Risk-Based Decision Making for the Remediation of Petroleum-Contaminated Sites

Revision History

Update to risk-based concentration calculation spreadsheet and RBC tables (November 2011)

This update is relatively routine to be consistent with currently accepted toxicity values. Since the most recent (2009) RBC revision, EPA updated its regional screening levels (RSLs, created by EPA Regions 3, 6, and 9). The current regional screening levels are available at:

• www.epa.gov/reg3hwmd/risk/human/rb-concentration_table/

The following are the more substantive changes to DEQ’s RBC spreadsheet.

Consideration of Breastfeeding Exposure Pathway
To be consistent with 2010 revisions to DEQ’s human health risk assessment guidance, infant exposure through breastfeeding has been added for all long-term exposure pathways used to develop RBCs. Working with EPA Region 10, DEQ found that breastfeeding is an important pathway for some bioaccumulating chemicals, particularly polychlorinated biphenyls. The PCB non-cancer risk to an infant is up to 25 times the risk calculated for the mother, resulting in correspondingly lower RBCs. This makes PCB RBCs nearly equivalent for non-cancer and cancer endpoints. As a consequence, including this exposure pathway on all sites will not substantially affect cleanup decisions. However, new PCB hotspot levels will be considerably lower than past levels.

An evaluation of breastfeeding risks is complicated by the known substantial health benefits of breastfeeding. Including the breastfeeding exposure pathway in developing screening values is important to ensure that remedial decisions consider the potential for infant exposure. Comparisons with screening values are not intended to advise women about whether or not to breastfeed their infants.

Consideration of TCE Toxicity
The EPA's Integrated Risk Information System completed a final evaluation of trichloroethylene toxicity in September 2011. The new toxicity values replace 2001 draft values previously used by DEQ in developing risk-based concentrations.

One issue that complicates derivation of RBCs for TCE concerns the incorporation of early-life exposure. There are three cancer endpoints considered in the development of the slope factor and inhalation unit risk factor for TCE: kidney cancer, liver cancer, and non-Hodgkins lymphoma. EPA determined that TCE was carcinogenic by a mutagenic mode of action for kidney cancer (renal cell carcinoma). Following EPA’s recommendations, DEQ is using the method presented in the following memorandum to derive RBCs for TCE. Because of the necessity to calculate TCE RBCs for three different toxicological endpoints and recombine them into a single numeric value, they're not computed in the spreadsheet. Rather, they were calculated manually, externally to the spreadsheet, and values were written into the final spreadsheet. Therefore, the cancer-based values cannot be updated automatically, or compared to CTE values. However, noncancer values can be computed, if desired, by removing the “NA” in the toxicity values column and replacing with an “nc.” To restore the cancer-based values, it is necessary to close the spreadsheet without saving, and re-open.

• Trichloroethylene Toxicity Values PDF

Definition of Volatile Chemicals
A volatile chemical was previously defined as having a Henry’s constant > 10-5 m3-atm/mol and a molecular weight < 200 g/mol. EPA no longer uses the molecular weight criterion -- only the Henry’s constant. To be consistent with EPA, DEQ removed the molecular weight criterion in the current version of its RBC spreadsheet. This results in calculated vapor transport RBCs for a number of heavier compounds with sufficiently high Henry’s constants.

Cancer and Non-cancer Endpoints
The 2011 revision (like the 2009 release) includes a “PDF” format table that shows the lowest default RBCs without regard to endpoint (cancer or non-cancer). When the spreadsheet is used to calculate RBCs for chemicals with both cancer and non-cancer toxicity values, RBCs corresponding to both endpoints should be calculated to verify that the lowest applicable RBC is being used. The default setting for the spreadsheet is whichever endpoint results in the lowest soil direct contact RBCs in the residential scenario.

Chemicals which have both cancer and non-cancer toxicity values, and may have lower non-cancer values in some scenarios include beryllium, cadmium, nickel, formaldehyde, hexachloroethane, PCBs, trichloroethene 2,4,6-trichlorophenol, and 1,1,2-trichloroethane.

Chromium VI, or hexavalent chromium, now has an oral cancer slope factor. It is also now considered mutagenic so that early life exposure must be evaluated for residential exposures. These changes result in RBCs that are significantly lower than in the previous tables.

Total Petroleum Hydrocarbon RBCs
The updated 2011 DEQ TPH spreadsheet incorporates the first substantive modifications since 2003. The 2011 TPH spreadsheet includes the following changes:

• Computational methodology was changed to be consistent with EPA’s Risk Assessment Guidance for Superfund; Volume I: Human Health Evaluation Manual (Part F, Supplemental Guidance for Inhalation Risk Assessment) EPA 540-R-070-002, OSWER 9285.7-82, January 2009.
• Soil vapor TPH RBCs are now available to assess the vapor intrusion pathway.
• Toxicity Value updates. Toxicity values previously used since 2007 have been withdrawn by EPA and are no longer available. Therefore, toxicity values for petroleum fractions were updated to be consistent with those provided in EPA’s PPRTVs for TPH Mixtures and Xylenes. Attachment 1: Provisional Peer-Reviewed Toxicity Values for Complex Mixtures of Aliphatic and Aromatic Hydrocarbons (CASRN various). Attachment 2: Provisional Peer-Reviewed Toxicity Values for Xylenes (CASRN 1330-20-7). Nov. 10, 2009.

Implementation Schedule (2011)
The revised RBCs should be implemented according the following guidelines:

1. For all newly initiated risk-based decision-making screening evaluations at a site, the recently revised RBCs and EPA regional screening values will be used from this point forward;
2. For RBDM screening evaluations currently being performed, the revised RBCs and EPA regional screening values will be used on a case-by-case basis; and
3. For completed RBDM screening evaluations submitted before November 2011, reassessments may be performed on a case-by-case basis when a new decision is required that needs to be supported by a screening evaluation or updated risk assessment. Until that time, the current completed and approved screening evaluation will continue to be considered valid based on the guidance used when the assessment was completed.

EPA updates its screening table semi-annually. DEQ will evaluate updates to EPA screening values when they occur and will consider the significance of the EPA updates as they relate to DEQ’s RBCs. DEQ therefore will update RBCs periodically when warranted by substantive updates to EPA’s screening tables. Under special circumstances, DEQ may at any time require a revised screening value for a chemical based on new toxicity information.

• Click here for RBDM revisions history archives

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