The risk model used for the external radiation exposure pathway in this guidance assumes that an individual is exposed to a source geometry that is effectively an infinite slab. The concept of an "infinite slab" means that the thickness of the contaminated zone and its aerial extent are so large that it behaves as if it were infinite in its physical dimensions. In practice, soil contaminated to a depth greater than about 15 cm and with an aerial extent greater than about 1,000 m² (i.e., one-quarter acre) will create a radiation field approaching that of an infinite slab.

This infinite slab assumption has been used in the calculation of radionuclide slope factors presented in Section 2.1. For very small areas of contamination, this will result in overly conservative estimates of risk. For calculation of SSLs for a residential setting, an adjustment for source area is considered to be an important modification for Superfund sites. Thus, an area correction factor, ACF, has been added to the SSL calculation.

Table 5.1 provides recommended ACFs for radionuclides as a function of source area calculated using MicroShield V5.01.1 Since the default source size is 0.5-acre (i.e., 2,000 m²), the default ACF for SSL equations is set at 0.9. The calculations assume a uniform layer of contamination 15 cm deep with a soil density of 1.6 g/cm³. A single recommended value is considered suitable for all radionuclides over the range of source areas, since EPA's analysis shows that ACFs vary little from one radionuclide to another. For other source areas, recommended ACFs are presented in Table 5.1.

Table 5.1 Recommended Area Correction Factors as Function of Source Area

Source Area (m2)	ACF
10,000	1.00
5,000	0.94
2,000	0.90
1,000	0.88
500	0.86
100	0.75
50	0.66
10	0.4

EPA's analysis of ACFs is found in Table 5.2, which provides examples of ACFs for several radionuclides as a function of source area calculated using MicroShield V5.01. The calculations assume a uniform layer of contamination 15 cm deep with a soil density of 1.6 g/cm³. Strong gamma-ray emitters, like ⁶⁰Co, have relatively large slope factors for this pathway compared to the slope factors for weak photon emitters, like ²³⁹Pu. As noted in the table, however, ACFs vary little from one radionuclide to another over the range of source areas shown. Users that have one of the radionuclides in Table 5.2 as a contaminant at their site may use the radionuclide-specific ACF that is appropriate for their source area rather than the value found in Table 5.1.

Table 5.2 Area Correction Factors as Function of Source Area for Selected Radionuclides Calculated Using MicroShield