Long-term trends of radiocesium activity concentrations in vegetation in Irish semi-natural ecosystems.

137Cs activity concentrations were determined in vegetation from four Irish blanket bog ecosystems during the summer months of 1989-1997. The dominant and abundant vegetation species were sampled at each site and the data were used to estimate long-term trends of 137Cs activity concentrations. A general decline in the 137Cs activity concentrations in vegetation was observed from all sites sampled. The fastest and most consistent long-term rates of 137Cs decline were observed in the bryophyte and lichen vegetation group with 137Cs effective half-life (T ef) values of 2.2 to 10.7 y. The ericoid group with the exception of one case also showed a significant long-term decline in 137Cs activity concentrations with T ef values of 3.5 to 12.4 y as did the rush species Juncus squarrosus (T ef range 9.3 to 12.8 y). The sedge and grass groups showed the slowest and the least consistent rates of 137Cs decline, with T ef values of 2.9 to 59.8 y. Preliminary evaluation of 137Cs activity concentrations in sheep from some of the studied sites suggests that the decline found in vegetation is reflected in sheep.