(a) The person shall base the RBELs developed in accordance with §350.74 of this title (relating to Development of Risk-Based Exposure Limits) and the PCLs developed in accordance with §350.75 of this title (relating to Tiered Human Health Protective Concentration Level Evaluation) on the following carcinogenic risk level and hazard quotient.
  (1) Carcinogenic COCs. The RBEL and PCL for each carcinogenic COC, including those PCLs based on combined exposure pathways, shall be based on a carcinogenic risk level of 1 x 10-5 (1 in 100,000) except when other standards shall be used as RBELs as discussed in §350.74 of this title (relating to Development of Risk-Based Exposure Limits).
  (2) Noncarcinogenic COCs. The RBEL and PCL for each noncarcinogenic COC, including those PCLs based on combined exposure pathways, shall be based on a hazard quotient of 1 except when other standards shall be used as RBELs as discussed in §350.74 of this title (relating to Development of Risk-Based Exposure Limits).
(b) The person shall evaluate whether the PCLs for a human health exposure pathway need to be adjusted to lower concentrations to meet the cumulative carcinogenic risk level and hazard index criteria in subsection (c) of this section when there are more than 10 carcinogenic COCs and/or more than 10 noncarcinogenic COCs within a source medium. The person shall conduct this evaluation separately for each individual and combined human health exposure pathway for which PCLs must be developed in accordance with §350.71(c) of this title (relating to General Requirements). This cumulative evaluation shall include all COCs across all tiers for which the person is required to establish PCLs in accordance with §350.71(k) of this title. In cases where 10 or more carcinogenic COCs and 10 or more noncarcinogenic COCs are present in the source medium, the cumulative evaluation shall be conducted for both carcinogenic COCs and noncarcinogenic COCs by separately addressing the cumulative effects of multiple carcinogenic COCs and multiple noncarcinogenic COCs. The COCs which exhibit both carcinogenic and noncarcinogenic characteristics shall be counted as both a carcinogenic COC and a noncarcinogenic COC and evaluated as required by this subsection. This evaluation shall be modified as specified in paragraphs (1)-(5) of this subsection.
  (1) For the groundwater ingestion exposure pathway, the person shall not include COCs with a primary maximum contaminant level (MCL) as provided in 40 Code of Federal Regulations Part 141, as amended, or the most currently available federal action level for drinking water (e.g., lead and copper) in the cumulative carcinogenic risk level or hazard index evaluation when that MCL or action level is the groundwater ingestion PCL. The person is also not required to include COCs with a secondary MCL as provided in 40 Code of Federal Regulations Part 143, as amended, in the cumulative carcinogenic risk level or hazard index evaluation when the secondary MCL is used as the groundwater PCL and is based on a RBEL established in accordance with §350.74(f)(3) of this title (relating to Development of Risk- Based Exposure Limits) for that COC.
  (2) The person is not required to comply with subsection (c) of this section for the class 3 groundwater PCL GW GWClass 3 .
  (3) The person is not required to conduct an additional cumulative check in accordance with subsection (c) of this section for the soil-to-groundwater PCL GW Soil. The cumulative check is already addressed when establishing GW Soil to meet the groundwater PCLs which have been adjusted to comply with the criteria specified in subsection (c) of this section.
  (4) The person is not required to comply with subsection (c) of this section for the groundwater-to-surface water PCL SW GW.
  (5) The person shall not include the PCL established in §350.76 of this title (relating to Approaches for Specific Chemicals of Concern to Determine Human Health Protective Concentration Levels) for lead, dioxins, or polychlorinated biphenyls (only exclude polychlorinated biphenyls when the soil PCL is based on requirements of the Toxic Substances Control Act as specified in §350.76(d)(4) of this title) in soil in the cumulative carcinogenic and hazard index evaluation.
(c) The person shall use the following criteria for the cumulative carcinogenic risk level and hazard index when determining if the evaluation in subsection (b) of this section requires PCLs for individual COCs to be adjusted to a lower concentration.
  (1) Carcinogenic COCs. The cumulative carcinogenic risk level for multiple carcinogenic COCs shall not exceed 1 x 10 -4 .
  (2) Noncarcinogenic COCs. The hazard index for multiple noncarcinogenic COCs shall not exceed 10.
(d) The person shall use the equation in the following figure to adjust PCLs to a lower concentration as required in subsection (b) of this section to achieve the cumulative carcinogenic risk level or hazard index established in subsection (c) of this section. The person shall adjust the PCL for one or more COCs to a lower concentration (carcinogens and noncarcinogens are treated separately) such that the conditions of the equation are met. The person shall choose which PCLs are adjusted downward and the magnitude of the reduction. The PCLi shall remain constant in the denominator. The PCL- adji , which is the final human health PCL for a particular COC and exposure pathway, shall be less than or equal to PCLi .
Attached Graphic

Source Note: The provisions of this §350.72 adopted to be effective September 23, 1999, 24 TexReg 7436