636 TRANS URANJC ELEMENTS IN THE ENVIRONMENT 



The existing mean inventory of plutonium in the sediments (to 10 cm) is 1.7 x 10^ 

 nCi (Tables 3 and 4). Tliis amount of plutonium represents the total accumulation from 

 all sources minus the losses via the various routes of export. The validity of this can be 

 examined by calculating a theoretical accumulation of plutonium in the sediments using 

 annual rates of plutonium deposition via sedimenting seston and annual accumulation of 

 plutonium by pond biota. 



The mean concentration of plutonium in U-Pond seston measured over the study 

 period is 5.6 X 10° nCi/m'^. Thus the pond's water mass (2.27 X 10'* m^) contains an 

 average of 1.3 X 10^ nCi of plutonium. This mass passes through the pond's basin at an 

 average rate of 219 times per year (i.e., 40-hr retention time). If we assume that the 

 flushing rate and plutonium content of U-Pond water remain constant, then 2.8 X 10^ 

 nCi of sestonic plutonium is deposited as sediments each year. Proceeding with the same 

 assumptions, the 33-yr total theoretical accumulation of plutonium in the sediments is 

 9.2 X 10* nCi. The mean amiual accumulation of plutonium by pond biota is 2.4 x 10^ 

 nCi (Tables 3 and 4), which suggests a historic total deposition of 7.9 X 10^ nCi if we 

 assume that each year biotic accumulation of plutonium is the same. This supply 

 increases the theoretical accumulation of plutonium in the pond's sediments to 1.0 X 10^ 

 nCi. This quantity is also a theoretical expression of the historic supply of the plutonium 

 to the pond. 



The Export 



Percolation. No experiments were undertaken to measure the percolative loss of 

 plutonium from U-Pond, and defmitive conclusions about the movement of plutonium 

 from the sediments into the ground below the pond cannot be made. However, there are 

 indications that nearly all the plutonium that has reached the pond has been retained by 

 its sediments. In the above discussions, we concluded that the pond has deposited 

 about 10 cm of sediments since its formation in 1944. This was based on present-day 

 measurements of sedimentation processes occurring in the pond. It was also theorized 

 that about 1 Ci of plutonium has reached these sediments during the Ufetime of the pond. 

 Intensive samphng of the pond's sediments to a depth of 10 cm has shown that about 

 1.7 Ci of plutonium presently resides there. This agreement between theoretical and 

 observed accumulation of plutonium in U-Pond sediments suggests that the pond has 

 received about 1 Ci of plutonium and that most of it has been retained by its sediments. 

 The downward migration of plutonium in Hanford soils has been studied by several 

 workers to assess the seepage of reprocessing wastes from crib sites (Crawley, 1969; 

 Ames, 1974; Price and Ames, 1976). Their findings indicate a vertical reduction of 

 plutonium concentrations over the upper 10 m of the vadose zone (i.e., soil lying above 

 the water table). Price and An-^s (1976) found that particulate plutonium (>2 /im) was 

 deposited within the upper 1 m of the vadose zone. The nonparticulate plutonium (<2 

 )um), which was less than 0.5% (by weight) of the plutonium entering the vadose zone, 

 showed deeper penetration and was eventually deposited in association with the silicate 

 hydrolysis of sediment particles. Brown (1967), studying the vertical migration of other 

 long-hved radionuclides below disposal facilities, found that more than 99.9% (by 

 activity) of these materials was deposited within the upper 10 m of the vadose zone. In 

 addition to this, Myers, Fix, and Raymond (1977) indicate that plutonium concen- 

 trations in the groundwater below Hanford (at =^50 m) are not significantly different 

 from those of other areas. 



