Chemistry and age of groundwater in bedrock aquifers of the Piceance and Yellow Creek watersheds, Rio Blanco County, Colorado, 2010-12

Fourteen monitoring wells completed in the Uinta and Green River Formations in the Piceance Creek and Yellow Creek watersheds in Rio Blanco County, Colorado, were sampled for chemical, isotopic, and groundwater-age tracers to provide information on the overall groundwater quality, the occurrence and distribution of chemicals that could be related to the development of underlying natural-gas reservoirs, and to better understand groundwater residence times in the flow system. Methane concentrations in groundwater ranged from less than 0.0005 to 387 milligrams per liter. The methane was predominantly biogenic in origin, although the biogenic methane was mixed with thermogenic methane in water from seven wells. Three BTEX compounds (benzene, toluene, and ethylbenzene) were detected in water from six of the wells, but none of the concentrations exceeded Federal drinking-water standards. The presence of thermogenic methane in the aquifers indicates a connection and vulnerability to chemicals in deeper geologic units. Helium-4 data indicate that groundwater had ages ranging from less than 1,000 years to greater than 50,000 years. The presence of old groundwater in parts of the aquifers indicates that these aquifers may not be useful for large-scale water supply because of low recharge rates.