Jackson et al. sampled 81 drinking water wells and combined their results with information from 60 previously-collected samples. They measured the concentrations of dissolved methane, ethane, and propane in the water samples and the distance to the nearest gas well from each sample. Other possible sources of natural gas contamination—valley bottom streams and the Appalachian Structural Front—were ruled out using multiple regression analysis, linear regression, and Pearson and Spearman coefficients. In addition, the authors tested to see if the gas came from biogenic sources, i.e. produced by microorganisms, or from thermogenic sources, i.e. with a potential connection to shale gas production. Indicators of thermogenic gas include the presence of ethane or propane, certain isotopic signatures in methane (δ13C-CH4), and the ratio of helium isotope (4He) to methane.
by Shannon Julius
Shale gas is an unconventional source of natural gas recently made accessible by horizontal drilling and hydraulic fracturing. Shale and other unconventional sources of natural gas have caused overall U.S. production of methane to increase 30% since 2005. Despite their increasing importance, the environmental implications of producing unconventional natural gas have not yet been studied extensively. Jackson et al. explored the possibility of stray gas contamination by testing for concentrations of methane, ethane, and propane in drinking water wells near homes in the Marcellus shale region of Pennsylvania. In general, they found higher amounts of dissolved gases in sources less than one kilometer from a natural gas well. Statistical analysis showed that distance from gas wells was a more significant factor for raised levels of natural gas than other potential sources of contamination. Closer analysis into the chemistry of the samples showed that at least some of the natural gases present in drinking water wells came from a thermogenic source, which includes gas wells. The authors suggest that the stray gases could be due to wells with faulty steel casings or cement sealing.
The study found dissolved methane at 115 of 141 homes (82%), ethane at 40 of 133 homes (30%), and propane at 10 of 133 homes (8%). Methane had a far higher average than other natural gases in all cases, but homes within one kilometer of a natural gas well had 6 times the amount of methane as homes farther away. The 12 highest concentrations of methane were above the U.S. Department of Interior level for hazard mitigation, and 11 of those houses were within one kilometer of a gas well. In addition, homes within one kilometer of a gas well had 23 times the amount of ethane as homes farther away, and propane was only detected at homes within one kilometer of a gas well. Ethane and propane only derive from thermogentic sources, so their presence is evidence that the natural gas contamination is likely from a gas well.
Another way to determine if gas is from a thermogenic source is to look at the isotopic signature. Yet again the strongest evidence for thermogenic sources (the most δ13C-CH4 signatures greater than –40‰) were within one kilometer from natural gas wells. There is also a trend of shale gas in which isotopes (δ13C) of methane become heavier than those of ethane, though in other cases it is the reverse. Six out of 11 houses where sampling was possible showed this shale gas trend, indicating that those gas samples came from shale gas production.
The helium isotope 4He is a component of thermogenic natural gas. The ratio of this isotope to methane (4He to CH4) in the dataset was fairly consistent, except for the points with elevated levels of methane. These had a ratio of 4He to CH4 that was consistant with Marcellus production gases, somewhat lower than normal drinking water levels.
The authors contend that poor well construction led to this contamination of drinking water. In particular, stray gases could have escaped through faulty protective steel casings or from imperfections in the cement sealer between the casings and rock outside the well. Faulty steel casings would lead to gas from inside the well leaking out to the surroundings, followed by metals, fracking fluids, or other evidence of gas extraction. Faulty cement would lead to any gas in the spaces around the well to escape upwards into drinking water, meaning the gases would not be easily identifiable with the well itself.
The authors would like to see further research to understand more about how drinking water near the Marcellus shale gas production area compares to drinking water near other shale gas sources. They also suggest gathering predrilling data, even making detailed studies of water quality before, during, and after drilling and hydraulic fracturing.
Jackson, R.B., Vengosh, A., Darrah, T.H., Warner, N.R., Down, A., Poreda, R.J., Osborn, S.G., Zhao, K., Karr, J.D., 2013. Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction. Proceedings of the National Academy of Sciences 110, 11250-11255