The Uncompahgre Watershed Partnership (UWP) has completed its first report for the Ouray Hydrodam Sediment Release Study, detailing the water quality impacts of the annual release into the Uncompahgre River. Due to the release’s increased flows and concentrations of heavy metals compared to pre-release flows in the river,the study compares metals concentrations to water quality standards for various beneficial uses of water. Additionally, the study can help determine if there are any substantial environmental or public health issues created by the release.
UWP conducted the initial Ouray Hydrodam sediment release study in March 2017, during an early snowmelt period where flows were substantially higher than typical for March. Observations from this study are specific to this year’s event and additional study should occur to improve certainty.
Here is a link to the full Study Report, published in November 2017. In summary, the March 2017 study found the following:
Change in metal concentrations during the release: Total metal concentrations increased during the release and returned to concentrations similar to the pre-release concentrations. Dissolved metal concentrations decreased for most metals during the release. Dissolved metal concentrations generally returned to pre-release concentrations following the sediment release. Both patterns are expected given the large volume of water and sediment released from the sluice gate during the sediment release.
Metal concentrations and aquatic life standards: All metal concentrations attained aquatic life standards during the release.
Metal concentrations and human-health standards during the sediment release: The EPA classifies arsenic as a Class A carcinogen (most dangerous classification). This classification results in a very low human-health standard (0.02 ug/L of total arsenic). An ambient concentration of less than 10 ug/L is considered acceptable for raw drinking water supplies. Arsenic concentrations measured during the sediment release below the dam and near Ouray exceeded the human-health and raw water supply criteria; concentrations at CR-24 near Ridgway exceeded the human-health criterion.
The arsenic concentrations measured at all three locations exceeded the human-health criterion before and after the sediment release. Arsenic concentrations measured in the Uncompahgre River near Ouray and Ridgway occasionally exceeded the human-health and raw water supply criteria during the 15-year period of record.
Although the hydrodam sediment releases influence arsenic concentrations, there may be other sources and controls within the watershed. Additional study may be warranted to better understand arsenic concentrations and sources within the Uncompahgre River Watershed.
Other metals did not exceed human-health criteria.
Follow-up study required for better understanding: UWP plans to coordinate with local stakeholders to design a follow-up study. We secured partial funding for a 2018 study from Ouray County and the Town of Ridgway, and are seeking additional funding to implement the study.