Temporal changes and factors influencing Cs-137 concentration in vegetation colonizing an exposed lake bed over a three-year period

Activity concentrations of Cs-137 in sediments, as well as extractable sedi ment concentrations of K, Na, Mg, Ca, Mn, Zn and P, pH, percent organic mat ter and cation exchange capacity, were used as independent variables in an incomplete principal component analysis to identify factors affecting Cs-13 7 activity concentrations in 12 species of native wetland and terrestrial p lants invading a recently exposed lake bed. Sediments in the lake had been contaminated 35 years previously from discharges at a nuclear production re actor. Cs-137 activity concentrations in plants were positively correlated with sediment concentrations of Na and Cs-137, and inversely correlated to K and pH. Significant decreases in concentrations of sediment constituents (from 3 to 77%), as well as a 42% decline in Cs-137 activity concentrations in plants, occurred during the three-year period. Significant differences in Cs-137 activity concentrations among plant species, driven by low concen trations in cattails (Typha latifolia), were observed. Terrestrial species had significantly lower activity concentrations of Cs-137 than species clas sified as wetland (arithmetic mean +/- S.E. of 1069 +/- 151. and 2602 +/- 3 94, respectively). Concentration ratios (Bq kg(-1) dry plant/Bq kg(-1) dry soil) were among the highest reported in the literature (arithmetic mean +/ - S.D. = 12.5 +/- 28.9; geometric mean = 1.6, geometric S.D. = 1.4) and wer e attributed to kaolinitic sediments of pH < 5, organic matter < 5%, K conc entrations < 15 ppm and cation exchange capacity < 5 meg (100 g(-1)). Even though Cs-137 was released into the system over 35 years ago, lowering of t he water caused Cs-137 in the newly formed terrestrial system to behave lik e a fresh, rather than aged, deposit (initial uptake rates by plants were h igh and subsequent reductions in bioavailability were rapid). Implications for management of contaminated reservoirs from a public risk perspective ar e discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.