Mobility of Chernobyl-derived 137Cs in a peatbog system within the catchment of the Pripyat River, Belarus.

The behaviour of Chernobyl-derived 137Cs in a hydrologically isolated bog system in the catchment of the Pripyat River, Belarus was investigated. Measurements were made of 137Cs activities in the solids and pore waters of the bog soils, as well as the variability in activity in water draining from the bog. It was found that the radiocaesium activity of the pore water, and hence the measured distribution coefficient, Kd, was dependent upon the pressure at which the water was removed from the soil. Measured values of Kd were of the order 10(2) l/kg at an extraction pressure of 0.8 MPa, approximately one order of magnitude lower than those measured in a similar system, Devoke Water, in the UK [1]. Results of comparative measurements suggested that this was a result of the different pore water extraction techniques used. The vertical migration of radiocaesium was modelled using a solution of the advection-diffusion equation. Using a mass balance approach, it was estimated that 137Cs was removed from the system at a rate of 0.3% of the catchment inventory per year, approximately 8 years after the Chernobyl accident. It was shown that both vertical migration and removal of 137Cs is best modelled using a Kd based on a measurement of pore water held at low pressure in the soil, approximately 10(3) l/kg. 137Cs activities in soil pore waters and in drainage waters were very strongly related to the aqueous potassium concentration, and both showed concentration minima in drainage water during the spring. It was shown that runoff coefficients of radiocaesium from peat bogs 8 years after the Chernobyl accident were approximately one order of magnitude greater than those from unsaturated soils of higher mineral content.