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Characterization of selenium concentrations and loads in select tributaries to the Colorado River in the Grand Valley, western Colorado

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Characterization of selenium concentrations and loads in select tributaries to the Colorado River in the Grand Valley, western Colorado

Study Area: Colorado River in the Grand Valley, western Colorado
Period of Project: 2005-2007
Project Number: BSD00
Project Chief: Ken Leib
Cooperator: Mesa County, City of Grand Junction

BACKGROUND:

Selenium impairment of stream segments from non-point sources in western Colorado is of concern to local, state, and federal governments, local water providers, and local land users.

As a result of elevated selenium concentrations, many western Colorado rivers and streams are on the 2004 Colorado 303(d) list including the main stem of the Colorado River, from the Gunnison River confluence to the Utah border. Selenium is a trace metal that bioaccumulates in aquatic food chains and has been known to cause reproductive failure, deformities, and other adverse impacts in birds and fish, including some threatened and endangered fish species.

The Colorado River and portions of Colorado River tributaries in the Grand Valley, located within the 100-year floodplain of the Colorado River, are designated critical habitat for four fish species listed under the Endangered Species Act (ESA)(Colorado Pikeminnow, Razorback Sucker, Bonytail, and Humpback Chub).

Selenium exists naturally in the Mancos Shale derived soils common to western Colorado. Studies in the Grand and Gunnison Valley region of western Colorado suggest that selenium mobilization occurs primarily in shallow aquifers, which are present as a result of irrigation and water delivery through unlined canal networks. Water in shallow aquifers is a diffuse source of return flows to tributaries and the Colorado River, thus making it difficult to determine where major loading sources of selenium occur. Irrigation is common in the Grand Valley in both agricultural and urban settings. In the Grand Valley, there is a steady transition from agricultural land use to urban land use that is expected to continue indefinitely. How the transition will affect selenium levels in rivers and streams of the Grand Valley has not been determined.

OBJECTIVES:

To characterize sources of selenium loads to three Colorado River tributaries that drain watersheds with varying land uses: agricultural, urban, or transitional agricultural to urban land use.

• Tracer injection techniques will be used to measure streamflow.
• Synoptic water-quality sampling will be done in conjunction with the tracer injection study to obtain the selenium concentration data needed to calculate selenium loads.