Geochemistry and age of groundwater in the Williston Basin, USA: Assessing potential effects of shale-oil production on groundwater quality

Thirty water wells were sampled in 2018 to understand the geochemistry and age of groundwater Williston Basin assess potential effects shale-oil production from Three Forks-Bakken petroleum system (TBPS) on quality. Two geochemical groups are identified using hierarchical cluster analysis. Group 1 represents younger (median 4He = 21.49 × 10?8 cm3 STP/g), less chemically evolved water. 2 older 1389 more At least two samples each group contain elevated Cl concentrations (>70 mg/L). Br/Cl, B/Cl, Li/Cl ratios indicate multiple sources account for concentrations: septic-system leachate/road deicing salt, lignite beds aquifers, Pierre Shale beneath associated with TBPS (one sample). 3H 14C data that 10.8, 21.6, 67.6% modern (post-1952), mixed age, premodern (pre-1953), respectively. Lumped-parameter modeling 3H, SF6, 3He, indicates mean ages fractions range ~1 30 years 1300 >30,000 years, contains highest CH4 (0.0018–32 ?13C–CH4 C1/C2+C3 (?91.7 ?70.0‰ 1280 13,600) is biogenic origin not thermogenic shale gas. Four volatile organic compounds (VOCs) detected samples. One mixed-age sample chloroform (0.25 ?g/L) dichloromethane (0.05 ?g/L), which probably septic leachate. butane (0.082 n-pentane (0.032 gas a nearby oil well. The hydrocarbon activities do currently (2018) widely affect Cl, CH4, VOC groundwater. predominance recharge aquifers moves relatively slowly, could inhibit widespread chemical movement overlying TBPS. Comparison groundwater-age five major unconventional hydrocarbon-production areas aquifer zones used supply area have lower risk than similar Fayetteville (Arkansas) Marcellus (Pennsylvania) areas, but higher Eagle Ford (Texas) Haynesville (Texas, Louisiana) areas.