Scientific Investigations Report 2009–5082
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
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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
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
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