UPTAKE OF TRANS URANIC NUCLIDES FROM SOIL BY PLANTS 349 



Soil processes involving soil OM decomposition (Cataldo et al., 1976; Wildung and 

 Garland, tliis volume); microbial, particularly fungal, growth (Wildung and Garland, this 

 volume); chelation; hydrolysis; and oxidation likewise influence the phytoavailability of 

 the nuclides. 



The long-time phytoavailability of the transuranic nuclides, once on the soil, will 

 depend on the long-time soil processes. Leaching and capillary rise will cause the nuclides 

 to move in the profile. Animal activity will also do the same. 



The slow process of sheet erosion likewise can cause the nuclides to move in the 

 profile. Particularly with plowing, which moves nuclides to greater depths, sheet erosion 

 can result in movement of the nuclides up in the profile because of a decrease in the level 

 of the soil surface. Hundreds of years may be involved in this process, but it must be 

 considered in the long-time availability of the nucHdes. 



Plant properties influence the uptake of the transuranic elements. It is well known 

 that plant roots excrete protons, organic and amino acids, chelators, and other 

 substances. These liave profound effects on tlie uptake translocation of many metals, 

 including transuranic elements. Also, roots have a reducing capacity that is exceptionally 

 important in the physiology of iron uptake by plants. At least in some species, Fe^"" must 

 be reduced to Fe^"" before absorption can occur. This process is believed to be of 

 importance in the various oxidation states of some transuranic elements. Decomposition 

 of plant residues influences both uptake and recycling. 



The depth of rooting of various plant species, a characteristic of both the plant and 

 the soil in which it is growing, is a factor in transuranium-element uptake. This is 

 especially important in areas that are or will be plowed or in areas where wastes have been 

 buried. Nuclides in lower horizons of soil may be mobilized by deep roots. 



The aboveground contamination of plants and subsequent leaf absorption— 

 translocation is covered elsewhere in this volume. Leaf uptake does depend on the 

 physiological and anatomical characteristics of the leaves. 



Environmental conditions other than soil characteristics may be more important in 

 leaf uptake of transuranic elements tlian in root uptake of them. The effect of soil 

 moisture is as yet unknown for root uptake, but soil moisture is a factor in wind 

 resuspension and subsequent deposition of contaminated materials on leaf surfaces. 



The sources of transuranic elements are of much importance to phytoavailability. The 

 particle size greatly determines the availability. Small particles generally are more subject 

 to weathering and release nuchdes faster than do large particles. Oxides are less available 

 than other forms. Transuranic elements in wastes containing acids and chelator chemicals 

 probably are more available than those from other sources because of the complexation 

 processes involved. Oxidation state has been referred to previously. The transuranic 

 elements themselves differ in phytoavailability (Pu < Am < Cm < Np) (Price, 1972b). 



The various factors that influence transuranic -element uptake by plants result in a 

 very wide range covering several orders of magnitude in the CR. Data have been compiled 

 from the literature and are shown in Table 7. One contributing factor is the very low level 

 of the nuclides that move from soil to roots and then to shoots of plants. The root uptake 

 is generally believed to be very much lower than that which comes from the atmosphere. 

 Tliis is correct only so long as the nuclides can be resuspended or released to the air. On a 

 long-term basis, root uptake would exceed that from the atmosphere. 



(Text continues on page 357.) 



