FIELD MEASUREMENTS OF URANIUM USING GAMMA SPECTROMETRY IN SHALE OF THE UTICA SHALE AND SCHENECTADY FORMATION, LOWER MOHAWK VALLEY NY

The lower Mohawk Valley is underlain by shale and sandstone of the Upper Ordovician Utica Shale and Schenectady Formation, and thus shale is a common bedrock lithology covered by a thin mantle of glacial till. This study is aimed at documenting the distribution of uranium in shale units to understand causes of variation of radioactivity in shale and radon potential. Fieldwork for the study was conducted using a portable gamma spectrometer (Radiation Solutions, RS-230 BGO) with a lead (Pb) collar. Measurements on 140 rocks were made in the field to understand radioactivity (K, Th, and U) in outcrops over a wide area in Montgomery and Schenectady counties. We are particularly interested in uranium because it can occur in elevated concentrations in ground water, and the decay chain includes radon gas, known to cause cancer. For all units, uranium ranges from ~2 to 9 ppm and the highest values are from the upper part of the Utica Shale. Average uranium content for all stratigraphic units of the Utica Shale and Schenectady Formation is approximately 5 ppm. In the Schenectady Formation, uranium concentrations are consistent and uniform whereas concentrations in the Utica are highly variable, and generally higher. The Utica Shale shows increasing radioactivity and uranium upsection. High values of uranium in the upper part of the stratigraphy may be associated with increasing amounts of clastic sediments, redox reactions in the depositional setting, or secondary remobilization of uranium. The highest values of uranium in the Utica Shale and Schenectady Formation range between 6 and 9 ppm. The mean Th/U ratio for the Utica Shale and Schenectady Formation is 1.9 and 3.0, respectively. Anomalously high concentrations of uranium were observed in the upper Utica Shale (8-9 ppm) downstream of Buttermilk Falls on Yates Creek, adjacent to the Noses Fault. Shale units along the Hoffman’s fault do not appear to have anomalous values. An important finding of this work is that there is a strong stratigraphic dependence of uranium in the Utica Shale and that much of the shale of the Schenectady Formation has relatively high uranium (equal to or greater than the lower Utica). These observations can be used to better inform decisions about radon hazards, and radionuclides in groundwater