A new study by the U.S. Geological Survey (USGS) said natural gas drilling in the Eagle Ford, Fayetteville and Haynesville shales is not a significant source of methane or benzene found in drinking water wells located in areas of drilling operations.
However, USGS said it could take decades before the effects of unconventional oil and natural gas production on groundwater in Arkansas, Louisiana, and Texas is fully known. An article detailing the study, “Methane and Benzene in Drinking-water Wells Overlying the Eagle Ford, Fayetteville, and Haynesville Shale Hydrocarbon Production Areas,” is published online in the journal Environmental Science & Technology.
The study is the first of these areas “to systematically determine the presence of benzene and methane in drinking water wells near unconventional oil and gas production areas in relation to the age of the groundwater,” USGS said.
“Understanding the occurrence of methane and benzene in groundwater in the context of groundwater age is useful because it allows us to assess whether the hydrocarbons were from surface or subsurface sources,” said USGS hydrologist Peter McMahon, study lead. “The ages indicate groundwater moves relatively slowly in these aquifers. Decades or longer may be needed to fully assess the effects of unconventional oil and gas production activities on the quality of groundwater used for drinking water.”
USGS said it examined 116 domestic and public-supply water wells in Arkansas, Louisiana and Texas that were as close as 360 feet 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 milligrams per liter. The Department of the Interior Office of Surface Mining, Reclamation, and Enforcement proposed this threshold to minimize explosion risks.
Most of the methane detected in groundwater was from naturally occurring microbial sources at shallow depths rather than from deep shale gas, USGS said.
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 (5 micrograms per liter). Benzene was detected about 1.5-8 times more frequently in the study area groundwater than in national data sets of benzene in groundwater, USGS said.
“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,” USGS said.
“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.”
Earlier this year the U.S. District Court for the Middle District of Pennsylvania reversed a federal jury's March 2016 decision that required Cabot Oil & Gas Corp. to pay damages to two families in rural Dimock Township that had alleged contamination from area drilling operations. A new trial was ordered.