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Scientific Investigations Report 2009–5082

Prepared in cooperation with Eagle County, the Eagle River Water and Sanitation District,
the Town of Eagle, the Town of Gypsum, and the Upper Eagle Regional Water Authority

Groundwater Quality, Age, and Probability of Contamination, Eagle River Watershed Valley-Fill Aquifer, North-Central Colorado, 2006–2007

By Michael G. Rupert and L. Niel Plummer


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The Eagle River watershed is located near the destination resort town of Vail, Colorado. The area has a fastgrowing permanent population, and the resort industry is rapidly expanding. A large percentage of the land undergoing development to support that growth overlies the Eagle River watershed valley-fill aquifer (ERWVFA), which likely has a high predisposition to groundwater contamination. As development continues, local organizations need tools to evaluate potential land-development effects on ground- and surface-water resources so that informed land-use and water management decisions can be made. To help develop these tools, the U.S. Geological Survey (USGS), in cooperation with Eagle County, the Eagle River Water and Sanitation District, the Town of Eagle, the Town of Gypsum, and the Upper Eagle Regional Water Authority, conducted a study in 2006–2007 of the groundwater quality, age, and probability of contamination in the ERWVFA, north-central Colorado.

Ground- and surface-water quality samples were analyzed for major ions, nutrients, stable isotopes of hydrogen and oxygen in water, tritium, dissolved gases, chlorofluorocarbons (CFCs), and volatile organic compounds (VOCs) determined with very low-level laboratory methods. The major-ion data indicate that groundwaters in the ERWVFA can be classified into two major groups: groundwater that was recharged by infiltration of surface water, and groundwater that had less immediate recharge from surface water and had elevated sulfate concentrations. Sulfate concentrations exceeded the USEPA National Secondary Drinking Water Regulations (250 milligrams per liter) in many wells near Eagle, Gypsum, and Dotsero. The predominant source of sulfate to groundwater in the Eagle River watershed is the Eagle Valley Evaporite, which is a gypsum deposit of Pennsylvanian age located predominantly in the western one-half of Eagle County.

First posted July 23, 2009

For additional information contact:

USGS Colorado Water Science Center
Box 25046, MS 415
Denver, CO 80225

Or visit the Colorado Water Science
Center Web site at:

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Suggested citation:

Rupert, M.G., and Plummer, L.N., 2009, Groundwater quality, age, and probability of contamination, Eagle River watershed valley-fill aquifer, north-central Colorado, 2006–2007: U.S. Geological Survey Scientific Investigations Report 2009–5082, 59 p.





Purpose and Scope

Description of Study Area

Methods of Investigation

Water-Quality Sampling and Measurement of Groundwater Levels

Quality Assurance and Quality Control

Groundwater Age



Dissolved Gases

Low-Level Volatile Organic Compounds

Geographic Information System Data

Delineating Contributing Areas to Sampled Wells

Logistic Regression Statistical Method

Construction of the Probability Maps

Water Chemistry and Groundwater Age

Quality Assurance and Quality Contro

Major Ions



Dissolved Gases

Chlorofluorocarbons and Groundwater Age

Low-Level Concentrations of Volatile Organic Compounds

Groundwater Probability Modeling

Development of Nitrate Model

Development of Chlorofluorocarbon-11 and Tritium Model

Development of Volatile Organic Compound Model

Verification of Probability Models

Comparison of Probability Models

Appropriate Uses of the Probability Maps



References Cited

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