Hydrocarbons in Groundwater Overlying the Eagle Ford, Fayetteville, and Haynesville Shale UOG Development Areas

Groundwater overlying the Eagle Ford, Fayetteville, and Haynesville Shale unconventional oil and gas (UOG) development areas was analyzed for chemical, isotopic, and groundwater-age tracers to evaluate sources of selected hydrocarbons in groundwater. Methane isotopes and hydrocarbon gas compositions indicate most of the methane detected in these areas was biogenic and produced by the CO₂ reduction pathway, not from thermogenic shale gas. Two samples contained methane from the fermentation pathway that could be associated with oil or fuel degradation based on their co-occurrence with fuel hydrocarbons such as ethylbenzene, butane, and MTBE. Benzene was detected at low concentrations (<0.15 µg/L), but at relatively high frequencies (2.4–13.3% of samples), in the study areas. Eight of nine samples containing benzene had mean groundwater ages >2500 years, indicating the benzene was from subsurface sources. One sample from the Fayetteville study area contained benzene that might be from a surface source associated with UOG production activities based on its mean groundwater age (10±2.4 years) and proximity to UOG wells. Overall, our results suggest groundwater in the Fayetteville study area was the most vulnerable to potential surface sources of contamination based on estimates of mean groundwater age and fractions of post-1950s water in the samples.