GEOLOGY, GROUNDWATER RESOURCES, AND WATER QUALITY OF PARK COUNTY, COLORADO

A multi-year study by the Colorado Geological Survey (CGS) and the Coalition for the Upper South Platte (CUSP) was conducted in Park County to develop a groundwater quality baseline prior to potential oil and gas development. The study initially focused on the central part of the South Park Basin, a north-south trending, elongated, intermontane basin recharged primarily from the surrounding mountain ranges, but was expanded to the whole county.

The most recent Park County geologic mapping and interpretations were compiled in ArcGIS. Water sample locations were attributed by hydrogeologic unit using locations within known unit outcrop and subcrop patterns and well depth information. Groundwater quality sampling was conducted at 99 water wells and 15 springs, some of which were sampled more than once. Analyte groups included anions, cations, metals, total dissolved solids, alkalinity, and radionuclides (gross alpha/beta and radon). Additional analyte groups tested specific to the year or geologic formation included dissolved gasses (methane, ethane and ethylene), volatile organic compounds, total volatile petroleum hydrocarbons (TVPH), and semi-volatile organic compounds.

A comprehensive county-wide groundwater-quality database was compiled from: CUSP/CGS water data, National Water Quality Database, other USGS publications not in the database, Colorado Department of Parks and Wildlife, and a Colorado School of Mines thesis. From this data, general water type and quality maps were prepared for each hydrogeologic unit.

Water type patterns (e.g., calcium-bicarbonate-sulfate versus sodium-chloride dominated) were observed to be geologic formation specific. Formations containing evaporites, high iron or sulfates were problematic, requiring extensive water filtration systems. Well yields were generally adequate for residential and stock use, except in shale dominated formations. This study represents a unique combination of the latest detailed mapping and GIS techniques to assess regional water quality. These data are useful in planning future development.