Geochemical evidence for possible natural migration of Marcellus Formation brine to shallow aquifers in Pennsylvania

Abstract

The debate surrounding the safety of shale gas development in the Appalachian Basin has generated increased awareness of drinking water quality in rural communities. Concerns include the potential for migration of stray gas, metal-rich formation brines, and hydraulic fracturing and/or flowback fluids to drinking water aquifers. A critical question common to these environmental risks is the hydraulic connectivity between the shale gas formations and the overlying shallow drinking water aquifers. We present geochemical evidence from northeastern Pennsylvania showing that pathways, unrelated to recent drilling activities, exist in some locations between deep underlying formations and shallow drinking water aquifers. Integration of chemical data (Br, Cl, Na, Ba, Sr, and Li) and isotopic ratios ((87)Sr/(86)Sr, (2)H/H, (18)O/(16)O, and (228)Ra/(226)Ra) from this and previous studies in 426 shallow groundwater samples and 83 northern Appalachian brine samples suggest that mixing relationships between shallow ground water and a deep formation brine causes groundwater salinization in some locations. The strong geochemical fingerprint in the salinized (Cl > 20 mg/L) groundwater sampled from the Alluvium, Catskill, and Lock Haven aquifers suggests possible migration of Marcellus brine through naturally occurring pathways. The occurrences of saline water do not correlate with the location of shale-gas wells and are consistent with reported data before rapid shale-gas development in the region; however, the presence of these fluids suggests conductive pathways and specific geostructural and/or hydrodynamic regimes in northeastern Pennsylvania that are at increased risk for contamination of shallow drinking water resources, particularly by fugitive gases, because of natural hydraulic connections to deeper formations.

Fig. 1.

Digital elevation model (DEM) map of northeastern PA. Shaded brown areas indicate higher elevations and blue-green shaded areas indicate lower elevations (valleys). The distribution of shallow (< 90 m) groundwater samples from this study and previous studies (18, 19) are labeled based on water type. Two low salinity (Cl < 20 mg/L) water types dominated by Ca-HCO₃ (type A = green circles) or Na-HCO₃ (type B = blue triangles) were the most common, and two higher salinity (Cl > 20 mg/L) water types were also observed: Br/Cl < 0.001 (type C = pink squares) and brine-type groundwater Br/Cl > 0.001 (type D = red diamonds). Type D groundwater samples appear associated with valleys (Table S1) and are sourced from conservative mixing between a brine and fresh meteoric water. The DEM data were obtained from NASA’ Shuttle Radar Topography Mission http://srtm.usgs.gov/.

Fig. 2.

Generalized stratigraphic section in the subsurface of western and eastern PA plateau adapted from (14, 15, 18, 19) and Sr isotope data of Appalachian brines and type D saline groundwater. Variations of ⁸⁷Sr/⁸⁶Sr ratios in Appalachian Brine and type-D groundwater samples show enrichment compared to the Paleozoic secular seawater curve (dashed grey line) (49). Note the overlap in values of type-D shallow ground water with ⁸⁷Sr/⁸⁶Sr values in Marcellus brines or older formations (21, 22, 24) but no overlap with the Upper Devonian brines in stratigraphically equivalent formations (Table S2) (21, 24).

Fig. 3.

Bromide vs. chloride concentrations (log-log scale) in shallow groundwater in NE PA and Appalachian brines from this and previous studies (18, 19). The linear relationship (type D: r² = 0.99, p < 1 × 10^-5; sample types A–C: r² = 0.14) between the conservative elements Br and Cl demonstrates that the majority of the higher salinity samples of type D are derived from dilution of Appalachian brines that originated from evaporated seawater. Even with a large dilution of the original brine, the geochemical signature of type-D waters are still discernable in shallow groundwater from other high salinity (Cl > 20 mg/L) groundwater with low Br/Cl ratios (type C). Type C water likely originated from shallow sources such as septic systems or road deicing. Seawater evaporation line is from (25).

Fig. 4.

Ternary diagrams that display the relative percent of the major cations (A) and anions (B) in shallow groundwater samples from this and previous studies (18, 19). The overlap indicates that Na-Ca-Cl type saline water was present prior to the recent shale-gas development in the region and could be from natural mixing.

Fig. 5.

δ²H vs. δ¹⁸O in shallow groundwater from this study and Appalachian brines. The water isotope composition of the shallow groundwater samples including the Salt Spring appear indistinguishable from each other and the local meteoric water line (LMWL) (23) and do not show any apparent trends toward the stable isotope ratios of the Appalachian brines (6, 22). The data indicate that dilution of the type-D waters likely occurred on modern (post-glacial) time scales. Symbol legend is provided in Fig. 3.

Fig. 6.

⁸⁷Sr/⁸⁶Sr vs. Sr concentrations (log scale) of Appalachian Brines (21, 24) and shallow groundwater samples in the study area. The shallow groundwater samples are divided in the figure based on water types. Increased concentrations of Sr in the shallow aquifers are likely derived from two component mixing: (i) A low salinity, radiogenic ⁸⁷Sr/⁸⁶Sr groundwater sourced from local aquifer reactions; and (ii) A high salinity, less radiogenic ⁸⁷Sr/⁸⁶Sr water consistent with Marcellus Formation brine. The Marcellus Formation ⁸⁷Sr/⁸⁶Sr appears lower in western Bradford than in Susquehanna and Wayne counties. Other brine sources such as the Upper Devonian formations have a more radiogenic ⁸⁷Sr/⁸⁶Sr ratio that does not appear to show any relationship to the salinized shallow groundwater. Symbol legend is provided in Fig. 3.