472 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



intervals of 10, 15, 30, and 60 days after fallout. Factoring out the loss of ' ^ ' I due to 

 vaporization and the losses of both radionuclides due to radioactive decay, the data 

 indicated that the weathering (environmental) half-time for these two radionuclides 

 increased with respect to time after fallout. During the interval from 5 to 15 days after 

 fallout, the average weathering half-time was about 20 days. The value obtained for the 

 interval from 15 to 30 days after fallout was about 30 days, and it increased for the 

 interval from 30 to 60 days postdetonation to about 130 days. Tlie D+5-day 

 concentrations can be compared with assumed D+0 concentrations, which would 

 reconcile the difference between Martin's estimate of the plant interception factor and 

 Miller and Lee's estimate of the foliage contamination factor. This procedure suggests an 

 average weathering half-time of approximately 1.4 days during the interval from to 5 

 days after fallout. 



These observations lead us to the hypothesis that tlie decay-corrected concentration 

 of a radionuclide in fallout-contaminated plant material is a very rapidly declining 

 exponential function of time at times soon after the contaminating event but approaches 

 a lower asymptote. Since this hypothesis appears to be correct, the effective rate at which 

 a radionuclide is removed from surfaces following external deposition cannot be 

 expressed precisely by a single coefficient because the weathering half-time increases as a 

 function of time after contamination. If the initial deposition is a heavy one, a significant 

 fraction of it (perhaps as much as 90%) can be removed by weathering in a matter of 

 hours, or a few days at most. A portion of what remains after this initial period of fast 

 weathering (something in the range of 10 to 60%) is so tightly trapped that it cannot be 

 removed even by vigorous washing (Romney et al., 1963). Presumably, this nonremovable 

 fraction is composed predominantly of particles that are small and mechanically trapped 

 on plant surfaces. 



The situation in the plutonium-contaminated area at NTS is one in which foliar 

 deposition of resuspended particles and the loss of these particles from foliage is a more 

 or less continuous process. If the turnover rate is rapid, a foliage/soil steady state would 

 be quickly established. 



Plant Growth Rates. As indicated earlier, the growth of new plant tissue may dilute both 

 the external and the internal concentrations of plutonium or other transuranium elements 

 in plant materials. Since different plant parts may grow at different rates, it is obvious 

 that the growth rate of interest with respect to external contamination is the growth rate 

 of leaves (and other edible parts formed above ground). If we assume that internal 

 plutonium due to root uptake is uniformly distributed to all parts of the plant, the 

 growth rate of interest with respect to dilution of root uptake is the overall growth rate, 

 i.e., the growth rate of leaves plus the growth rate of all other plant parts. 



Plant growth is not a continuous process, nor is it the same for all species in a given 

 area or for all the parts of a given plant. In the temperate zone, at least, plant growth is 

 confined to the warm season, and the rate of growth is not uniform througliout the 

 growing season because different plant organs develop at different times. Ignoring the 

 morphogenic aspects of plant growth (i.e., the differentiation and development of 

 structure), growth is most simply conceived as an increase in biomass (i.e., dry weight of 

 tissue per unit area). For armuals, the biomass at the beginning of the growing season 

 consists of seeds; for herbaceous perennials, for which the aboveground parts die back 

 during the winter, it consists mostly of roots and other belowground parts; for woody 

 perennials, it consists of roots and stems (mostly dead tissue) plus, in the case of 



