mis 500 g for lung,
Y
eq
= 0.002 Cy,
8
y is the lung burden after 50 years = 134.34 pCi per pCi/day
inhaled (see Tables 12 and A-1).
0.002 ¢ dis the plutonium inhalation rate (pCi/day), l.e.,
20 mtair) /day x 1074 g(soil)/m3(air), and Ce is the
average soil concentration (pCi/g).
0.2 km* (about 50 acres) of the 11.4 km? (about 2,812 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 less than
0.4 km? (100 acres).
In other words, the plutonium contamination at NTS is so
concentrated in areas near ground zero sites that decontamination‘of from 2 to
4 percent of the total soil inventory area would reduce average soil concentrations by 40 to 50 percent. However, it should be noted that effective decontamination of these areas may be extremely difficult (Wallace and Romney,
1975; Rhoads, 1976).
DISCUSSION
COMPARISON OF ASC AND SOIL INVENTORY DATA
Based on the results derived above (ASC = 2,800 pCi/g) and the mass loading
factor of 100 ug(sotl)/mi(air), the expected air concentration would be 2.8 x
Gilbert et al. (1975, p. 379) have summarized the estimated inventortes of
In each contaminated
239+240py in the surface soils (0-5 cm depth) at NTS.
study area, soil sampling was stratified according to contour intervals (strata)
previously established by field instrument (FIDLER) surveys. The pertinent
results for Area 13 are given in Table 12.
Table 12.
‘
Estimated Inventory of 235+240py in Surface
Soil (0-5 cm Depth) in Area 13
107!3 yeoifem?,
The maximum permissible concentration in air (MPC ) indicated
by ICRP Publication 2 (107!? yci/cm?) is higher than this by a factor of 3.6.
Based on MPC = 1071? woi/cm? and a mass loading factor of 100 pg/m3, the
acceptable soil concentration would 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 ug
(soil)/m3 (air).
Another conservative factor in our estimate of ASC is that the lung deposition
factor (D, = 0.31) is based on the assumption that the mean size of resuspended
soil particles of 0.5 um (AMAD). The value obtained from cascade impactor
studies in the GMX area was 3 pm (AMAD) which would indicate D, < 0.2.
Changing
only this parameter would increase the estimate of ASC, by a factor of 1.55,
to 4,266 pCi/g.
,
Area (m2)?
Strata’?
874,500
1
2,552,400
2
3
4
5
6
Z
Total
{a)
(b)
(a)
Mean Pu (uCi/m?)
(pCi/g)
34
1.9
103
5.8
411
952
1,847
14,245
23.0
81,100
54.0
73,700
110.0
20,900
23,400|820.0
3,626,000
Soil Conggntration
Mean
213
From Gilbert et al. (1975, p. 379).
From Table 1, this report.
Complete decontamination of stratum 6
Only stratum 6 exceeds ASC = 2,800 pCi/g.
of the total plutonium
(0.65 percent of Area 13) would remove about 43 percent
213 to 121 pCi/g.
in Area 13 and réduce the average soil concentration From
in which the average soil
If it were decided to decontaminate all areas at NTS
for about
concentration exceeds 2 nCi/g, decontamination would be required
The least conservative factor involved in arriving at ASC = 2.8 nCi/g is the
assumed masa loading factor of 100 ug(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 high winds or mechanical
disturbances such as vehicular traffic, plowing, excavation, etc., might
increase the mass loading factor to several mg/m?, If we assume, for example,
that the hypothetical Standard Man at NTS were exposed, for one reason or
another, to mass loading factors of 5,000 g/m? during 30 days each year, the
average mass loading factor would increase to about 500 ug/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” ig not Fixed, but very much dependent on how man plans to utilize 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 which would tend to increase the average
mass loading factor, a lower ASC would be indicated.