Deposition near a reprocessing plant

Washout by rain becomes fully effective only when the cloud of activity extends up to the height of the rain-forming clouds. Within less than 10 km from a near-ground source, dry deposition is usually more effective than washout. 

Iodine is lost from herbage by the same processes which cause field loss of Sr, 137Cs and other nuclides (Section 2.13). There is also the possibility of revolatilisation of iodine. If XG is the rate constant of field loss (fraction of iodine per unit area of ground lost from vegetation per second) and X1 the rate constant of radioactive decay, the combined apparent or effective loss rate is XE = XG + Xt. The effective half-life is Te = 0.693/A . The use of the term half-life implies that field loss is exponential and TE invariant with time, which is not always true. 

Volatilisation may be more significant when appreciable quantities of stable iodine carrier are present. Heinemann Vogt (1980) measured 

If a constant airborne concentration of 131I is maintained until the concentration reaches equilibrium, the air/grass transfer factor defined  

If vD = 0.1 m3 kg-1 s-1 and XE = 0.14 d 1 = 1.6 x 10-6 s 1, then the transfer factor air/grass is 6 x 104 m3 kg-1. Somewhat lower values, in the range 2 to 4 x 104 m3 kg-1 have been obtained by comparing measurements of 131I in air and herbage during periods when activity has been emitted from chemical separation plants (Marter, 1963 Eisenbud Wrenn, 1963). 

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