Interception and retention of radioactive aerosols by vegetation

Abstract

The efficiency of vegetation as a filter for nuclides deposited on the earth's surface is examined. The proportion of the fallout initially retained in the herbage, p, is assumed to be related to an uptake coefficient μ (m 2 kg −1) by the relation 1 − p = exp − μw, where w is the herbage density in kg m −2 dry matter. Experimental values of μ lie in the range 2.3 to 3.3 m 2 kg −1, and there does not seem to be any marked dependence on the physical form of the activity. The rate of loss of activity by weathering from foliage is next considered. The field loss coefficient λ b has been found to be of order 0.05 days −1 in field experiments during the growing season, but lower values (corresponding to longer retention periods on the foliage) have been observed in winter. Representative values of p and λ b are then used to calculate the normalised specific activity of herbage continuously exposed to fallout, where N.S.A. = Activity per kg dry matter on foliage Activity deposited day −1 m −2 of ground The calculations give N.S.A. values ranging from 27 to 55 m 2 days kg −1 depending on the value assumed for λ b and for the time T 0 between successive croppings of the herbage. Finally, surveys of the activity of 90Sr and 137Cs in the U.K. and abroad are used to deduce the N.S.A. of foliage exposed to world-wide fallout from weapon tests. Good agreement with the calculated values is obtained, but there is evidence of enhanced N.S.A. values in poor growing conditions.