Environmental impacts of shale-gas production

David Kramer, in the July 2011 issue of Physics Today (page 23) , presented a fairly thorough review of the status and issues associated with the extraction of shale gas in the US. Much is still unknown about the environmental effects of shale-gas production. Eight federal and state government agencies are currently working together to collect baseline data on a future shale-gas drill site in southwestern Pennsylvania, with the intent of monitoring environmental impacts through the drilling and fracturing process and for some time afterward. ¹

Several statements in Kramer’s news story are misleading or incorrect. The piece mentions gas industry claims that no case of groundwater contamination caused by hydraulic fracturing (fracking) has ever been documented. In the next paragraph, Anthony Ingraffea of Cornell University is quoted as stating that “thousands of cases” of groundwater contamination due to oil and gas drilling have been documented. Despite requests, Ingraffea has not shared that documentation or published it in the peer-reviewed literature.

Although the two statements regarding groundwater sound contradictory, they are, in fact, two separate issues. In most instances, fracking takes place at such great depths that it is highly unlikely to affect shallow aquifers in any way. Detailed microseismic data in both the Marcellus and Barnett shales ² showed that none of the induced fractures in the shales approached within several thousand vertical feet of the deepest freshwater aquifers overlying them. On the other hand, a surface spill of fracturing fluid, followed by ground infiltration and percolation down to an aquifer used for drinking water, is a much more likely contamination route. Indeed, that is how many common contaminants—from gasoline additives to agricultural fertilizers—have entered the groundwater.

The Kramer account also overstates that Marcellus shale fracking operators must find the means to dispose of up to 7 million gallons of wastewater generated per well. Drillers typically use 3 million to 5 million gallons of water to frac a multistage well, and only about a quarter to a third of that total volume of water is recovered. Because of higher disposal costs under new regulations, flowback is now essentially 100% recycled into the next well as a standard practice. After the final frac treatment, the recycled water is injected down a separate disposal well, as stated in the article.

The issue of methane in groundwater requires much more data and analysis before any conclusions can be drawn. Each case probably has unique circumstances and requires a forensic-type investigation to determine the source of the gas and the route by which it may have migrated into a domestic water well. The Duke University study ³ Kramer cites suffers from several flaws, including a lack of predrilling baseline data and no assessment of the local geology or hydrology. Alternative explanations, such as gas migration from shallow bedrock into aquifers, were not explored.

It is important for the scientific and regulatory communities to focus on protecting water resources, air quality, habitat, and ecosystems during shale-gas production. Objective data are needed to update state oil and gas regulations, identify environmental concerns, and define mitigation strategies for the production of this important resource. Misleading or inaccurate statements do little except shift focus away from the real problems and needlessly worry the public.