(a)
For regulated substances listed in Appendix A, Table 6 that are found in
groundwater, the minimum threshold MSC of 5 micrograms per liter in
groundwater, shall be used.
(b) For
regulated substances listed in Appendix A, Table 6 that are found in soil, the
lowest of one of the following values shall be used as the minimum threshold
MSC:
(1) An ingestion numeric value of 100
milligrams per kilogram in soil.
(2) The soil-to-groundwater pathway numeric
value throughout the soil column as determined by the methodology in §
250.308 (relating to
soil-to-groundwater pathway numeric values), but substituting 5 micrograms per
liter in groundwater for the groundwater MSC. The soil-to-groundwater pathway
numeric value shall be calculated by using a concentration in soil at the site
which does not produce a leachate in excess of the MSC for groundwater or by
using a value which is 100 times the MSC for groundwater, expressed in
milligrams per kilogram of soil. An equivalency demonstration under §
250.308(d) may be
substituted for the soil-to-groundwater numeric value.
(c) The minimum threshold MSC in subsection
(a) and the ingestion numeric value in subsection (b)(1) are calculated
according to the following exposure assumption and equation: 0.50 ppb dietary
intake corresponds to a 1x10
-6 risk (USFDA Threshold
of Regulation Final Rule July 17, 1995) assuming the substance is a carcinogen.
Correcting this value (or 5.0 ppb) to the 10
-5 risk
level, in Statewide health standard formulation, the threshold of regulation
concentrations are determined by the following exposure assumptions and
calculations:
Five µg/kg (substance of concern) threshold level
corresponding to 1x10-5 risk
Dietary intake 2 kg/day x 5 µg/kg (substance - 10
µg/day (daily intake of substance of concern)
For soil ingestion: 10 µg/100 mg soil or 100
mg/kg = Threshold concentration for soils
For groundwater ingestion: 10 µg/2L water = 5
µg/L - Threshold concentration for water
The 100 mg soil and 2L water factors are the default
ingestion rates from §
250.306(c)
(relating to ingestion numeric values).