COUPLING ELEMENT CONCENTRATIONS AND LEAD ISOTOPE RATIOS TO TRACK SHORT AND LONG TERM WASTE DISPOSAL CONCERNS FROM MARCELLUS SHALE NATURAL GAS EXTRACTION

Element concentrations and Pb isotope ratios from leached drill cuttings were used to assess the potential for short and long term environmental impacts from Marcellus Shale waste materials. Samples from 600 meters of dry drilled cuttings from the middle Devonian stratigraphic section from Tioga County, NY were used for water/rock leaching experiments, including 24 hour and 4 month deionized water (DI) and dilute hydrochloric acid (HCl) leaches, and a duplicate set of experiments on cuttings that had undergone accelerated oxidization.

The HCl leach mimicked acid addition during hydraulic fracturing preparation processes, and when contrasted with the DI leach, solubilized Pb, Cu, Al, Fe and REEs. Compared to the rest of the stratigraphic section, the most elevated U, V, Sb, Ni and Mo concentrations are from the lowermost Marcellus Shale. Microprobe characterization suggests some of the iron sulfides occurred in framboidal forms within the Marcellus Shale, likely controlling the solubility of Pb and other chalcophile elements during the 24 hour acid leaching experiments. In-situ radioactive decay of ²³⁸U resulted in lowering of ²⁰⁷Pb/ ²⁰⁶Pb isotope ratios, which was most noticeable in the 24 hour acid leach from the lowermost Marcellus. This finding suggests extensive basin scale brine interaction does not seem to have taken place within the Marcellus Shale at this location after early diagenesis. The 4 month leaching experiments mimic long term weathering processes, and further increased the concentration of many elements in the leachate, with the notable exception of Ba, because of barite precipitation following sulfide oxidation.

Coupled differences in metal concentrations and Pb isotope ratios provided evidence for acid induced changes in metal partitioning between solid and aqueous phases during the hydrofracturing process, and subsequent changes from oxidation during weathering. Differences in waste disposal techniques could result in changes in metal mobility over short and long time periods, necessitating site specific monitoring to minimize the potential for environmental impacts.