DISTINGUISHING NATURAL AND ANTHROPOGENIC SOURCES OF WATER QUALITY VARIABILITY, SOUTHEASTERN PICEANCE BASIN, COLORADO

Water resources in semi-arid western Colorado are scarce and especially vulnerable to impact by petroleum production activities. In Mamm Creek natural gas field in the southeastern Piceance Basin, gas well completion problems caused gas seepage into a tributary of the Colorado River. Using this case to define impact, hydrochemical data from the surrounding area was analyzed. Normalized and standardized inorganic data from 648 water samples were separated into five statistically distinct groups, representing water facies, using hierarchical clustering. One cluster was low TDS, Ca-Mg-HCO3 water consistent with natural background. Other clusters showed “impacted” signatures: high Fe-Mn, high TDS Na-Cl or high TDS Na-SO4-HCO3. Samples with high Fe-Mn signatures are found primarily at the methane seep location where reducing conditions dominate during active seeping. These samples also show elevated benzene and methane concentrations, indicating direct petroleum contamination. Samples with high Na-Cl signatures are indicative of formation water from the gas production zone. These samples have the highest average methane concentrations, but low benzene concentrations suggesting migration from the petroleum source has degraded the benzene. Samples with high Na-SO4-HCO3 signatures are not associated with hydrocarbon deposits, due to their characteristic reducing environment. The presence of this water facies suggests upward migration of formation waters from a deep source.

The positive correlation between average groundwater methane concentration by year and the total number of wells in the study area, in addition to the lack of correlation between water facies and structural trends, suggests that these impacted waters may be due to additional transport pathways caused by well drilling. Possible sources of methane include geologic deposits, gas wells leaking petroleum product, or bradenhead gas escaping from well casings. Geochemical modeling, including one-dimensional reactive transport and inverse mixing, is being conducted to evaluate sources of Na-Cl, Na-SO4, methane and benzene.