Fig. 2.

The ratios of CH₄/³⁶Ar [ratios are in units (cm³ STP/L)/(cm³ STP/L); A and C] and ⁴He/²⁰Ne (B and D) vs. Cl⁻ of domestic groundwater wells. The samples were collected in the Marcellus (MSA) (Left) and Barnett (BSA) (Right) study areas at distances >1 km (triangles) and <1 km (circles) from unconventional drill sites (Tables S1 and S2). [CH₄] is shown using grayscale intensity [0–60⁺ cm³ ([CH₄]) STP/L]. The dashed lines in the MSA are the regressions of all points collected >1 km from drill sites. In the MSA, all samples >1 km from drill sites had [CH₄] at or below saturation and showed significant correlations between Cl⁻ and CH₄/³⁶Ar (r² = 0.72; P < 0.01) or ⁴He/²⁰Ne (r² = 0.59; P < 0.01) defined as the normal trend. For samples <1 km from drill sites, one subset was consistent with the “normal trend” (P = 0.31; Chow test), whereas the other anomalous subset had supersaturated [CH₄] and high CH₄/³⁶Ar and ⁴He/²⁰Ne, even at low [Cl⁻] (green-rimmed circles in A and B). The natural Salt Spring in Montrose, PA, is shown as a square in all MSA figures, and samples targeted for microbial-sourced gases are distinguished by diamonds. In the BSA, 15 samples had [CH₄] at or below saturation and significant correlations between Cl⁻ and CH₄/³⁶Ar (r² = 0.59; P < 0.01) or ⁴He/²⁰Ne (r² = 0.48; P < 0.01) (dashed lines in C and D). Five samples, including two that changed between the first and second sampling periods (Fig. S4), had substantially higher CH₄/³⁶Ar and ⁴He/²⁰Ne independent of [Cl⁻]. The anomalous subset of samples from both locations with elevated CH₄ that do not fall along the normal trend (>1 km) regression lines are consistent with a flux of gas-phase thermogenic hydrocarbon gas into shallow aquifers.