The primary public concern surrounding fracking—the fracturing of shale rock layers with hydraulic pressure to release the natural gas and oil they contain—has been the perceived risk to drinking water. After all, the water used to fracture the rock is laced with chemicals that enhance the process, and some of them are hazardous. While those chemicals haven’t really shown up in water wells, natural gas has. If natural gas isn’t identified and vented, it could collect in buildings and pose an explosion hazard—videos of garden hoses turned into flame-throwers have made the rounds.
But tying that natural gas to fracking projects isn’t as straight-forward as many assume since there are natural sources of methane as well. One group of researchers has been studying this question for several years, focusing on Pennsylvania, where the Marcellus Shale has been targeted by the natural gas industry. A controversial analysis the group performed concluded that natural gas in well water was more common near active natural gas production wells, indicating that much of the contamination was related to recent human activities rather than natural conditions.
The researchers also looked for hints of natural migration of fluids from the Marcellus Shale, which is deep underground, to the well water, which is taken from sources closer to the surface. By analyzing elements like chlorine and strontium, they identified the fingerprint of briney Marcellus fluid in some of the water wells, which pull from an aquifer where concentrations of those elements are much lower. They concluded that some of those fluids were present, casting doubt on the idea that the Marcellus Shale was too tight a seal to allow fluid to escape upward into drinking water. That work also indicated that some of the methane-contaminated wells seemed to be impacted by naturally occurring methane, but typically the ones close to natural gas production wells weren’t.
Searching for nobles
That same research group has published a new study in the journal PNAS looking at what the analysis of noble gas isotopes in water samples can add to our understanding. That analysis includes 133 wells in the same area of Pennsylvania as well as 20 wells in Texas, where the Barnett Shale is being fracked for natural gas.
Noble gases have the potential to tell us where the gas in water samples came from partly because they are chemically nonreactive, making it easier to tell what they’ve been doing underground. Some isotopes, like neon-20 and argon-36, will be common in water recently exposed to the atmosphere. Others, like helium-4, neon-21, and argon-40, are produced by radioactive decay and will predominate in deep, old, isolated groundwater.
The researchers think that these isotopic signatures can help differentiate between gas that migrated slowly upward from natural gas sources above the Marcellus and gas from the Marcellus that took a shortcut through a production well before escaping, for example. Some of their well samples—the same ones that seemed influenced by deep brines—bore the fingerprint of the natural process of slow gas migration.
But some other wells less than a kilometer from natural gas production sites told a different story. The noble gas isotopic signature in those samples has been interpreted to match the shortcut scenario. Four clusters of Pennsylvania wells seemed to contain Marcellus gas that leaked out of the production well and into the drinking water aquifer. Three clusters looked more like gas from a shallower rock layer that escaped upward along the outside of a production well, where cement doesn’t always perfectly fill the space around the well pipe. An eighth cluster involved Marcellus gas leaking upward in a different manner, likely due to a mechanical problem in a nearby production well.
For the Texas wells, five of the 20 had a human-caused signature, probably relating to shallower gas migrating along the outside of the production well.
The fingerprinting these researchers are attempting is undeniably tricky, but it once again points to the seal in and around natural gas wells as the primary vulnerability raising contamination risk. That has always been more plausible than having the fracturing of the shale rocks allow the free flow of contaminants into overlying drinking water aquifers—at least for geological configurations like the Marcellus Shale. The researchers conclude, “In our opinion, optimizing well integrity is a critical, feasible, and cost-effective way to reduce problems with drinking water contamination and to alleviate public concerns accompanying shale gas extraction.”