USGS examined 116 US and public-supply wells as close as 360 ft to unconventional oil and gas wells. Methane was detected in 91% of the wells, and, of those, 90% had methane concentrations lower than the threshold of 10 mg/l., which was proposed by the Department of the Interior Office of Surface Mining, Reclamation, and Enforcement for the purposes of protection from explosive risk. Most of the methane detected in groundwater was from naturally occurring microbial sources at shallow depths rather than deep shale gas.
Although benzene was detected in 8% of the wells sampled, concentrations were low—the highest concentration was nearly 40 times lower than the federal standard for benzene in drinking water. Benzene was detected 1.5-8 times more frequently in the study area groundwater than in national data sets of benzene in groundwater, USGS notes.
Groundwater in the Louisiana and Texas study areas typically entered the aquifers several thousand years ago. Nearly all the benzene detected in those areas occurred in old groundwater, indicating it was from subsurface sources such as natural hydrocarbon migration or leaking oil and gas wells.
In Arkansas, groundwater was much younger—typically less than 40 years old. Benzene was detected in one sample of young groundwater in Arkansas that could be associated with a surface release associated with unconventional oil and gas production activities.