GEOCHEMICAL EVIDENCE FOR POSSIBLE NATURAL MIGRATION OF MARCELLUS FORMATION BRINE TO SHALLOW AQUIFERS IN PENNSYLVANIA

Shale gas exploration has generated an increased awareness of risks to drinking water quality amid concerns for the possible migration of stray gas or hydraulic fracturing fluid and brine to shallow aquifers. The degree to which shallow drinking water is at risk could depend upon the hydraulic connectivity between the shale gas formations and the surface. In this study, we analyzed the geochemistry of over 400 water samples located across northeastern Pennsylvania in the three principle aquifers that overlie the Marcellus Formation. Based on a detailed analysis of major (Br, Cl, Na, Mg, Ba, and Sr) and trace (Li) element chemistry, coupled with utilization of a specific spectrum of isotopic tracers (⁸⁷Sr/⁸⁶Sr, ²²⁸Ra/ ²²⁶Ra, ²H/H, ¹⁸O/¹⁶O), we identify a salinized (Cl> 20 mg/L) shallow groundwater type which suggests conservative mixing between shallow groundwater and an underlying brine.

Many of the brines in the northern Appalachian Basin likely share a common origin as the expelled remnants of the formation of the Silurian Salina evaporite deposits. To determine the ultimate source of the diluted brine we compared the observed geochemistry to over 80 brines produced from northern Appalachian Basin formations. The shallow salinized groundwater most closely resembles diluted produced water from the Middle Devonian Marcellus Formation. The ¹⁸O/¹⁶O and ²H/H of the salinized groundwater indicate that the brine is likely diluted with post-glacial (<10,000 ybp) meteoric water. Combined, these data indicate that hydraulic connections allowed cross formational migration of brine from deeper formations (1-2 kilometers below ground surface) and subsequent dilution. The occurrence of the saline water does not appear to be correlated with the location of shale-gas wells. Also, salinized groundwater with similar chemistry was reported prior to the most recent shale-gas development in the region. Instead brine likely migrated into the shallow aquifers and was recently diluted through natural pathways and processes. However, the presence of natural hydraulic connections to deeper formations suggests specific structural and hydrodynamic regimes where shallow drinking water resources are at greater risk of contamination with fugitive gases during drilling and hydraulic fracturing of shale gas.