634 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Sediments also have the highest concentrations of plutonium in the pond (Table 2). The 

 95% confidence interval of plutonium in sediment samples extended from 1.9 to 

 2.7 X 10^ pCi/g for 239,240p^ ^^^ ^^^^ 2.0 to 3.3 X 10^ pCi/g for ^^^Pu. Hence the 

 inventory of plutonium in the sediments is more than 99% of the inventory for the entire 

 system (Tables 3 and 4). 



Other studies of plutonium in aquatic systems show sediments playing the dominating 

 role in the plutonium inventory of their respective ecosystems (Johnson, Svalberg, and 

 Paine, 1974; Patterson et al., 1976; Trabalka and Eyman, 1976). In nearly all studies that 

 have reported inventories of plutonium in freshwater systems, the sediments contain at 

 least 99% of the total plutonium burden. This commonality among ecosystems having 

 widely different limnological characteristics suggests that the accumulation, retention, 

 and transport of plutonium is strongly associated with sediment and sedimenting 

 particles. This idea is supported by results of numerous studies in which particulate 

 plutonium concentrations were measured apart from those dissolved (or suspended) in 

 water and interstitial water (Bartelt, Wayman, and Edgington, 1975; Dahlman, Bondietti, 

 and Eastwood, 1975; Emery, Klopfer, and Weimer, 1974; Hakonson, Nyhan, and 

 Purtymun, 1976; Johnson, Svalberg, and Paine, 1974; Magno, Reaney, and Apidianakis, 

 1970; Noshkin, 1972; Singh and Marshall, 1977; Trabalka and Eyman, 1976). These 

 studies show that plutonium associated with particulates makes up more than 80% of the 

 total plutonium concentrations in water. This characteristic of plutonium distribution 

 and transport in freshwaters is the most significant aspect of its envirorrm'^ntal behavior. 



The pond is highly enriched with nutrients coming from laundry effluents via U-14 

 ditch (Fig. 1, Emery, Klopfer, and Weimer, 1974). Tliis nutrient supply supports 

 luxuriant growths of algae and macrophytes (Table 1) wliich eventually settle to the 

 bottom and decompose. The result of this process is the formation of a layer of organic 

 floe that rests on the surface of older floe and sediments. Although this material is 

 sedimentary, it has several special characteristics. The density of floe approaches that of 

 water, which causes it to be loosely compacted and easily resuspended. The large quantity 

 of floe generated each year serves as a source of food for many animal populations in and 

 around the pond. Perhaps most important, the floe is the primary concentrator of 

 plutonium in the ecosystem. 



Hydrologic considerations of the pond provide additional significance to the 

 functional role of the organic floe in the ecosystem. Since the pond has no surface 

 outflow and a short retention time (40 hr), there is a rapid deposition of suspended 

 material (seston). The sedimentation rate is approximately 1 kgm^^ yr^^ (dry). This 

 means that about 5.6 x 10'* kg of seston is deposited each year in this loosely compacted 

 floc/sediment. 



If U-Pond has sustained a continuous annual sedimentation rate of 5.6 x 10^ kg since 

 its formation in 1944, about 1.8 x 10^ kg of sestonic sediments has been deposited. The 

 total organic production of biomass in U-Pond is about 5 x lO'* kg/yr (Table 1), which is 

 approximately equal to the annual deposition of suspended matter. If losses of organic 

 matter in the sediments caused by decomposition are ignored, all sources of sedimentary 

 materials have deposited 3.5 x 10^ kg since 1944. The weiglit of U-Pond sediments down 

 to 10 cm is 3.4 X 10^ kg (dry. Table 1). This suggests that U-Pond has not deposited 

 more than about a 10-cm layer of sediments since its creation. The actual tliickness of 

 deposition would probably be smaller because of the decomposition of organic matter. 

 Tliis does not account for wind-blown dust accumulated in the pond. 



