-quality standard includes a beneficial use of warm-water permanent fish life propagation and a TSS numeric criterion for a single sample of 158 milligrams per liter (mg/L). A Total Maximum Daily Load (TMDL) watershed assessment was initiated in May 2004 focused on TSS to address this impairment. The project was a two-phase assessment with the first phase focused on historic data analysis. It became clear during the first phase that TSS concentrations routinely exceed current numeric criteria and that site-specific criteria may be necessary. The U.S. Environmental Protection Agency (EPA) requires physical, chemical, and biological data to support adoption of site-specific criteria. To meet these requirements, the second phase of the TMDL assessment focused on the collection and analysis of biological, physical, and water-quality data. Sixty-four sites were established for classification and assessment of physical habitat and the benthic macroinvertebrate community following the methodologies from the Environmental Monitoring and Assessment Program (EMAP) and from South Dakota's Standard Operating Procedure for biologic and physical habitat data collection. Sites were distributed throughout the watershed to characterize the habitat gradient from the upper reaches down to the confluence with the Missouri River. Measured and calculated physical habitat metrics were analyzed using principal component and cluster analysis to characterize the condition of the Cheyenne River Watershed and the variability contained therein. Benthic macroinvertebrates were collected and Index of Biotic Integrity (IBI) scores were calculated at all sampling locations. Biologic sampling of the benthic macroinvertebrate community suggests that there may be some level of impairment of the aquatic ecosystem in the watershed when compared to watershed reference condition; however, this impairment is most likely more closely related to other water-quality constituents than to sediment. Review of previous fisheries studies conducted in the watershed suggests that the Cheyenne River hosts a wide variety of fish species, suggesting that some level of the fisheries beneficial use is being met; however, biotic integrity based on vertebrate populations have not been calculated as part of previous fisheries assessments. Water-quality samples were collected at 19 sites over a 2-year period, with a mix of baseflow and storm event sampling. Thirteen sites were U.S. Geological Survey (USGS) sites where real-time discharge and water-quality data were available. Continuous-stage recorders were installed at six sites where stage/discharge measurements were recorded. Two sites were South Dakota Department of Environment and Natural Resources (SD DENR) Water-Quality Monitoring (WQM) stations where discharge was the only added data collection. These data were used for a statistical analysis of water quality as well as for calibration/validation of the watershed model. The second phase of the TMDL assessment confirmed that natural background for TSS far exceeds the current numeric criteria. Based on the findings of this project, it was determined that a site-specific criterion is appropriate for the Cheyenne River for TSS. The final product of this assessment will be a recommended new numeric criterion for TSS.