by Mary Caperton Morton Wednesday, April 23, 2014
In 2008, when dangerously high levels of methane were found in drinking water wells in Dimock, Pa., fingers were pointed at nearby hydraulic fracturing operations as the source of the contamination. Six years later, the debate still rages over whether the town’s methane problem was exacerbated by drilling operations or if it was there all along. Now a new study is adding some much-needed baseline data for methane levels in groundwater in the natural gas-rich Marcellus Shale region of New York. The results suggest that, in some cases, methane occurs at naturally high levels in groundwater, without the influence of gas exploration and production.
Hydraulic fracturing, also called fracking or hydrofracking, involves pumping water, sand and chemicals deep underground into horizontal gas wells at high pressure to crack open hydrocarbon-rich shale and extract oil and natural gas. The controversial practice has sparked widespread fears in public forums about drinking water contamination, but few systematic scientific studies have been published to date, says Paul Heisig, a hydrologist with the U.S. Geological Survey (USGS) based in Troy, N.Y., and lead author of the new study, published by the USGS.
“When methane problems arose in Dimock’s drinking water, the big question was: Is this natural or not natural?” he says. “But they didn’t have enough pre-drilling baseline data to say whether the methane was naturally occurring or whether there was some form of contamination from nearby gas wells.”
To establish a baseline for methane in groundwater in the region of the Marcellus Shale along the border between New York and Pennsylvania, Heisig and colleague Tia-Marie Scott, also with the USGS in Troy, sampled 66 household drinking water wells across five counties in south-central New York. The team picked wells located at least 1.5 kilometers from any existing gas operations, and those with adequate subsurface data to classify each well within a particular hydrogeologic setting, Heiser says.
They found that 15 percent of the wells tested had methane levels high enough to warrant monitoring. And half of those were dangerous enough to require mitigation, with concentrations higher than 10 milligrams per liter. Methane is not known to be toxic, but it does carry a risk of flammability or explosiveness if it accumulates without proper ventilation, Heiser says. Mitigation typically consists of enhanced venting of well casings.
The team also characterized the most common geologic setting in which methane contamination was found: Higher concentrations were found in wells located in valleys rather than uplands, and valleys capped by a thick layer of glacial till had the highest levels. Isotopic data showed that methane in valley groundwater was mostly thermogenic in origin, produced over millions of years by geologic processes deep underground. In contrast, methane found in upland groundwater was mostly biogenic, generated more recently by bacteria.
“Sixty-some wells isn’t a huge number, but it’s a start. There’s a real need for more of these kinds of baseline studies,” says Dennis Risser, a hydrologist at the Pennsylvania Water Science Center (part of USGS) in New Cumberland who was not involved in the new study. “If you query the USGS database or the Environmental Protection Agency database, you don’t find much dissolved gas data and hardly any isotopic data for this area, and most of the testing that has been done only happens after there’s been a reported problem.”
The lack of data is due mainly to a lack of funding, Heisig says. “We’d like to launch other baseline sampling programs to see if our findings can be applied to a broader region, but right now, funding is the major obstacle.”
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