GROUNDWATER AGES AND MIXING IN THE PICEANCE BASIN NATURAL GAS PROVINCE, COLORADO

Groundwater monitoring activities in the United States are increasing in response to the rapid expansion of energy development. Few monitoring programs assess groundwater age even though it can provide unique information for assessing water availability, aquifer susceptibility, and chemical transport pathways. Chemical, isotopic, and age tracers were used to estimate groundwater ages and mixing with deeper saline water in three areas of the Piceance Basin natural gas province. Area 1 was dominated by pre-1950s recharge (some ages >50,000 years) that generally was not mixed with saline water. Area 2 was dominated by unmixed post-1950s recharge. Area 3 had a complex array of ages and mixing fractions, from unmixed post-1950s recharge to highly mixed pre-1950s recharge that was several thousand years old. The broad range of ages and mixing fractions reflects differences in well locations in the flow systems, land use, aquifer confinement, and geologic structure between the study areas. Methane concentrations in groundwater were related to groundwater age and mixing. Old groundwater in some of the aquifers implies millennium-scale contaminant flushing times that could have implications for groundwater monitoring programs. Moreover, aquifers with old groundwater may not be useful for large-scale water supply because of low recharge rates.