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

摘要

Unusually radiogenic Pb has been found in sediments of a portion of the Ashtabula River (80o48’ W, 41o54’ N)near its confluence with Lake Erie. Pb isotopic compositions have been determined by quadrupole ICPMS, revealing thatthe Pb results from mixing of three sources: crustal Pb, common Pb from industrial pollutant sources, and highlyradiogenic Pb associated with U-containing accessory minerals in a Ti ore refining process discharge. Crustal Pb andanthropogenic pollutant common Pb exhibit 206Pb/204Pb ratios of ~ 19, in agreement with findings from published LakeErie sediment studies. Horizons containing > 10 ppm U exhibit elevated 206Pb/204Pb ratios (23.88 ± 2.57, n = 11) whichcannot be accounted for by anthropogenic common Pb sources. Increases in 206Pb/204Pb are not accompanied bycorresponding increases in 208Pb/204Pb, with the exception of one horizon that contains 96 ppm Th. The 206Pb/204Pb iscorrelated with the U concentration (r2 = 0.902). A small tributary of the Ashtabula River, Fields Brook, is the apparentpoint of origin of the U, Th and radiogenic Pb in the Ashtabula River sediments. These findings demonstrate that the Pbisotopic composition in the environment is subject to local influences from naturally occurring radioactive materialsources.