Arsenic levels in the Uncompahgre River in Ouray County continue to exceed state water-quality standards for human health, according to a recently released Uncompahgre Watershed Partnership (UWP) study. 

Though not a direct source of drinking water for homes and businesses in Ouray, Ridgway, Loghill and other downstream neighborhoods, the river is used for agriculture and recreation, and may be connected to underground sources that feed nearby wells.

The Uncompahgre River is known to have relatively high concentrations of several heavy metals, such as manganese, aluminum and iron, since it has many tributaries that pass through both naturally high mineral content in the mountains and minerals exposed by past mining activity. The water flowing through the river between Red Mountain Pass and Ridgway Reservoir turns various shades of green, yellow and orange at different times throughout the year due to human-caused and natural events that increase the flows of heavy metals, according to the report. 

Over the years, the Ouray County government has fielded calls from concerned people when the river’s color was brightest. One annual event that elicits such a public response is the sluicing of the Ouray Hydrodam, when a gate at the bottom of the dam is opened to release sediment from the reservoir. The sediment flows into and builds up in the reservoir each year, and must be released to improve operations. This release, usually once per year, sends an orange plume down the river. 

“The hydrodam has a storage capacity of less than one acre-foot, which fills quickly with sediment and precipitated metals from the inflow. The annual sluice event releases accumulated sediment and metals in hours rather than slowly, over the period of a year,” said UWP Board Member Dennis Murphy, a retired Bureau of Land Management hydrologist who volunteered for the study.

Some community members have wondered if the plume with its higher concentrations of metals has negative impacts on the Uncompahgre River. Last March, a group of volunteers with hydrology expertise, led by UWP Project Manager Agnieszka Przeszlowska, took water and sediment samples before, during and after the dam release at three different locations along the river.

Analysis of the sampling data showed that the water and sediment released from the hydrodam raised water levels in the river for a short period of time. The streamflow of the Uncompahgre River near Ouray increased from 141 cubic feet per second (cfs) to 174 cfs for less than 30 minutes. Downstream, near Ridgway, the streamflow peaked at 170 cfs for approximately three hours and 30 minutes, only 2 cfs higher than the 168 cfs peak the previous day.

During the release, measurements showed substantially raised total metal concentrations, including manganese, aluminum, arsenic, cadmium, copper, iron, lead, nickel, selenium, silver and zinc. All metal concentrations met aquatic life standards and most metals met human health standards, according to the state water-quality criteria.

However, both manganese and arsenic exceeded water quality standards for human health. The hydrodam release is not suspected to be an original source of the manganese and arsenic concentrations, so UWP recommends additional study to better understand sources and concentrations within the watershed. 

Manganese exceeded state standards before, during and after the release at the sampling location below the dam, but attained levels within safety standards at the other two sampling locations at certain times around the release. Drinking water sources, including wells, are not located near the dam, and the overall manganese concentrations were considered relatively benign.

The arsenic concentrations, which exceeded the human health criterion before, during and after the sediment release at all three sampling locations, have been shown to cause more negative health impacts at low levels. 

“The EPA classifies arsenic as a Class A carcinogen, meaning it may pose the highest risk of cancer. This classification results in a very low human health standard (0.02 microgram per liter of total arsenic),” according to the report produced for UWP by Ashley Bembenek and Julia Nave of Alpine Environmental Consultants in Crested Butte.

The arsenic concentrations are not new in the Uncompahgre River near Ouray and Ridgway, which have occasionally exceeded the human-health and raw water supply criteria in other measurements taken over the past 15 years. 

The UWP study did not directly investigate the potential effect of the sediment release on public water supplies, because the raw source waters for local utilities are all upstream from the Uncompahgre River and do not receive any flows from the releases. While those supplies would be unaffected by the sediment release, wells in the area may be effected. They were not studied in 2017, but there are plans to study them in the future.

“This initial study was conducted under significant time, labor and financial constraints, so it did not provide as complete a picture as we had hoped. However, using what we learned from this study will be beneficial to better design future studies and monitor potential water quality issues in the Upper Uncompahgre Valley,” said Murphy, who made a presentation of the report’s findings to the Ouray County Board of Commissioners on Jan 29. “Sampling the water quality of domestic wells in the valley bottom, that may be pumping water connected to the river, might expose some potential health issues previously undetected.”

If a well is properly made, it shouldn’t be impacted by river water, but all wells should be tested on a regular basis to check water quality, explained Ouray County Public Health Director Elisabeth Lawaczeck, adding that her office has been working with Delta County to help property owners in a six-county area to test wells through a Center for Disease Control grant. She is collaborating with UWP to follow up with property owners in the county and assist in well tests in the area of the Uncompahgre River. 

As far as the health impacts of arsenic on recreational users, the Colorado Department of Public Health and Environment put out an advisory after the 2015 Gold King Mine spill into the Animas River, stating that it “does not anticipate adverse health effects from exposure to contaminants detected in the sediment during typical recreational activities or through incidental contact with the sediment.” 

The department recommends prudent public health practices when coming into contact with sediment and surface water containing heavy metals: Don’t drink untreated water from the river. Wash hands thoroughly with soap and water after contact. Avoid contact in areas where there is visible discoloration in sediment or river water. Wash clothes after contact. Supervise young children to make sure they follow these recommendations.

Ouray County Commissioner Ben Tisdel commented, “It’s important to remember this is one of the most mineral-soaked environments on the crust of the Earth. Whether it’s the sluicing or a chronic background situation, it’s important to know what’s in the water.”

The full report on the Ouray Hydrodam Sediment Release is available online at

Editor’s Note: Tanya Ishikawa is the Uncompahgre Watershed Partnership communications director.