Unusually Radiogenic Lead (Pb) in Ashtabula River Sediments: An Isotopic Tracing Case Study Using Quadrupole ICP-MS

Abstract

Unusually radiogenic Pb has been found in sediments of a portion of the Ashtabula River (80 o 48' W, 41 o 54' N) near its confluence with Lake Erie. Pb isotopic compositions have been determined by quadrupole ICPMS, revealing that the Pb results from mixing of three sources: crustal Pb, common Pb from industrial pollutant sources, and highly radiogenic Pb associated with U-containing accessory minerals in a Ti ore refining process discharge. Crustal Pb and anthropogenic pollutant common Pb exhibit 206 Pb/ 204 Pb ratios of ~ 19, in agreement with findings from published Lake Erie sediment studies. Horizons containing > 10 ppm U exhibit elevated 206 Pb/ 204 Pb ratios (23.88 ± 2.57, n = 11) which cannot be accounted for by anthropogenic common Pb sources. Increases in 206 Pb/ 204 Pb are not accompanied by corresponding increases in 208 Pb/ 204 Pb, with the exception of one horizon that contains 96 ppm Th. The 206 Pb/ 204 Pb is correlated with the U concentration (r 2 = 0.902). A small tributary of the Ashtabula River, Fields Brook, is the apparent point of origin of the U, Th and radiogenic Pb in the Ashtabula River sediments. These findings demonstrate that the Pb isotopic composition in the environment is subject to local influences from naturally occurring radioactive material sources.