Geochemistry of the Union Springs Member, Marcellus Formation in Central Pennsylvania and environmental implications of hydraulic fracturing

The black shales of the Appalachian Basin have gained renewed and widespread attention over the last several years. While gas has been produced from the Appalachian Basin for over one hundred years, low recovery had made these source rocks uneconomical to companies in the past. In the last several decades, advances in drilling technology and an increase in demand have renewed interest in the Marcellus Formation as a source of natural gas; however, concerns over the safety of hydraulic fracturing persist. Additionally, debate continues over the conditions that persisted when the Marcellus Formation was deposited, specifically, over the relative importance of primary productivity versus the preservation of organic matter.;To better understand the depositional conditions that persisted when the Marcellus Formation was deposited, a high-resolution geochemical data set was constructed for the Union Springs Member from more proximal to more distal from the paleo-shoreline in the Appalachian Basin. Results suggest that during deposition, the sediment-water interface and a portion of the water column was anoxic to sulfidic. As deposition continued, euxinia was periodically interrupted by dysoxia and even oxic conditions, and a greater influx of clastic material occurred. Such variations were likely related to variations in water depth and progradation of deltaic complexes from the eastern margin of the Appalachian Basin.;Sequential extractions were performed to determine the phase in which the metals reside in the Union Springs Member, both to confirm geochemical conditions under which the Marcellus Formation was deposited, and also to gain a better understanding of what conditions may mobilize and transport metals from Marcellus Formation after hydraulic fracturing. It has generally been believed that the elevated concentrations of total dissolved solids (TDS) are due to the dissolution of constituents from the shale by the water that is injected during hydraulic fracturing. Most notably, results show that uranium was solubilized from the sample during the first extraction step involving hydrochloric acid. This most likely is related to the presence of acid-soluble uraninite in the samples, suggesting that uranium may be solubilized from the Marcellus Formation after hydraulic fracturing.;To assess the mobility of uranium from the Marcellus Formation, a small sample set of produced waters was analyzed for the presence of naturally occurring radionuclides. Uranium was not detected above the method detection limit in the flowback waters. It is still difficult to conclude that hydraulic fracturing is the cause of mobilization of uranium reported in previous data sets, as little published data is available, and radium and uranium do not appear to be in secular equilibrium. However, an accurate method for the determination of uranium in produced and flowback water samples is required to assess the relationship between hydraulic fracturing and uranium concentrations in flowback and produced waters. Preliminary research shows that the use of Kinetic Phosphorescence Analysis (KPA) may be a viable method for the determination of uranium at low concentrations. Use of such techniques may allow better understanding of the presence of uranium and other radionuclides in flowback and produced waters.