Ecological half-lives of ¹³⁷Cs in fishes from the Savannah river site

Ecological half-lives (T(e)'s) were estimated for ¹³⁷Cs in largemouth bass, sunfishes, and bullheads from two reservoirs and three streams on the Savannah River Site, a nuclear weapons material production facility in South Carolina. Ecological half-life is the time required for a given contaminant concentration to decrease by 50% as a result of physical, chemical, and/or biological processes that remove it from an ecosystem or render it biologically unavailable. T(e)'s were estimated from whole-fish ¹³⁷Cs concentrations in samples collected during 1972-1996, following radionuclide releases that occurred primarily during the 1960's and early 1970's. T(e)'s ranged from 3.2 to 16.7 y, and all were shorter than expected from the half- life for radioactive decay (T(p) = 30.2 y) alone. Fish taxa from the same locations differed in mean ¹³⁷Cs concentrations (highest in largemouth bass and lowest in sunfishes) but, in most cases, exhibited similar ¹³⁷Cs T(e)'s. Rates of ¹³⁷Cs removal in fishes were strongly correlated with rates of ¹³⁷Cs removal in water. The shortest T(e)'s occurred in the upper portions of the streams. T(e)'s in lower portions of the streams were longer, as were T(e)'s in one of the reservoirs. T(e)'s in the second reservoir, which had a much shorter water residence time, were nearly comparable to those in the upper portions of the streams until 1991. At that time, ¹³⁷Cs concentrations in fishes began to increase following drainage and refilling of the reservoir, which apparently resuspended ¹³⁷Cs buried in the sediments.