474 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



move in the soil and its availability for root uptake, may increase with time as a result of 

 chelation due to bacterial decay of soil organic matter. 



The evidence that the soil-uptake concentration ratio may increase exponentially as 

 a function of time is scanty but deserves consideration because of its implications. 

 Romney, Mork, and Larson (1970) grew ladino clover (Trifolium repens L.) on a 

 plutonium-contaminated soil under greenhouse conditions and cropped it repeatedly over 

 a period of 5 yr. The resulting estimates of Crv were as follows: 



Crv. 

 (d/min)g"'' plants (dry weight) 



Year (d/min) g"' soil (dry weight) 



A least-squares fit of these data to an exponential function yields 



Crv = 1.31 X 10"^ exp (0.452 t) 



where t is time (years) and 0.452 is the apparent growth rate coefficient; i.e., the data 

 indicate that Crv would be expected to double in about 1.5 yr. By extrapolation to 20 

 yr, the concentration ratio would be 0.1 1, a value within the range of field observations 

 for perennials (see below) but misleading nonetheless. Romney, Mork, and Larson (1970) 

 attribute the ''apparent" increase of Crv to root growth during the time the experiment 

 was being conducted and not to any change in the biological availability of the plutonium 

 contained in the contaminated soil. 



In fact, the plant/soil ratios observed under field conditions are generally too high to 

 be explained by root uptake. Romney et al. (1975) collected soil and plant samples from 

 contaminated areas at NTS. The mean plant/soil ratios for different groups of paired 

 samples ranged from 0.004 to 0.44 and were inversely related to the mean soil 

 contamination of each group, which ranged from 5.9 x 10~^ to 0.12 luCi/g. The 

 weighted mean ratio for 506 paired samples was 0.096 ± 0.004. The plants in this study 

 were desert shrubs growing in areas that were contaminated with plutonium as a result of 

 nuclear safety tests conducted from 1953 to 1964. Ratios obtained by growing plants in 

 these same soils were on the order of 10""^ to 10""*, which indicates that no more than 

 1% of the plutonium in plant samples from contaminated areas at NTS is likely to be due 

 to root uptake. 



Environmental monitoring data from Hanford (Bramson and Corley, 1973; Nees and 

 Corley, 1974; Fix, 1975) indicate ratios in the range of 0.05 to 1.0 for soil concentrations 

 in the range of 2 x 10~^ to 4 x 10~^ fJ^Ci/g. Similar data from Savannah River 

 (McLendon et al., 1976) indicate ratios from 0.009 to 0.97 for soil concentrations in the 

 range of 1.3 X 10"^ to 1.6 x 10"^ juCi "^Pu/g and 2x lO"''^ to 4.6 x 10"^ [JiCi 

 ^^*Pu/g. The higlier plant/soil ratios are usually found at lower soil concentrations. 



