Matrix-Derived Radium Mobilization in Shallow Aquifers Near Unconventional Oil and Gas Operations.

Previous studies in Pennsylvania identified localized "hotspots" of elevated salinity (Cl-, Ba2+, Sr2+) in shallow groundwater thought to be associated with unconventional oil and gas (UOG) operations. Because produced water generated during UOG operations is both highly saline and enriched in radium, accidental releases could increase groundwater salinity and radium activity. Elevated salinity from other sources such as road salt or septic tanks could also release radium from aquifer solids through cation exchange. Therefore, we measured combined radium activity (226+228Ra) in shallow groundwater (n = 91) and tested for associations with UOG wells, historical surface impoundments, or reported spills within 1, 3, or 5 km. All samples were below U.S. EPA maximum contaminant levels [0.185 Bq/L (5 pCi/L)] and consistent with regional background values. Radium activities were statistically greater within 3 km of spills and impoundments and correlated positively with the density of UOG wells within 3 km. Although radium correlated with Cl-, Ba2+, and Sr2+, other geochemical indicators, Na/Cl, Cl/Br, and 228/226Ra, indicate that radium was likely not directly derived from UOG fluids. Only six samples were consistent with produced water mixing with shallow groundwater. Instead, most of the salinity was primarily attributed to shallow sources that released matrix-bound radium.