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Risk Based Decision Making for the Remediation of Contaminated Sites

Current guidance documents

Current Risk-Based Concentrations

Generic Risk-Based Concentrations 

Site-Specific Risk-Based Concentrations

Background

Oregon environmental cleanup law and regulations require a risk-based approach for assessing and managing environmental contamination. The Risk Based Decision Making Guidance and associated tables of risk-based concentrations provide a consistent, streamlined decision making process for evaluating risks to human health and the environment. Other models or approaches may be used to evaluate site risks provided they meet risk-based cleanup standards specified in Oregon Revised Statute 465.315.

In March 2024, DEQ published new information on vapor intrusion acure and chronic risk based concentrations. The most recent revisions are summarized below under Update history. Visit the Vapor Intrusion web page for more information

DEQ periodically updates RBCs when EPA makes substantive updates to screening tables. EPA updates its screening table semi-annually. DEQ evaluates those updates to determine how they relate to DEQ’s RBCs. Under special circumstances, DEQ may at any time require a revised screening value for a chemical based on new toxicity information. If a chemical does not have a DEQ RBC, then the EPA regional screening level should be used.

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Update history

To ensure Risk-Based Concentration screening levels are based on the most current science, DEQ updated the RBCs for vapor intrusion from soil vapor and groundwater by incorporating information from EPA's November 2023 regional screening levels tables. 

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A prior version of the RBDM spreadsheet had incorrect RBCs for cyanide because cyanide was incorrectly indicated as non-volatile. The June 2023 amended spreadsheet had correct calculations, but the cyanide RBCs were not updated in the PDF version of the RBC table. The PDF table has been corrected.​​

To bring RBCs up to date with current science, DEQ removed RBCs for vapor intrusion from soil, and calculated RBCs for vapor intrusion from soil vapor and groundwater in a separate spreadsheet developed using EPA’s vapor intrusion screening level (VISL) calculator.​

​In addition, the May 2018 RBC Excel spreadsheet and PDF tables were amended to reflect the changes to vapor intrusion RBCs.​

Please email Vapor Intrusion Work Group to request June 2023 RBC updates. 

DEQ updated RBC tables to reflect updates to toxicity factors for b​enzo[a]pyrene (and by extension the other carcinogenic polycyclic aromatic hydrocarbons), dichlorodiphenyldichloroethane (DDD), 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene. These tables supersede DEQ's previously released RBC tables. Parties should use the updated tables for new screenings and risk assessments, unless DEQ and parties agree to use of alternative approaches.​​

Updated Toxicity Factors

  • Updated toxicity factors for the carcinogenic polycyclic aromatic hydrocarbon (cPAH) for benzo[a]pyrene (B[a]P), consistent with the U.S. Environmental Protection Agency’s 2017 update. This update generally results in risk-based screening levels that are higher by about a factor of 7. In addition, no cancer effects (including developmental effects) can now be quantitatively evaluated for benzo[a]pyrene.
  • Updated toxicity factors for dichlorodiphenyldichloroethane (DDD), 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene. New noncancer RBCs for DDD are generally lower than previous RBCs. RBCs for 1,2,4-trimethylbenzene are now generally about 2 to 4 times higher than previous RBCs, and RBCs for 1,3,5-trimethylbenzene are now generally about half the previous RBCs.

Fixed Formatting Errors

  • Updated the generic table of Risk-Based Concentrations for Individual Chemicals:
    • Updated to show the lowest cancer or noncancer RBCs are provided. In preparing the 2015 table, the lowest of the cancer or noncancer RBCs were not always selected. The May 2018 table accurately shows the lowest cancer or noncancer RBCs.
  • Updated the site-specific table of Risk-Based Concentrations for Individual Chemicals:
    • Unlocked spreadsheet cells and other formatting errors that prevented site-specific calculations for new chemicals. The update allows parties to modify assumption parameters. 
    • Unlocked spreadsheet cells for early-life exposure assumptions. The update allows parties to modify related assumption parameters. Changes to standard exposure assumptions will not be automatically incorporated into early-life exposure assumptions. Note that RBC calculations for TCE are contained in the March 2018 Calculation of Trichloroethene RBCs using Early-Life Exposure Memorandum.
    • Updated to show the correct soil leaching to groundwater (RBCsw). In preparing the 2015 table, an error occurred that result in these values being listed incorrectly. The May 2018 spreadsheet contain the correct values.


Implementation schedule

Parties should use the updated tables for new screenings and risk assessments, unless DEQ and parties agree to use of alternative approaches. In general, the updated RBCs should be implemented as follows:

  • New screenings and risk assessments should use the updated RBC tables, unless DEQ and parties agree to use of alternative approaches.
  • For screenings and risk assessments currently underway, the updated RBCs and EPA regional screening values apply on a case-by-case basis.
  • For completed and approved evaluations, reassessment may be performed on a case-by-case basis. Typically approved screenings and risk assessments do not require reassessment unless a new decision is requested or required, and that decision needs the support of a current screening evaluation or risk assessment.
  • When updated RBCs are used, they should be used for all chemicals of interest for a facility.

