26 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



The deposition of plutonium on biological surfaces can be inferred directly from 

 concentration ratios 



_ Transuranic concentration in receptor 

 Transuranic concentration in donor 



based on field data (Hakonson, 1975; Little, 1976; this volume; Romney and Wallace, 

 1977; Dalilman, Bondietti, and Eyman, 1976). Such ratios are much higher than those 

 derived from greenliouse studies (Francis, 1973; Price, 1973; Schulz, 1977) and imply 

 that root uptake cannot account for concentrations measured in field samples. Physical 

 processes are evidently more important than chemical processes in transporting 

 plutonium to vegetation. 



Wind is apparently more important in contaminating vegetation in dry regions than it 

 is in humid regions, as shown by plutonium CR's. These ratios decrease from about 10~* 

 in United States deserts (Hakonson, 1975; Little, 1976; Romney and Wallace, 1977) to 

 10" ■^ in mesic ecosystems of the southeast (Dalilman, Bondietti, and Eyman, 1976). 



Additional observations implicate wind-driven processes in contaminating vegetation 

 with plutonium. For example, plutonium concentrations are inversely correlated with 

 height of plants above tlie ground (Hakonson and Johnson, 1974; Dahlm an, Garten, and 

 Hakonson, this volume). Tlius low-growth forms, such as grasses, forbs, lichens, and 

 mosses, generally' exhibit higlier plutonium concentrations than shrubs or trees. This 

 pattern is consistent with soil flux— heiglit relationships, which show that most of the soil 

 mass transported by wind is within 1 m of the ground surface (Selimel, 1978; Gillette, 

 Blifford, and Fenster, 1972; Phelps and Anspauglr, 1977). 



Water Erosion. Physical transport of transuranic elements by raindrop splash or surface 

 runoff has received Uttle attention in terrestrial ecosystems, althougli these processes 

 certainly occur (Romney and Wallace, 1977; Hakonson, Nyhan, and Purtymun, 1976; 

 Sprugel and Bartelt, 1978; Muller, Sprugel, and Kohn, 1978). For example, Beasley 

 (1972) has shown that a 5-cm rainstorm causes disaggregation of 200 metric tons of soil 

 per hectare by raindrop splash and surface-water runoff. The importance of soil splash up 

 from raindrops in contaminating vegetation is unknown, although the process certainly 

 occurs. 



In certain cases (e.g., intermittent streams) water movement of sediments may be the 

 dominant mechanism of plutonium transport (Hakonson, Nyhan, and Purtymun, 1976). 

 The process is primarily the physical transport of plutonium sorbed on soil particles 

 rather than movement of dissolved plutonium (Hakonson, Nyhan, and Purtymun, 1976; 

 MuUer, Sprugel, and Kohn, 1978). 



The relationship of plutonium concentration to soil particle size is also important in 

 assessing transport because water movement preferentially sorts soil according to particle 

 size (Hakonson, Nyhan, and Purtymun, 1976; Muller, Sprugel, and Kohn, 1978). For 

 example, as water velocity decreases, successively smaller soil size fractions remain in 

 suspension. Hence silt— clay fractions, wltich usually contain higher concentrations of 

 plutonium, are probably carried greater distances than larger size fractions. 



Water transport of soil across landscapes redeposits plutonium within local watershed 

 soils and stream channel sediments (Hakonson, Nyhan, and Purtymun, 1976), down- 

 stream ponds (Muller, Sprugel, and Kohn, 1978), rivers (Hayes and Horton, this volume; 

 Sprugel and Bartelt, 1978), lakes, and oceans. Studies of intermittent streams at Los 



