RBCA Soil Equations

Section 742 Section 742.715  RBCA Soil Equations a)         This Section presents the RBCA model and describes the equations and parameters used to develop Tier 2 soil remediation objectives. b)         Ingestion, Outdoor Inhalation, and Dermal Contact 1)         The two sets of equations in subsections (b)(2) and (b)(3) shall be used to generate Tier 2 soil remediation objectives for the combined ingestion, outdoor inhalation, and dermal contact with soil exposure routes. 2)         Combined Exposure Routes of Soil Ingestion, Outdoor Inhalation of Vapors and Particulates, and Dermal Contact with Soil A)        Equations R1 and R2 form the basis for deriving Tier 2 remediation objectives for the set of equations that evaluates the combined exposure routes of soil ingestion, outdoor inhalation of vapors and particulates, and dermal contact with soil using the RBCA approach.  Equation R1 is used to calculate soil remediation objectives for carcinogenic contaminants.  Equation R2 is used to calculate soil remediation objectives for noncarcinogenic contaminants.  Soil remediation objectives for the outdoor inhalation exposure route from subsurface soils must also be calculated in accordance with the procedures outlined in subsection (b)(3) of this Section and compared to the values generated from Equations R1 or R2.  The smaller value (i.e., R1 and R2 compared to R7 and R8, respectively) from these calculations is the Tier 2 soil remediation objective for the combined exposure routes of soil ingestion, outdoor inhalation, and dermal contact with soil. B)        In Equation R1, numerical values are calculated for two parameters: i)          The volatilization factor for surficial soils (VF ss ) using Equations R3 and R4; and ii)         The volatilization factor for surficial soils regarding particulates (VF p ) using Equation R5. C)        VF ss uses Equations R3 and R4 to derive a numerical value. Equation R3 requires the use of Equation R6.  Both equations must be used to calculate the VF ss . The lowest calculated value from these equations must be substituted into Equation R1. D)        The remaining parameters in Equation R1 have either default values listed in Appendix C, Table D or toxicological-specific information (i.e., SF o , SF i ), which can be obtained by following the guidelines in OSWER Directive 9285.7-53, as incorporated by reference in Section 742.210 or requested from the program under which the remediation is being performed. E)        For Equation R2, the parameters VF ss and VF p are calculated. The remaining parameters in Equation R2 have either default values listed in Appendix C, Table D or toxicological-specific information (i.e., RfD o , RfD i ), which can be obtained by following the guidelines in OSWER Directive 9285.7-53, as incorporated by reference in Section 742.210 or requested from the program under which the remediation is being performed. F)         For chemicals other than inorganics which do not have default values for the dermal absorption factor (RAF d ) in Appendix C, Table D a dermal absorption factor of 0.5 shall be used for Equations R1 and R2.  For inorganics, dermal absorption may be disregarded (i.e., RAF d = 0). 3)         Outdoor Inhalation Exposure Route from Subsurface Soils (soil below one meter) A)        Equations R7 and R8 form the basis for deriving Tier 2 remediation objectives for the outdoor inhalation exposure route from subsurface soils using the RBCA approach.  Equation R7 is used to calculate soil remediation objectives for carcinogenic contaminants.  Equation R8 is used to calculate soil remediation objectives for noncarcinogenic contaminants. B)        For Equation R7, the carcinogenic risk-based screening level for air (RBSL air ) and the volatilization factor for soils below one meter to ambient air (VF samb ) have numerical values that are calculated using Equations R9 and R11, respectively.  Both equations rely on input parameters from a variety of sources. C)        The noncarcinogenic risk-based screening level for air (RBSL air ) and the volatilization factor for soils below one meter to ambient air (VF samb ) in Equation R8 have numerical values that can be calculated using Equations R10 and R11, respectively. c)         Soil Component of the Groundwater Ingestion Exposure Route 1)         Equation R12 forms the basis for deriving Tier 2 remediation objectives for the soil component of the groundwater ingestion exposure route using the RBCA approach.  The parameters, groundwater at the source (GW source ) and Leaching Factor (LF sw ), have numerical values that are calculated using Equations R13 and R14, respectively. 2)         Equation R13 requires numerical values that are calculated using Equation R15. 3)         Equation R14 requires numerical values that are calculated using Equations R21, R22, and R24.  For non-ionizing organics, the Soil Water Sorption Coefficient (k s ) shall be calculated using Equation R20.  For ionizing organics and inorganics, the values for k s are listed in Appendix C, Tables I and J, respectively. The pH-dependent k s values for ionizing organics can be calculated using Equation R20 and the pH dependent K oc values in Appendix C, Table I.  The remaining parameters in Equation R14 are field measurements or default values listed in Appendix C, Table D. d)         The default value for GW comp is the Tier 1 groundwater remediation objective.  For chemicals for which there is no Tier 1 groundwater remediation objective, the value for GW comp shall be the concentration determined according to the procedures specified in 35 Ill. Adm. Code 620, Subpart F.  As an alternative to using the above concentrations, GW comp may be developed using Equations R25 and R26, if approved institutional controls are in place as may be required in Subpart J.