​

DEQ renamed the ​guidance to “Risk-Based Decision Making for the Remediation of Contaminated Sites" to clarify it wasn't limited to petroleum contaminated sites.​

The following are substantive changes to DEQ’s tables of RBCs . DEQ also updated toxicity factors for some chemicals and made other minor changes.
 

Updated exposure parameter values

In September 2011, EPA issued the current version of its Exposure Factors Handbook (EPA/600/R-090/052F). EPA’s regions adopted many of the updated values in their November 2014 regional screening level table. DEQ is incorporating the exposure value changes in this table, which will make Oregon consistent with exposure assumptions in national risk assessments. To be consistent with EPA’s approach, DEQ now includes two separate event times for duration of skin exposure to water for adults and children, where previously DEQ used only one event time. In addition, DEQ updated skin exposure parameters for consistency with national risk assessment.
 

Dermal exposure to tapwater

In developing the tapwater scenario for DEQ’s original, 1999 risked-based decision making guidance, it was reasonable to assume exposure could occur through ingestion, inhalation and skin contact. However, there was not an established approach for including skin contact. DEQ decided to be consistent with EPA and not include skin contact with tapwater exposure. In 2004, EPA developed Part E to Risk Assessment Guidance for Superfund (Supplemental Guidance for Dermal Risk Assessment), and included exposure factors necessary for evaluating the water-skin pathway. In November 2011 updates to EPA regional screening level tables, EPA added the skin pathway to tapwater exposure. DEQ is now incorporating EPA’s approach for evaluating skin exposure to tapwater. For pathways involving skin exposure to water, DEQ includes a check to determine if a chemical is within the effective predictive domain of EPA’s model, and does not calculate skin exposure for chemicals outside the predictive domain. EPA now calculates non-carcinogenic screening levels based on exposure to children, not adults. Because it’s important to protect the most sensitive populations, DEQ also revised its calculations to use child exposure factors for all tapwater exposure routes.
 

Definition of volatile chemicals

Previously, DEQ defined volatile chemicals as those with a Henry’s Law constant greater than or equal to 1.0 x 10-5 atm-m3/mole. This definition inappropriately identified some chemicals, such as formaldehyde, as non-volatile. DEQ is now consistent with EPA by also defining volatile chemicals as those with a vapor pressure greater than or equal to 1 mm mercury.
 

Chemical-specific parameter value data sources

DEQ now uses EPA’s hierarchy of data sources to determine chemical-specific parameter values. This is DEQ’s first major update of parameter values since 2003.
 

Chemical classes

DEQ has clarified the definition of chemical classes to include such chemicals as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), chlorinated dibenzo-p-dioxins and chlorinated dibenzofurans (dioxins/furans), chlordanes and total petroleum hydrocarbons (TPH). These chemical classes should be evaluated as a single hazardous substance for determining risk and potential hot spots. The acceptable cancer risk level for individual hazardous substances of one-in-one-million and non-cancer hazard quotient of one applies to each chemical class, and potential hot spot determinations will be made accordingly.
 
Carcinogenic PAHs should be evaluated as summed benzo[a]pyrene equivalents. Dioxins/furans should be evaluated as the sum of 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents. PCBs should be evaluated as total PCBs, either derived from congeners or aroclors. Similarly, total petroleum hydrocarbons and chlordanes should be evaluated as totals. This approach for evaluating chemical classes replaces Section 3.3.5 of DEQ’s October 2010 Human Health Risk Assessment Guidance.
 

Implementation schedule 

The revised DEQ and EPA risk-based concentrations will be implemented according the following guidelines:
 
For all newly started site screening evaluations, DEQ will use the recently revised risk-based concentrations from this point forward:
  1. For risk assessments currently being performed, the revised risk-based concentrations may be used on a case-by-case basis provided all the revisions are considered; and
  2. Completed risk assessments submitted for approval before Jan. 1, 2016 will be considered valid based on guidance used when the assessment was completed.
 

​This update accommodates changes to toxicity values for tetrachloroethene (PCE). On Feb. 10, 2012, EPA updated its integrated risk information system with new values for PCE.
 
Implementation schedule
 
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 are now in effect;
  2. For RBDM screening evaluations currently being performed, the revised RBCs and EPA regional screening values apply on a case-by-case basis; and
  3. For completed RBDM screening evaluations submitted before April 2012, reassessments may be performed on a case-by-case basis when a new decision is required that needs the support of a screening evaluation or updated risk assessment. Until then, the current completed and approved screening evaluation is considered valid based on the guidance used when the assessment was completed.
 

​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 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.
 
 

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.


Contact

For more information, contact the Environmental Cleanup Program contact:

Mike Poulsen 
Cleanup Toxicologist
503-229-6773

To submit a report to DEQ, use Your DEQ Online.

Applicable regulations

OAR 340-122-0100 through 340-122-0115
Releases of hazardous substances including petroleum under the state’s hazardous substance remedial action rules 

Oregon Administrative Rules 340-122-0205 through 340-122-0260
Cleanup of petroleum releases from regulated underground storage tanks under Oregon cleanup rules for Leaking Petroleum Underground Storage Tank Systems