Isotopic Studies of Sources of Uranium in Sediments of the Ashtabula River, Ohio, U.S.A.

Abstract

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 > 10× 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 ...