In a new study led by Duke University, Durham, USA, scientists analyzed 141 drinking water samples from private water wells across the region of the Marcellus Shale in Pennsylvania. Overall they have found an increased methane concentration in private water wells up to 1 km from areas of shale gas extraction. Besides methane, ethane and propane have also been detected at elevated concentration levels within a 1 km distance.

The study was published in PNAS on 24 June, 2013.

Dissolved methane was detected in 115 out of 141 drinking water wells; from 133 wells sampled, higher ethane levels were found in 40 wells, and higher propane levels in 10 wells. For homes within 1 km of a gas well, the average methane concentration was six times higher (in 59 out of 141 samples), and for ethane 23 times higher than in water wells located further away. Moreover, 12 water samples from these houses contained methane concentrations higher than 28 mg/L. The U.S. Department of Interior sets the threshold for immediate remediation of dissolved methane in groundwater at higher than 28 mg/L.

By analyzing the isotopic signature  (δ¹³C and δ²H) of the methane, the source of these gases could be distinguished as being either of biogenic or thermogenic origin. For homeowners living less than 1 km from natural gas wells, both origins were identified. However, all the drinking water wells with elevated methane concentrations (> 10 mg/L) have isotopic signatures consistent with thermogenic natural gas. In order to provide a detailed characterization of the source of these thermogenic gases, to determine whether they derive from the Marcellus shale (Middle Devonian) or from the overlaying younger Upper Devonian gases, additional isotopic signatures (δ¹³C) of ethane were measured. 6 out of 11 samples exhibited signatures similar to the Marcellus shale gas and 5 out of 11 samples had signatures similar to the Upper Devonian production gases. 

These findings led the scientists to the conclusion that gases from the Upper Devonian were “introduced into the shallow crust either by natural processes over geological time or through leakage around the casing in the annulus space of the production well”, and gas that “likely stems from Marcellus production gases or a mixture of Marcellus gases and other annulus gases, likely migrated to the surface during drilling, well completion, or production.”

The scientists of this study propose that some homeowners living within 1 km of shale gas extraction areas have contaminated drinking water containing stray gas from drilling operations, likely caused by poor well construction.

The study continues a study published in 2011, in which 60 drinking water samples were analyzed. An additional 81 drinking water samples were now analyzed in this new study.