Geochemical and γ ray characterization of Pennsylvanian black shales: Implications for elevated home radon levels in Vanderburgh County, Indiana

Radon (²²²Rn) is a radioactive gas that results from the decay of uranium (²³⁸U) in the Earth's crust. This study characterizes the presence and relative quantity of radon precursors in the Pennsylvanian black shales of southwest Indiana. Cores were drilled on the campus of the University of Southern Indiana to a depth of 237.7 m (780 ft) during exploration for coal-bed methane. Gamma ray logs were taken to measure radioactive activity as a function of depth in the bore hole. Activity readings of 270, 467, 555, and 388 GAPI (American Petroleum Institute γ ray units) were measured at depths of 124.3 m (408 ft), 154.0 m (505 ft), 187.1 m (614 ft) and 214.0 m (702 ft) in four separate shale layers of the Pennsylvanian stratigraphic column. GAPI units are used in the petroleum industry when drilling to represent the relative intensities of γ radiation from ⁴⁰K, ²³²Th, and ²³⁸U in bore holes (Belknap et al., 1959). For purposes of this study, the high activity readings on the gamma ray logs were used only to identify at which depths further gamma ray spectroscopy of the cores would be completed in the laboratory. Gamma ray spectroscopic studies of these cores were conducted with a large volume NaI crystal detector to observe γ rays of specific energies. Characteristic γ rays from various isotopes were identified confirming the presence and relative quantity of radon precursors in core samples. Geochemical analysis of cores was also conducted to measure presence and quantity of trace metals and radon precursors. Of 744 homes tested in Vanderburgh County from 2007 to 2013, 169 homes (22.7 percent) had elevated radon levels greater than 148 mBq L⁻¹ (4.0 pCi L⁻¹). Additionally, 246 homes (33.1 percent) had measured radon levels of 74–145 mBq L⁻¹ (2.0–3.9 pCi L⁻¹). About 80 percent of elevated radon levels greater than 148 mBq L⁻¹ (4.0 pCi L⁻¹) are located in proximity to depositional contacts between the Dugger and Shelburn formations, or the Shelburn and Patoka formations. These formational contacts are stratigraphically associated with Pennsylvanian black shales, which are interpreted to be the ultimate source of radon in Vanderburgh County, Indiana. Moreover, high radon levels also occurred in homes built on alluvium, terrace deposits, and outwash adjacent to the Ohio River. These elevated levels are probably due to transmission of radon through soil gas in highly permeable sands and gravels sourced from buried bedrock exposures of Pennsylvanian black shales. - Highlights: • Relationship between bedrock and surficial geology and elevated radon in homes. • Spectral gamma ray identification of radon precursors in black shake. • Residential radon data from 2007 to 2013 provided for Vanderburgh County, Indiana. • Importance of highly permeable sediments and soil gas to radon distribution