National Academy of Sciences
Increased stray gas abundance in a subset of drinking water
wells near Marcellus shale gas extraction
24 June, 2013 - Horizontal drilling and
hydraulic fracturing are transforming energy production, but their
potential environmental effects remain controversial.
We analyzed 141 drinking water wells
across the Appalachian Plateaus physiographic province of
northeastern Pennsylvania, examining
natural gas concentrations and isotopic signatures with proximity to
shale gas wells.
Methane was detected in 82% of drinking
water samples, with average concentrations six times higher for
homes <1 km from natural gas wells (P = 0.0006).
Ethane was 23 times higher in homes <1
km from gas wells (P = 0.0013); propane was detected in 10 water
wells, all within approximately 1 km distance (P = 0.01).
Of three factors previously proposed to
influence gas concentrations in shallow groundwater (distances to
gas wells, valley bottoms, and the Appalachian Structural Front, a
proxy for tectonic deformation), distance to gas wells was highly
significant for methane concentrations (P = 0.007; multiple
regression), whereas distances to valley bottoms and the Appalachian
Structural Front were not significant (P = 0.27 and P = 0.11,
Distance to gas wells was also the most
significant factor for Pearson and Spearman correlation analyses (P
For ethane concentrations, distance to
gas wells was the only statistically significant factor (P < 0.005).
Isotopic signatures (δ13C-CH4,
δ13C-C2H6, and δ2H-CH4), hydrocarbon ratios (methane to ethane and
propane), and the ratio of the noble gas 4He to CH4 in groundwater
were characteristic of a thermally postmature Marcellus-like source
in some cases.
Overall, our data suggest that some
homeowners living <1 km from gas wells have drinking water
contaminated with stray gases.