Quality and Age of Shallow Groundwater in the Bakken Formation Production Area, Williston Basin, Montana and North Dakota

P.B. McMahon, R.R. Caldwell, J.M. Galloway, J.F. Valder, A.G. Hunt

The quality and age of shallow groundwater in the Bakken Formation production area were characterized using data from 30 randomly distributed domestic wells screened in the upper Fort Union Formation. Comparison of inorganic and organic chemical concentrations to health based drinking-water standards, correlation analysis of concentrations with oil and gas well locations, and isotopic data give no indication that energy-development activities affected groundwater quality. It is important, however, to consider these results in the context of groundwater age. Most samples were recharged before the early 1950s and had (14) C ages ranging from <1000 to >30,000 years. Thus, domestic wells may not be as well suited for detecting contamination associated with recent surface spills as shallower wells screened near the water table. Old groundwater could be contaminated directly by recent subsurface leaks from imperfectly cemented oil and gas wells, but horizontal groundwater velocities calculated from (14) C ages imply that the contaminants would still be less than 0.5 km from their source. For the wells sampled in this study, the median distance to the nearest oil and gas well was 4.6 km. Because of the slow velocities, a long-term commitment to groundwater monitoring in the upper Fort Union Formation is needed to assess the effects of energy development on groundwater quality. In conjunction with that effort, monitoring could be done closer to energy-development activities to increase the likelihood of early detection of groundwater contamination if it did occur.