GEOCHEMISTRY AND ORIGIN OF FLOWBACK WATERS FROM MARCELLUS GAS WELLS IN PENNSYLVANIA

Large quantities of highly saline water flow back from fracked Marcellus gas wells in Pennsylvania and surrounding regions. Concentrations of most inorganic components of flowback water (Cl, Br, Na, K, Ca, Mg, Sr, Ba, Ra, Fe, Mn, total dissolved solids, and others) increase with time from a well after hydraulic stimulation from levels in the injected fracking fluid up to salinities many times that in seawater. Chloride in flowback water reaches at least 151,000 mg/L, about 8 times the concentration in seawater. Later flowback waters from Marcellus wells resemble brines produced from conventional gas wells that tap into other Paleozoic formations in the region. The Br/Cl ratio and other parameters indicate that both types of brine formed by the evaporation of seawater, probably during deposition of the Silurian Salina formation, followed by dolomitization and subsurface mixing with seawater and/or freshwater. Major proportions of the flowback may reach the fracked wells from overlying and underlying formations. The flowback contains concentrations of Ra and Ba that are commonly hundreds of times the U.S drinking water standards. Further research is needed on fate of toxic elements such as Ra, Ba and Br, on regional patterns of flowback composition, and on improved methods of disposal.