The unconventional oil and gas (UOG) industry expansion in the United States has been proposed to increase the release of shale-derived methane, a highly potent greenhouse gas, into the environment (i.e., shallow groundwater aquifers with potential for subsequently transfer to the atmosphere). However, controversy exists on whether methane intrusion into shallow aquifers is caused by the UOG development or is present by natural mechanisms. In this work, we collected 94 water samples from residential wells in Bradford county, PA and evaluated their methane concentrations alongside other water quality indicators. No statistically significant correlation was observed between methane concentration and distance to the nearest UOG well (Spearman test, p-value = 0.78). Similarly, dissolved methane concentration was not significantly correlated with the distance to the nearest gas well with documented casing and/or cementing violation, or the number of UOG wells or gas wells with violations near the groundwater samples. Methane-to-higher chain hydrocarbon (i.e., ethane + propane) ratios and δ<sup>13</sup>C suggested that methane more likely originated from shallow Upper Devonian formations than from the deeper Marcellus shale. Moreover, the statistically significant correlation between methane concentration and geochemical water type (determined by inorganic ion concentrations and ratios) as well as topographical location of sampling (e.g., valley versus slope) supported the natural migration of thermogenic methane from shallow reservoirs to groundwater aquifers (Kruskal-Wallis tests and post-hoc Conover’s tests, p-values &lt; 0.05). Our findings indicate that the elevated groundwater methane level was dominantly attributable to natural methane migration rather than the influence of UOG development in Northeastern Pennsylvania.

Geochemical indicators in northeastern Pennsylvania groundwater are consistent with natural methane sources

ACS Spring Meeting 2021

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