Transport and Storage of 137Cs and 210Pb in Sediments of Lake St. Clair

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Abstract
In 1985 cores were collected by diver from areas with fine-grained sediments in Lake St. Clair. Although the lake is shallow, rapidly flushed, and possesses only a thin layer of postglacial sediment (ca. 30 cm max.), 8% of the estimated, 137Cs loading from atmospheric nuclear testing in the mid-1960s and 13% of the potential standing crop of excess 210Pb were retained. A sediment column transport model including eddy diffusive mixing, advection and resuspension, acceptably described the vertical distribution of these radionuclides as well as stable lead and implied that such efficient retention may be of recent origin, occurring with the onset of net sedimentation about 100 yr ago. The model showed that, at selected sites, the history of lake loading by particle-associated contaminants can be reconstructed from sediment profiles. Horizontally averaged characteristics of the deposit indicate a surface mixed layer mass of 5 g∙cm−2 and tracer residence time of 3 yr in accord with residence times of surficial Hg, PCBs, and DDT. Trap-collected materials from two sites show markedly contrasting seasonal variations in,137Cs activity reflecting differing proportions of particles derived from inflow (ca 300 mBq∙g−1) and resuspension (< 30 mBq∙g−1).