CHARLESTON, W.Va. -- A new study by researchers in Texas has documented high levels of metals in drinking water supplies near natural gas production sites, a finding they say shows the need for more research into the impacts of the nation's gas-drilling boom.
Elevated concentrations of arsenic, selenium and strontium were discovered in drinking water wells located closest to natural gas extraction sites, according to the study, by a team of scientists from the University of Texas at Arlington.
Researchers did not pinpoint the exact source of the contamination, and said their findings were not strong enough to suggest "systematic contamination of groundwater" by the natural gas industry.
"We suggest that episodic contamination by private water wells could be due to a variety of natural and anthropogenic factors such as the mobilization of naturally occurring constituents into private wells through mechanical disturbances caused by intense drilling activity, reduction of the water table from drought or groundwater withdrawals, and faulty drilling equipment and well casings," said the study, published online Thursday by the journal Environmental Science and Technology.
The study is latest in a series of scientific assessments that are just beginning to examine the potential water quality impacts of a nationwide natural gas boom driven by technological advances in hydraulic fracturing and horizontal drilling.
"This study alone can't conclusively identify the exact causes of elevated levels of contaminants in areas near natural gas drilling, but it does provide a powerful argument for continued research," said lead author Brian Fontenot, a UT Arlington graduate who now works for the U.S. Environmental Protection Agency.
The effort focused on water quality in the Barnett Shale, a gas-rich geologic formation that underlies a 5,000-square-mile area in 17 counties of north Texas.
Researchers sampled 100 water wells from the Trinity and Woodbine aquifers, overlying the Barnett Shale and, as "reference sites" from the Nacatoch aquifer east of the Barnett Shale.
One piece of potential good news was that the study detected none of the family of BTEX chemicals - benzene, toluene, ethylbenzene and zylenes - in the drinking water, a possible indication that chemicals used in the hydraulic fracturing, or "fracking" process had not migrated into the water wells.
But, researchers detected the highest levels of metal contaminants within 3 kilometers of natural gas wells, including several samples that had arsenic and selenium above concentrations considered safe by EPA. Areas lying outside of active drilling areas or outside the Barnett Shale did not contain the same elevated levels for most of the metals.
"At minimum, these data suggest that private wells located over natural gas wells may be a higher risk for elevated levels of constituents than those located further from natural gas wells," the study concluded.
Duke University scientist Robert Jackson, who has done some widely cited work on gas drilling impacts, said one limitation of the new Texas study was its small number of "reference" sites - just nine, with five outside the Barnett Shale and four within the shale but not near active drilling.
Still, Jackson said, "It's an important study that should be followed up."
As they push for more natural gas, drilling operators are increasingly using a process called hydraulic fracturing, or fracking, which shoots vast amounts of water, sand and chemicals deep underground to break apart rock and release the gas. Much of the modern gas boom also involves drilling down and then turning horizontally to access more gas reserves.