Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction

Abstract

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, respectively). Distance to gas wells was also the most significant factor for Pearson and Spearman correlation analyses (P < 0.01). For ethane concentrations, distance to gas wells was the only statistically significant factor (P < 0.005). Isotopic signatures (δ(13)C-CH4, δ(13)C-C2H6, and δ(2)H-CH4), hydrocarbon ratios (methane to ethane and propane), and the ratio of the noble gas (4)He 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.

Keywords: carbon, hydrogen, and helium isotopes; fracking; geochemical fingerprinting; groundwater contamination; hydrology and ecology.

Fig. 1.

Concentrations of (Upper) methane, (Lower) ethane, and (Lower Inset) propane (milligrams liter⁻¹) in drinking water wells vs. distance to natural gas wells (kilometers). The locations of natural gas wells were obtained from the Pennsylvania DEP and Pennsylvania Spatial Data Access databases (54). The gray band in Upper is the range for considering hazard mitigation recommended by the US Department of the Interior (10–28 mg CH₄/L); the department recommends immediate remediation for any value >28 mg CH₄/L.

Fig. 2.

The ratio of ethane to methane (C₂/C₁) and (Inset) propane to methane (C₃/C₁) concentrations in drinking water wells as a function of distance to natural gas wells (kilometers). The data are plotted for all cases where [CH₄], [C₂H₆], and [C₃H₈] were above detection limits or [CH₄] was >0.5 mg/L but [C₂H₆] or [C₃H₈] was below detection limits using the detection limits of 0.0005 and 0.0001 mg/L for [C₂H₆] and [C₃H₈], respectively.

Fig. 3.

(A) Methane concentration, (B) δ²H-CH₄, and (C) methane to ethane + propane ratio plotted against δ¹³C-CH₄. The grayscale shading refers to (A) distance to nearest gas wells and (B and C) methane concentration. The solid lines in B distinguishing natural gas sources are from ref. ; the mixed line in B comes from the standard mixing equations in ref. . C shows two hypothetical trajectories: simple mixing between thermogenically and biogenically derived gas (lower curve) and either diffusive migration or a three-component mixture between Middle and Upper Devonian gases and shallow biogenic gases (upper curve).

Fig. 4.

Stable isotope signatures (‰ VPDB) of methane (δ¹³C-CH₄) vs. δ¹³C for methane minus ethane (Δ¹³C = δ¹³CH₄ − δ¹³C₂H₆); 6 of 11 drinking water samples exhibited isotopic reversals and δ¹³C-CH₄ values consistent with Marcellus production gas (14, 28, 55). In contrast, five drinking water samples and the salt spring at Salt Springs State Park (filled square) had δ¹³C-CH₄ and Δ¹³C < 0 consistent with Upper Devonian production gases (14, 55). Eleven drinking water samples had sufficient ethane concentrations for isotopic determinations. Ten of the samples were <1 km distance from shale gas wells, and one sample is at 1.1 km distance (the point in the lower left corner of the plot).

Fig. 5.

The ratio of ⁴He:CH₄ concentrations in drinking water wells vs. distance to gas wells (kilometers). The values are compared with water samples (mean ± SE) from the salt spring at Salt Springs State Park (n = 3) and Marcellus (n = 4) and Upper Devonian (n = 5) production gases (15).