Geochemical evidence for fugitive gas contamination and associated water quality changes in drinking-water wells from Parker County, Texas

Extensive development of horizontal drilling and hydraulic fracturing enhanced energy production but raised concerns about drinking-water quality in areas of shale-gas development. One particularly controversial case that has received significant public and scientific attention involves possible contamination of groundwater in the Trinity Aquifer in Parker County, Texas. Despite extensive work, the origin of natural gas in the Trinity Aquifer within this study area is an ongoing debate. Here, we present a comprehensive geochemical dataset collected across three sampling campaigns along with integration of previously published data. Data include major and trace ions, molecular gas compositions, compound-specific stable isotopes of hydrocarbons (delta C-13-CH4, delta C-13-C2H6, delta(2)-CH4), dissolved inorganic carbon (delta C-13-DIC), nitrogen (delta N-15-N-2), water (delta O-18, delta H-2, H-3), and noble gases (He, Ne, Ar), boron (delta B-11) and strontium (Sr-87/Sr-86) isotopic compositions of water samples from 20 drinking-water wells from the Trinity Aquifer. The compendium of data confirms mixing between a deep, naturally occurring salt-(Cl >250 mg/L) and hydrocarbon-rich groundwater with a low-salinity, shallower, and younger groundwater. Hydrocarbon gases display strong evidence for sulfate reduction-paired oxidation, in some cases followed by secondary methanogenesis. A subset of drinking-water wells contains elevated levels of hydrocarbons and depleted atmospherically-derived gas tracers, which is consistent with the introduction of fugitive thermogenic gas. We suggest that gas originating from the intermediate-depth Strawn Group (Strawn) is flowing along the annulus of a Barnett Shale gas well, and is subsequently entering the shallow aquifer system. This interpretation is supported by the expansion in the number of affected drinking-water wells during our study period and the persistence of hydrocarbon levels over time. Our data suggest post-genetic secondary water quality changes occur following fugitive gas contamination, including sulfate reduction paired with hydro-carbon oxidation and secondary methanogenesis. Importantly, no evidence for upward migration of brine or natural gas associated with the Barnett Shale was identified. Published by Elsevier B.V.

Automated summary

Extensive development of horizontal drilling and hydraulic fracturing has increased energy production but also raised concerns about drinking-water quality in areas of shale-gas development. This study focuses on the potential contamination of groundwater in the Trinity Aquifer in Parker County, Texas, presenting a comprehensive geochemical dataset collected across three sampling campaigns.