USING STRONTIUM ISOTOPES TO IDENTIFY MARCELLUS SHALE DERIVED FLUIDS IN ALLEGHENY RIVER WATERSHED, PENNSYLVANIA, USA

Potential water quality issues associated with accelerating natural gas production from the Marcellus Shale in New York, Pennsylvania, and West Virginia point to the need for geochemical monitoring of stream water in the producing regions. However, effects from past and ongoing coal, oil, and gas production present a challenge in defining geochemical baselines that are necessary for accurate assessment of surface water impacts from unconventional shale gas development. In this study, focused on the Allegheny River basin of western Pennsylvania within the Marcellus exploration region, we use strontium (Sr) isotopes combined with elemental analysis to assess its ability to distinguish between sources such as Marcellus-derived produced waters, abandoned coal mine drainage (AMD), fluids from conventional Upper Devonian gas production, and fly ash impoundments.

Preliminary results from stream water collected from approximately 45 sites along the Allegheny River and seven of its tributaries show that Sr varies across the watershed, with lower concentrations (mean [Sr] = 82 µg/L) in the upper reaches and tributaries, and increasing concentrations downstream, reaching approximately 200 µg/L where the Allegheny River enters the Ohio River near Pittsburgh. Anomalous Sr concentrations (as high as 2,500 µg/L) are observed in two tributaries with high densities of centralized wastewater treatment (CWT) facilities. Preliminary Sr isotope data for two seasons indicate that most of the Allegheny River and its tributaries cluster around a narrow range of ⁸⁷Sr/⁸⁶Sr values, from 0.7125 to 0.7130 (= +47.3 to +53.8, where is the deviation of the ⁸⁷Sr/⁸⁶Sr from that of seawater in parts per ten thousand). However, values as high as +62 and as low as +35 are observed in tributaries with high Sr concentrations. While Sr concentrations are expected to vary across the watershed due to differences in silicate and carbonate weathering sources (as indicated by Mg/Na and Ca/Na ratios), in some cases anomalous Sr concentrations and isotopic compositions are consistent with mixing with fluids derived from natural gas production. The results also suggest that Sr isotope analysis combined with geochemical data can distinguish influx of produced water from Marcellus shale vs. Upper Devonian sources into streams.