Since 2006, horizontal drilling and multi-stage hydraulic fracturing have been used to extract natural gas from the Marcellus Shale Formation. Although these unconventional methods have increased natural-gas production, their potential impacts on freshwater resources remain controversial. This controversy stems, in part, from the lack of data appropriate for distinguishing between legacy contamination and contamination caused by shale-gas development. To help address this issue, we evaluated water-quality changes in Susquehanna County, Pennsylvania, where the Marcellus Shale is being developed for natural gas. We measured groundwater and stream water chemistry throughout three phases of shale-gas development: (1) prior to the commencement of gas extraction activities, (2) during drilling and hydraulic fracturing, (3) and during natural-gas production. We selected an area (15 km<sup>2</sup>) encompassing four shale gas well pads, where seven horizontal wells were to be drilled. In winter 2015, we installed multi-level groundwater monitoring wells at hilltops adjacent to gas-well pads and in valleys above the vertical projection of gas-well laterals, which were subsequently drilled and hydraulically fractured during the spring, summer, and fall of 2015. Water samples were collected from the monitoring wells and from four streams in the study area on a monthly basis. These water samples were analyzed for concentrations of major cations, major anions, and trace metals, as well as for the isotopic composition of groundwater water and dissolved strontium. We also analyzed the chemistry of hydraulic fracturing fluids and flow-back fluid from gas wells within our study area. Findings from our study show that groundwater chemistry exhibits considerable variability in time and over aquifer depth. We will present our interpretation of these data in context of local geology, hydrology, and the timing of shale gas extraction stages.

Evaluating changes in groundwater and stream-water chemistry during natural-gas development of Pennsylvania's Marcellus shale

253rd National Meeting (2017)

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