Isotopic studies of sources of uranium in sediments of the Ashtabula River, Ohio, U.S.A.

Uranium contamination of anthropogenic origin has been identified in unconsolidated sediments of a 1.5 km portion of the Ashtabula River near its confluence with Lake Erie. Uranium concentrations as high as 188 μg/g dry sediment are present. A small tributary of the Ashtabula River, Fields Brook, is the apparent point of origin of the uranium in the Ashtabula River sediments. ¹³⁷Cs dating of a sediment core indicates that the U contamination occurred during the post-1964 time frame. The horizons of elevated U concentration also exhibit > 10x elevations in Zr, Nb, Hf, Ta, and W. ²³⁸U/²³⁵U isotopic ratios indicate that the uranium is largely but not exclusively of natural composition. Distinct horizons of slightly ²³⁵U-depleted (²³⁸U/²³⁵U > 137.88) and slightly ²³⁵U-enriched (²³⁸U/²³⁵U < 137.88) uranium are also present. ²¹⁰Pb activities and ²³²Th/²³⁰Th isotopic measurements indicate that a significant portion of the uranium contains ²³⁸U daughters in approximate secular equilibrium. It is inferred that at least two distinct sources of anthropogenic U contamination exist: (A) discharges from the processing of enriched and depleted U metal by a DOE contractor facility and (B) U-bearing wastes from the production of TiO₂ from ilmenite and associated minerals. These isotopic methodologies are potentially useful in settings where releases of nonnatural ²³⁸U/²³⁵U composition materials and/or 'naturally occurring radioactive material' (NORM) have taken place.