Big Groundwater Data Sets Reveal Possible Rare Contamination Amid Otherwise Improved Water Quality for Some Analytes in a Region of Marcellus Shale Development

Abstract

Eleven thousand groundwater samples collected in the 2010s in an area of Marcellus shale-gas development are analyzed to assess spatial and temporal patterns of water quality. Using a new data mining technique, we confirm previous observations that methane concentrations in groundwater tend to be naturally elevated in valleys and near faults, but we also show that methane is also more concentrated near an anticline. Data mining also highlights waters with elevated methane that are not otherwise explained by geologic features. These slightly elevated concentrations occur near 7 out of the 1,385 shale-gas wells and near some conventional gas wells in the study area. For ten analytes for which uncensored data are abundant in this 3,000 km rural region, concentrations are unchanged or improved as compared to samples analyzed prior to 1990. Specifically, TDS, Fe, Mn, sulfate, and pH show small but statistically significant improvement, and As, Pb, Ba, Cl, and Na show no change. Evidence from this rural area could document improved groundwater quality caused by decreased acid rain (pH, sulfate) since the imposition of the Clean Air Act or decreased steel production (Fe, Mn). Such improvements have not been reported in groundwater in more developed areas of the U.S.

Detecting and explaining why aquifers occasionally become degraded near hydraulically fractured shale gas wells

“…Although not all water-quality data are released to the public in the Appalachian Basin, the rate of incidence of problems such as described in this paper appears to be relatively low compared with the number of shale gas wells that have been drilled (2,3,18). Not to Scale Fig.…”

“…For example, drilling into a shale formation at a shallow depth along the axis of a large fold, such as described in this study area, may produce wells that intersect fractures that interconnect to form good pathways for upwardmigrating contaminants. In addition, future research is needed to determine why some anticlines (e.g., the Nittany and Towanda anticlines) are associated with higher CH 4 concentrations while others (the Rome and Wilmot anticlines in Bradford County) apparently are not (18). Such research could provide maps of areas where drilling should be precluded if CH 4 migration is to be eliminated entirely or could lead to better management practices for drilling such areas.…”

“…1 and 2). Predrilling data are water-quality measurements made by commercial laboratories on samples collected by consultants hired by hydrocarbon-extraction companies before drilling oil or gas wells nearby (3,18). Waterquality data in Sugar Run are also compared with waters reported to have been contaminated in other areas of Pennsylvania since shale gas drilling began in the state in 2004 (35).…”

“…Recently, other anticlines in Pennsylvania have also been shown to be associated with CH 4 -containing groundwater. Specifically, inspection of groundwater data has revealed that CH 4 concentrations increase slightly near the Towanda Anticline to the northeast of Lycoming County in Bradford County, PA (18). Several cementing/casing-related violations were also issued by the PA DEP to shale gas wells along that anticline (www.depreportingservices.state.pa.us/ReportServer/Pages/ ReportViewer.aspx?/Oil_Gas/OG_Compliance).…”

“…In addition, thermogenic CH 4 -CH 4 produced at depth at higher temperatures-can migrate into aquifers through natural mechanisms that might include transport as a dissolved solute in waters accompanied by salts from formation brine or, perhaps, as a separate free phase (8)(9)(10)(11)(12)(13). To add to the complexity of determining the source of gas in aquifers in the Marcellus and other shale plays, CH 4 can travel for kilometers along boreholes, fractures, faults, and bedding-plane openings and through porous sandstones (14)(15)(16)(17)(18).…”