Epsilon Archive for Student Projects

Abstract

Vertical migration of 137Cs and 137Cs activity concentrations in plants growing on a raised bog in Central Sweden were investigated. Raised bogs are among the most nutrient poor ecosystems. Blocks of soil (peat) and samples of plants were collected from two sites on the bog, the open bog site where no trees were growing and the low pine site with slowly growing Scots pine. The peat at both sites was composed of living Sphagnum mosses in the upper 5 cm layer and dead and more or less decomposed Sphagnum material in deeper layers. Samples were also collected from a forest on mineral soil close to the bog. The soil blocks from the bog were cut in 2 cm layers down to a depth of about 40 cm. The 137Cs activity concentrations (Bq kg-1 dw) in plant samples and the 137Cs activity per m-2 in each peat layer were determined and the migration centers and the migration rates were calculated.

In 2005, the migration center at the open bog site was 13.42 cm and the migration rate was
0.70 cm y-1, at the low pine site the migration center was 15.53 cm and the migration rate
0.81 cm y-1. The corresponding values in the forest soil were 7.47 cm and 0.39 cm y-1. At
the open bog site about 35 % of the total 137Cs activity in the soil profile was found in the upper 5 cm (living part). The corresponding percentage at the low pine site was 20 % and 50% at the forest site. The results from 2005 were compared with results from 1989. In
1989, the migration center was 5.0 cm and the migration rate was 1.67 cm y-1 at the open
bog site. Obviously, the migration rate was higher in 1989 compared to 2005.

In 2005, the highest 137Cs activity concentrations in plants collected from the open bog site were found in heather – about 20 000 Bq kg-1. The corresponding value in heather
collected from the low pine site was about 10 000 Bq kg-1 and at the forest site 5 000 Bq
kg -1. A much lower level, 418 Bq kg-1, was found in crowberry growing at the forest site.
In 1989, the 137Cs activity concentration in heather growing at the open bog site was
44 000 Bq kg-1. The 137Cs activity concentrations thus seem to have decreased by about 50
% from 1989 to 2005.