2.23 SOIL EXTERNAL RADIATION
Soils contaminated with radioactive material provide
a potential for external exposure to nearby individuals. Estimates of soil
contamination may be available from atmospheric deposition calculations
or measurements of soil contamination levels may be known.
The considerations given to estimation of the external
radiation dose from exposure to radionuclides in soil from atmospheric
transport and deposition are as follows.
- Transport Medium:
- soil concentration at the location of the exposed individual, Casi, pCi/m2, expressed as an annual deposition accumulation value
- Special Process:
- accumulation of pollutants in soil over the exposure duration for the soil ingestion pathway
- Exposure Factors:
- exposure time to soil, and exposure duration.
As for the other pathways involving deposition of
atmospherically transported pollutants, the soil concentration is provided
from the transport analysis as an average annual deposition amount averaged
over a 70-year exposure period. The accumulation of pollutants in soil
over a multiple-year exposure duration is estimated using the deposition
and accumulation Equations (19) and (20). The total lifetime dose for radionuclides
is evaluated as follows for the atmospheric transport and deposition pathway:
(108)
where
Ddei = total lifetime dose for radionuclide pollutant i from external exposure to soil (rem)
DPsi = annual average deposition rate to soil for pollutant i (pCi/m2/d)
SAFi = soil deposition and accumulation factor for the soil external exposure pathway over the exposure period (d)
Ude = exposure time to contaminated ground (h/d)
DEgi = dose conversion factor for external exposure to radionuclides on a ground plane for radionuclide i (rem/h per pCi/m2)
SHh = shield factor for exposure to soil while inside a home (dimensionless)
FTh = fraction of time spent inside a home (dimensionless)
SHo = shield factor for exposure to soil while outside (dimensionless)
FTo = fraction of time spent outside (dimensionless)
Fde = fraction of days per year that soil ingestion occurs (dimensionless)
EDde = exposure duration for the external exposure to soil pathway (yr)
and 365.25 is as previously defined.
The considerations given to estimation of the external
radiation dose from exposure to radionuclides in soil when measurements
of soil radionuclide contamination levels are available are as follows.
- Transport Medium:
- measured soil concentration at the location of the exposed individual, Cmsi, pCi/kg
- Special Process:
- decay of pollutants over time following the measurement of contamination levels
- Exposure Factors:
- exposure time to soil and exposure duration.
The total lifetime dose from external exposure to
radionuclides in soil for the measured soil pathway is evaluated as follows:
(109)
where
Ddei = total lifetime dose for radionuclide pollutant i from external exposure to measured soil levels (rem)
10-3 = units conversion factor (kg/g)
106 = units conversion factor (cm3/m3)
SMFi = average measured soil concentration for radionuclide i (pCi/kg)
rms = density of soil at the location of soil measurement (g/cm3)
tms = thickness through which contamination is mixed in soil at the location of soil measurement (m)
and other terms are as previously defined. The exponential term with division
by ldi represents the time integral
of activity over the exposure duration, accounting for radioactive decay
during this period.