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

Activity concentrations of 137Cs in sediments, as well as extractable sediment concentrations of K, Na, Mg, Ca, Mn, Zn and P, pH, percent organic matter and cation exchange capacity, were used as independent variables in an incomplete principal component analysis to identify factors affecting 137Cs activity concentrations in 12 species of native wetland and terrestrial plants invading a recently exposed lake bed. Sediments in the lake had been contaminated 35 years previously from discharges at a nuclear production reactor. 137Cs activity concentrations in plants were positively correlated with sediment concentrations of Na and 137Cs, 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 137Cs activity concentrations in plants, occurred during the three-year period. Significant differences in 137Cs activity concentrations among plant species, driven by low concentrations in cattails (Typha latifolia), were observed. Terrestrial species had significantly lower activity concentrations of 137Cs than species classified as wetland (arithmetic mean±S.E. of 1069±151 and 2602±394, 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 were attributed to kaolinitic sediments of pH<5, organic matter <5%, K concentrations <15 ppm and cation exchange capacity <5 meq (100 g-1). Even though 137Cs was released into the system over 35 years ago, lowering of the water caused 137Cs in the newly formed terrestrial system to behave like a fresh, rather than aged, deposit (initial uptake rates by plants were high and subsequent reductions in bioavailability were rapid). Implications for management of contaminated reservoirs from a public risk perspective are discussed.