The Marcellus shale is generally characterized by low to irreducible water saturation, and gas wells in this formation typically produce small volumes of produced water compared to the volume of fracturing fluids injected. However, some Marcellus wells produce larger volumes of high-salinity fluids months or years into production, pointing to the possibility of out-of-zone water production. To identify the origin of these high-salinity fluids, we utilize a combination of 228Ra/226Ra activity and 87Sr/86Sr ratios measured in Marcellus produced water samples. Key findings show that: i) bulk rock Th/U and Rb/Sr mass ratios, in concert with fresh water 87Sr/86Sr leachates, can be used to develop predicted 228Ra/226Ra and 87Sr/86Sr ratios of pore waters within different formations, ii) the 228Ra/226Ra and 87Sr/86Sr ratios of Marcellus produced water are dominated by the Ra2+ and Sr2+ content of downhole formation fluids and water-accessible fractions, and iii) Marcellus wells with higher water-to-gas production ratios (e.g. >1 bbl water/106 ft3 gas, or >5.6 m3 water/106 m3 gas) and elevated TDS (>100,000 mg/L) exhibit lower 87Sr/86Sr ratios, consistent with a significant contribution from exogenous Onondaga pore fluids. The approach developed here has potential utility for identifying out-of-zone water sources in other oil and gas basins.