METHANE AND BENZENE IN DRINKING-WATER WELLS OVERLYING THE EAGLE FORD, FAYETTEVILLE, AND HAYNESVILLE SHALE OIL AND GAS PRODUCTION AREAS

Drinking-water wells in aquifers overlying the Eagle Ford (EF), Fayetteville (FV), and Haynesville (HV) Shales were sampled to assess the occurrence of methane and benzene in groundwater in areas with extensive unconventional oil and gas (UOG) development. Methane concentrations ranged from <0.001 to 24 milligrams per liter (mg/L) in the EF, <0.001 to 25 mg/L in the FV, and 0.001 to 22 mg/L in the HV. Isotopic data for methane, water, and dissolved inorganic carbon (DIC) indicated the high methane concentrations were generally from biogenic production. Gas produced from the shales is typically thermogenic in origin. Samples with high methane concentrations were associated with low concentrations of dissolved oxygen, sulfate, and carbon-14 in DIC and elevated helium-4 concentrations, patterns consistent with evolution of groundwater toward methanogenic redox conditions. Methane concentrations were not significantly different in water wells located inside and outside a 1 km radius from oil and gas wells, nor were there significant correlations between concentrations and distance to oil and gas wells or density of oil and gas wells within 1 km of water wells. Benzene was detected in 1 (HV), 3 (EF), and 4 (FV) samples at concentrations ranging from 0.013 to 0.13 microgram per liter (μg/L), which are below the federal drinking-water standard of 5 μg/L. The HV benzene detection was not related to UOG development because benzene was also detected in that same well prior to development. One of the FV benzene detections occurred in young groundwater (<10 years old on the basis of tritium, helium-3, sulfur hexafluoride, and carbon-14 data) and co-occurred with PCE, indicating the benzene was probably from a land-surface source. An UOG source for that benzene cannot be ruled out because the age of the groundwater coincided with the period of FV UOG development and the water well was 0.4 km from an UOG well. Most of the remaining benzene detections occurred in old groundwater (>1,000 years old), indicating the benzene was from subsurface sources. An UOG source for some of those benzene detections cannot be ruled out because of their proximity to UOG wells (0.03 to 1.5 km). Overall, UOG development did not appear to have a substantial widespread effect on methane and benzene concentrations in the studied aquifers on the basis of this analysis.