Abstract Thirty water wells were sampled in 2018 to understand the geochemistry and age of groundwater in the Williston Basin and assess potential effects of shale-oil production from the Three Forks-Bakken petroleum system (TBPS) on groundwater quality. Two geochemical groups are identified using hierarchical cluster analysis. Group 1 represents the younger (median 4He=21.49×10-8 cm3 STP/g), less chemically evolved water. Group 2 represents the older (median 4He=1389×10-8 cm3 STP/g), more chemically evolved water. At least two samples from each group contain elevated Cl concentrations (>70 mg/L). Br/Cl, B/Cl, and Li/Cl ratios indicate multiple sources account for the elevated Cl concentrations: septic-system leachate/road deicing salt, lignite beds in the aquifers, Pierre Shale beneath the aquifers, and water associated with the TBPS (one sample). 3H and 14C data indicate that 10.8, 21.6, and 67.6% of the samples are modern (post-1952), mixed age, and premodern (pre-1953), respectively. Lumped-parameter modeling of 3H, SF6, 3He, and 14C concentrations indicates mean ages of the modern and premodern fractions range from ∼1 to 30 years and 1300 to >30,000 years, respectively. Group 2 contains the highest CH4 concentrations (0.0018 to 32 mg/L). δ13C-CH4 and C1/C2+C3 data in groundwater (-91.7 to -70.0‰ and 1280 to 13,600) indicate groundwater CH4 is biogenic in origin and not from thermogenic shale gas. Four volatile organic compounds (VOCs) were detected in two samples. One mixed-age sample contains chloroform (0.25 μg/L) and dichloromethane (0.05 μg/L), which are probably associated with septic leachate. One premodern sample contains butane (0.082 μg/L) and n-pentane (0.032 μg/L), which are probably associated with thermogenic gas from a nearby oil well. The data indicate hydrocarbon production activities do not currently (2018) widely affect Cl, CH4, and VOC concentrations in groundwater. The predominance of premodern recharge in the aquifers indicates the groundwater moves relatively slowly, which could inhibit widespread chemical movement in groundwater overlying the TBPS. Comparison of groundwater-age data from five major unconventional hydrocarbon-production areas indicates aquifer zones used for water supply in the TBPS area have a lower risk of widespread chemical movement in groundwater than similar aquifer zones in the Fayetteville (Arkansas) and Marcellus (Pennsylvania) Shale production areas, but have a higher risk than similar aquifer zones in the Eagle Ford (Texas) and Haynesville (Texas, Louisiana) Shale production areas.

中文翻译：

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美国威利斯顿盆地地下水的地球化学和年龄：评估页岩油生产对地下水质量的潜在影响

摘要 2018 年对 30 口水井进行了采样，以了解威利斯顿盆地地下水的地球化学和年龄，并评估三叉 - 巴肯石油系统 (TBPS) 的页岩油生产对地下水质量的潜在影响。使用层次聚类分析确定了两个地球化学组。第 1 组代表较年轻（中值 4He=21.49×10-8 cm3 STP/g）、化学释放量较少的水。第 2 组代表更老的（中值 4He=1389×10-8 cm3 STP/g）、更多化学释放的水。每组至少有两个样品含有升高的 Cl 浓度 (>70 mg/L)。Br/Cl、B/Cl 和 Li/Cl 比率表明 Cl 浓度升高的原因有多种：化粪池渗滤液/道路除冰盐、含水层中的褐煤床、含水层下方的 Pierre 页岩、和与 TBPS 相关的水（一个样品）。3H 和 14C 数据表明，10.8%、21.6% 和 67.6% 的样本分别是现代（1952 年后）、混合时代和前现代（1953 年前）。3H、SF6、3He 和 14C 浓度的集总参数模型表明现代和前现代部分的平均年龄范围分别为约 1 至 30 年和 1300 至 >30,000 年。第 2 组包含最高的 CH4 浓度（0.0018 至 32 mg/L）。地下水中的 δ13C-CH4 和 C1/C2+C3 数据（-91.7 到 -70.0‰ 和 1280 到 13,600）表明地下水 CH4 是生物成因的，而不是来自热成因页岩气。在两个样品中检测到四种挥发性有机化合物 (VOC)。一份混龄样品含有氯仿 (0.25 μg/L) 和二氯甲烷 (0.05 μg/L)，它们可能与化粪池渗滤液有关。一个前现代样品含有丁烷 (0.082 μg/L) 和正戊烷 (0.032 μg/L)，它们可能与附近油井的热成因气体有关。数据表明，碳氢化合物生产活动目前（2018 年）并未广泛影响地下水中的 Cl、CH4 和 VOC 浓度。含水层中前现代补给的优势表明地下水运动相对缓慢，这可能会抑制 TBPS 上方地下水中广泛的化学运动。来自五个主要非常规油气生产区的地下水年龄数据的比较表明，与费耶特维尔（阿肯色州）和马塞勒斯（宾夕法尼亚州）的类似含水层区相比，TBPS 地区用于供水的含水层带在地下水中发生广泛化学运动的风险较低页岩产区，