508 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



of Area 13) would remove about 48% of the total plutonium in Area 13 and reduce the 

 average soil concentration from 201 to 106 pCi/g. If it were decided to decontaminate all 

 areas at NTS in wliich the average soil concentration exceeds 2 nCi/g, decontamination 

 would be required for about 0.2 km^ (about 50 acres) of the 11.5 km^ (about 2850 

 acres) included in the soil inventory study (Table 1). If the decontamination criteria were 

 further reduced to 1 nCi/g, the area requiring decontamination would be about 0.5 km^ 

 (111 acres). In other words, the plutonium contamination at NTS is so concentrated in 

 areas near ground zero sites tliat decontamination of from 2 to 4% of the total soil 

 inventory area would reduce average soil concentrations by 40 to 50%. 



It should be noted that possible decontamination activities of these sites are 

 complicated by potential damage to the desert ecosystem. Further information on this 

 subject can be found in reports by Wallace and Romney (1975) and Rlioads (1976). 



Discussion 



On the basis of the preceding results (ASC = 2800 pCi/g) and the mass-loading factor of 

 100 jug soil/m^ air, the expected air concentration would be 2.8 x lO"^^'' juCi/cm^ . The 

 maximum permissible concentration in air (MPC)a indicated by ICRP Publication 2 

 (10~^^ juCi/cm^) is higlier than this by a factor of about 3.6. Using (MPC)a = 10"'^ 

 jLtCi/cm^ and a mass-loading factor of 100 jug/m^, we would find the acceptable soil 

 concentration to be 10 nCi/g instead of 2.8 nCi/g, which would be equivalent to assuming 

 a mass-loading factor of 355 instead of 100 jug soil/m^ air. 



Another conservative factor in our estimate of ASC is that the lung deposition factor 

 (D5 = 0.31) is based on the assumption that the mean size of resuspended soil particles is 

 0.5 /am (AMAD). The value obtained from cascade impactor studies in the GMX area was 

 3 /jm (AMAD), wliich would indicate D5 < 0.2. Changing only this parameter would 

 increase the estimate of ASC by a factor of 1 .55 to 4266 pCi/g. 



The least conservative factor involved in arriving at ASC = 2.8 nCi/g is the assumed 

 mass-loading factor of 100 iig soil/m^ air. As demonstrated by Shinn and Anspaugh 

 (1975) and Anspaugh et al. (1975), this estimate appears to be adequate for undisturbed 

 areas and normal winds, but liigh winds or mechanical disturbances, such as vehicular 

 traffic, plowing, excavation, etc., miglit increase the mass-loading factor to several 

 milligrams per cubic meter. If we assume, for example, that the hypothetical Standard 

 Man at NTS were exposed, for one reason or another, to mass-loading factors of 5000 

 jUg/m^ during 30 days each year, the average mass-loading factor would increase to about 

 500 A'g/m'^ , and our estimate of ASC would decrease to about 560 pCi/g. 



The point of this discussion is that the notion of an "acceptable soil concentration" is 

 not fixed but is very much dependent on how man plans to use a contaminated area. 

 Under present conditions the ASC for contaminated areas at NTS is 2.8 nCi/g. If these 

 same areas were to be used for agricultural purposes or for any other purpose that would 

 tend to increase the average mass-loading factor, a lower ASC would be indicated. In 

 other words, the notion of an "acceptable soil concentration" is an attractive one. It 

 implies the existence of a numerical criterion that can be used to make important 

 determinations concerning the need for or effectiveness of countermeasures to ensure 

 safety. How to determine an ASC value is a different matter, and how to determine 

 whether a particular ASC value is entirely appropriate under a given set of physical, 

 social, and political circumstances is a far different matter. 



