"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) ... For ethane concentrations, distance to gas wells was the only statistically significant factor (P < 0.005) ... Overall, our data suggest that some homeowners living <1 km from gas wells have drinking water contaminated with stray gases ... The two simplest explanations for the higher dissolved gas concentrations that we observed in drinking water are (i) faulty or inadequate steel casings, which are designed to keep the gas and any water inside the well from leaking into the environment, and (ii) imperfections in the cement sealing of the annulus or gaps between casings and rock that keep fluids from moving up the outside of the well ... For our dataset, there is a weak trend to higher methane concentrations with increasing age of the gas wells ... Horizontal drilling and hydraulic fracturing can stimulate fractures or mineralized veins, increasing secondary hydraulic connectivity. The upward transport of gases is theoretically possible, including pressure-driven flow through open, dry fractures and pressure-driven buoyancy of gas bubbles in aquifers and water-filled fractures. Reduced pressures after the fracturing activities could also lead to methane exsolving rapidly from solution ... We propose that a subset of homeowners has drinking water contaminated by drilling operations, likely through poor well construction ... Ultimately, we need to understand why, in some cases, shale gas extraction contaminates groundwater and how to keep it from happening elsewhere."

Zum Paper von Jackson et al., erschienen bei PNAS (24. Juni 2013) »