by Shannon Julius
Perhaps the biggest environmental and health concern related to shale gas development is the possibility of contaminants leaking from the well shaft into nearby groundwater supplies. The first sign of such leakage would be stray methane in groundwater, as methane is a small enough molecule to move through tiny spaces and easily dissolves in water. Jackson et al. explored the possibility of stray gas contamination by testing for concentrations of methane, ethane, and propane in drinking water wells of homes in the Marcellus shale region of Pennsylvania. The researchers generally found higher amounts of dissolved gases in drinking water wells less than one kilometer from a natural gas well. Statistical analysis showed that distance from gas wells was the most significant factor for raised levels of natural gas. 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 the chemical composition of the gas samples to see if their source was biogenic, i.e. produced by microorganisms, or thermogenic, i.e. with a potential connection to shale gas production. This chemical analysis showed that at least some of the natural gases present in drinking water wells came from a thermogenic source. The authors suggest that any stray gases could be due to wells with faulty steel casings or cement seals.
Jackson et al. sampled 81 drinking water wells and combined their results with information from 60 previously-collected samples. 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 concentrations were high in all cases relative to ethane and propane concentrations, and 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 level at which the US Department of Interior recommends 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 thermogenic 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 its 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. Shale gas has a specific trend in which heavy carbon isotopes in methane (δ13C) are more abundant than those in ethane, whereas the reverse is usually true. Six out of 11 houses where sampling was possible showed this trend, indicating that those gas samples came from shale gas extraction sites.
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 consistent with Marcellus production gases, which is somewhat lower than normal drinking water levels.
The researchers believe that poor well construction could have 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 sealing between the casings and rock outside of the well. Faulty steel casings would lead to gas from inside the well leaking out to the surroundings, followed by metals, fracturing fluids, or other evidence of gas extraction. Faulty cement would lead any gas in the spaces around the well to escape upwards into drinking water, meaning the gases would not be easily identifiable with a particular well, as was the case with the samples recorded in this study.
The authors would like to see further research go towards understanding more about how drinking water quality near the Marcellus shale gas production area compares to drinking water quality near other shale gas sources. They also suggest gathering pre-drilling data in order to make detailed studies of water quality before, during, and after drilling and hydraulic fracturing.
Jackson, R., Vengosh, A., Darrah, T., Warner, N., Down, A., Poreda, R., Osborn, S., Zhao, K., Karr, J., 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. Full article at: http://bit.ly/1rCRB03