by Mary Caperton Morton Tuesday, December 30, 2014
Hydraulic fracturing to harvest natural gas has been controversial due in large part to the potential for contamination of ground and surface waters by the pressurized fluids used to force open cracks in deep shale formations and by the so-called flowback fluids that re-emerge at the surface from fracking wells. Now, researchers have developed a geochemical method of tracing fracking fluids in the environment; it’s a tool that could be used to identify hazardous spills in the future and may lead to better use and disposal of fracking wastewater.
Fracking involves pumping mixtures of water, sand and chemicals underground at high pressures to crack open conduits through underground reservoirs of natural gas. The gas then flows up to the surface where it can be collected, along with the fracking fluids. Most of this fluid is captured in storage containers, but occasional spills and accidents mean that portions of the flowback fluid can find their way into the environment. And tracking the fluids through the environment has proved difficult, due to the proprietary nature of the mixtures.
“Developing tracers for fracking fluids has been tricky, given that the chemistry of fracking fluid is not usually revealed. It’s a trade secret,” says Avner Vengosh, a geochemist at Duke University and co-author of a study describing the new tracing technique, published in Environmental Science & Technology.
A handful of tracing techniques have been developed to date, including DNA-based tracers, but those methods usually require adding tracers to fracking fluids before they are injected. Other tracing methods cannot distinguish between fracking fluids and contaminated water generated by conventional oil and gas exploration.
In order to develop a better tracking technique, Vengosh and colleagues spent years building a database of the compositions of fracking fluids used by various companies in different shale environments from New York to Arkansas. “Getting samples for characterization was not easy. In many cases we were working with landowners who had found spills on their property after drilling operations,” Vengosh says.
Once the researchers assembled what they considered an “extensive” dataset – 15 samples of hydraulic fracturing flowback fluids – the team set about characterizing the fluids, looking for shared chemistries that could be used to fingerprint the fluids. They found that fracking fluids have distinctive isotopic ratios of boron and lithium, which are found naturally in clay minerals within the shale formations. As the fracking fluids react and mix with clays deep underground, they become enriched in boron and lithium. When the fluids return to the surface, they have distinctive isotopic fingerprints that are different from other types of wastewater, including wastewater from conventional gas or oil wells, as well as from naturally occurring background water.
So far, Vengosh and colleagues have tested their method at a fracking fluid spill site in West Virginia and downstream from an oil and gas brine wastewater treatment plant in Pennsylvania, both documented in the study. Using the tracers, the team determined where fracturing fluids had been released in the environment.
Ultimately, the researchers say that they hope the method will be used to identify ways to improve how shale gas wastewater is treated and disposed. “We’re being very careful, but we think we have a universal tool to identify the source of contamination and link to fracking fluid,” Vengosh says.
The development of a tracer for fracking fluid is a “critical step” in studying and regulating the use and disposal of fracking fluids, says Brian Lutz, a biogeochemist at Kent State University in Ohio who was not involved in the new research. “This seems like a very robust method because the isotopes can be detected at very low concentrations,” he says, meaning the method has “impressive power to identify small levels of contamination by volume in freshwater.”
The isotopic ratios of lithium and boron that the team found are also consistent across the fracking fluid samples from different shale environments, indicating the method may work in a wide range of places, Vengosh says. That’s an important feature, as fracking is currently taking place in a number of different geologic settings from New York to Texas to Nebraska to British Columbia.
However, the tracing method may end up fanning the controversy over fracking rather than quelling it, Lutz says. “It’s critical to be able to detect contamination, but it’s also important to continue to study the impacts of that contamination on human and environmental health,” Lutz says. “Just because you can detect fracking fluids in a place doesn’t mean they’re toxic.”
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