Determination of dose rate from Chernobyl-derived radiocaesium in Estonian soil

The deposition and vertical depth distribution of 134Cs and 137Cs in the natural undisturbed soil profiles down to 20-25 cm were studied at locations in the North Eastern Estonia, which were most strongly affected by the Chernobyl fallout in 1986. The total depositions were estimated based on summing the sampled and measured activities of 134Cs and 137Cs in all sections of soil profiles. The Chernobyl 137Cs deposition values varied considerably from site to site and the range was from 3.9 kBq m(-2) to 50.2 kBq m(-2), with the average of 22.8 kBq m(-2) for the region (reference data May 1, 1986). The ratio of total activities, A(134Cs)/A(137Cs), varied in the range from 0.47 to 0.55. Using a compartment model and the observed data on the 134Cs and 137Cs activity concentrations (Bq m(-2)) in four compartments, 0-1 cm, 1-5 cm, 5-15 cm and 15-30 cm, of soil collected in 1991-2003, the approximate residence half-times of radiocaesium in soil were determined. The latter increased from 3.7 y in the top-most compartment to 8.6 y and 36.4 y in the deeper compartments, respectively. The time dependence of the external gamma-dose rate at the height of 1 m above a flat ground area arising from the deposited and migrating radiocaesium was calculated using the modeled data on the 134Cs and 137Cs activity in soil compartments. Considering the total depositions of radiocaesium in soil from the Chernobyl 1986 accident, the 50 year effective doses caused by external gamma exposure varied in the range from 0.13 mSv to 1.74 mSv, with the mean of 0.79 mSv in the region.