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. Industry officials insist the process is safe. But the new Texas study notes a concern among scientists about the lack of hard data on the drilling boom's potential environmental effects. "Despite a number of recent investigations, the impact of natural gas extraction on groundwater quality remains poorly understood," the Texas study said. Another study, published in May by the journal Science, found that, "It is difficult to determine whether shale gas extraction in the Appalachian region since 2006 has affected water quality regionally, because baseline conditions are often unknown or have already been affected by other activities, such as coal mining." Last week, The Associated Press reported on what it said were the "preliminary" results of a U.S. Department of Energy study that found "no evidence" that drilling chemicals contaminated drinking water aquifers near at a western Pennsylvania site. The study marks the first time that a drilling company has allowed government scientists to inject special tracers into fracking fluid to monitor if the chemicals spread toward drinking water sources. Shelley Martin, a spokeswoman for DOE's National Energy Technology Laboratory, later told the Gazette, "While nothing of concern has been found thus far, the results are far too preliminary to make any firm claims." "We are still in the early stages of collecting, analyzing and validating data from this site," Martin said in an email earlier this week. "We expect a final report on the results by the end of the calendar year." Martin did not respond to a follow-up request for data from the "preliminary" results or for an interview with the scientists involved in the study. In West Virginia, business and political leaders are eager to expand natural gas drilling, to tap into the vast reserves contained in the Marcellus Shale, a formation that stretches from southern New York and into eastern Ohio. Between 2003 and 2011, West Virginia's natural gas production more than doubled to nearly 400 million cubic feet. Over roughly the same period, employment in the industry increased by 55 percent, to more than 10,000. The Obama administration has embraced shale-gas drilling, with the president saying in a major speech last month that natural gas is "the transition fuel that can power our economy with less carbon pollution" as the nation moves toward "the even cleaner energy economy of the future." Obama acknowledged that more needs to be done to make natural gas drilling safe, and noted the need for better control leaks of the potent greenhouse gas methane from gas production. At the same time, EPA has backed off major investigations of drilling impacts on water quality in Wyoming, Pennsylvania and Texas, and the agency doesn't plan to issue a draft of a nationwide study of the issue until late 2014. Reach Ken Ward Jr. at email@example.com or 304-348-1702.