Changing Cleanup Standards for TCE and 1,1-DCE

Jeffrey M. Pollock

Recent and proposed changes to the toxicity values for 1,1-dichloroethene (1,1-DCE) and trichloroethene (TCE) will result in changes to risk-based cleanup levels for these compounds. Proposed changes to TCE toxicity values reflect a greater concern about the risks passed by TCE, while recent changes in DCE toxicity values reflect lesser concerns about the risks associated with this compound. Because these compounds are often the focus of remedial activities, changes in the toxicity values for the compounds will have ramifications on remedial decision-making by regulatory agencies and private parties, whether under federal or state law. This article presents a discussion of the proposed changes to the toxicity values for these compounds and the potential impacts of the changes on remedial decision-making for hazardous waste sites.

Introduction
Regulations that govern many state and federal hazardous waste site cleanup programs incorporate a requirement for the site to be returned to a condition that does not pose a risk to human health or the environment. The evaluation of human health risk and decisions regarding the potential need for remediation are typically performed through calculation of health risks or through use of risk-based standards. In a Superfund-type Baseline Risk Assessment (BRA) or in an evaluation of ongoing exposures (such as for indoor air), remedial decisions are typically made based on the calculated human health risk associated with hypothetical exposure to measured media concentrations. In many state cleanup programs, remedial decisions are made through comparisons of measured media concentrations to risk-based cleanup standards that are developed for specific land use scenarios (typically residential and industrial/commercial). In both methods, final cleanup standards or site-specific remedial requirements are typically based on the higher of the concentrations associated with an acceptable level of health risk, background levels of contaminants, and detection limits or Practical Quantitation Limits (PQLs).

A fundamental health risk component of any remedial standard is the toxicity value which describes the safe dose of chemical and, for potential carcinogens, the carcinogenic potency of the chemical. Consequently, changes to toxicity values will cause changes to risk-based remedial levels. Importantly, in the cases of 1,1-DCE and TCE, the changes to toxicity values may result in order-of-magnitude changes to risk-based remedial levels.

TCE – Current and Proposed Changes to Toxicity Values
Currently, there are no published toxicity values for TCE for use in cancer or non-cancer risk assessment in U.S. EPA’s Integrated Risk Information System (IRIS) database (IRIS, www.epa.gov/iris). This database is the single most-used source of official U.S. EPA toxicity values. The carcinogenicity assessment in the TCE IRIS file was withdrawn on July 1, 1989 (rationale: new assessment in preparation; date unspecified), and has not been replaced since that time. There also has not been a Chronic Health Hazard Assessment for Noncarcinogenic Effects (associated with oral and inhalation exposures) during the period of 1989 to the present.

Not withstanding the absence of these toxicity values, most states and federal regulatory programs do regulate TCE based on potential health effects. Most states and federal programs have obtained oral Cancer Slope Factors (CSF) for TCE from one or more other (non-IRIS) U.S. EPA sources, while others have derived their own CSF from the available toxicology literature. Many states have elected not to evaluate non-cancer risks for TCE in their cleanup programs because there are no IRIS values and because the cancer risks are generally believed to be the more sensitive endpoint (and would therefore drive the derivation of cleanup standards).

In August 2002 U.S. EPA published TCE Health Risk Assessment: Synthesis and Characterization (Synthesis Document) as an External Review Draft (Cogliano, J., J. Cogliano and J. Cogliano, 2001. Trichloroethylene Health Risk Assessment: Synthesis and Characterization (External Review Draft). Office of Research and Development. EPA/600/P-01/002A. August). The Synthesis Document presented a comprehensive review and evaluation of toxicological data on TCE and recommended toxicity values for TCE. The U.S. EPA Science Advisory Board (SAB) recently completed a review of the Synthesis Document (December 2002). The SAB generally concurred with the Synthesis Document, and recommended that the U.S. EPA address comments from SAB and the public, then move forward with finalizing the document. Once the Synthesis Document has been finalized, U.S. EPA will be in a position to propose changes to the IRIS trichloroethylene file.

The changes and additions with respect to TCE toxicity values presented in the Synthesis Document are summarized in bulleted form below.

• A range of values for an Oral Cancer Slope Factor (CSF) is proposed. The range is 0.02 (mg/kg/day) –1 to 0.4 (mg/kg/day) –1. The lower end of the range is roughly two-times greater than the values commonly in use. The upper end of the range is roughly 20 times higher than values commonly in use.
• Inhalation Cancer Unit Risk (UR) values have been presented based on an occupational study. Values are 7 X 10-5 per ug/m3 (central tendency) and >9 X 10-5 per ug/m3 (upper bound), roughly 40 times higher than values commonly in use.
• An Oral Reference Dose (RfD) is proposed at 0.0003 mg/kg/day.
• An Inhalation Reference Concentration (RfC) is proposed at 40 ug/m3.

The Synthesis Document suggests that “risk assessors use the upper end of the slope factor range for susceptible populations having risk factors for TCE-induced cancer.” This suggestion will likely encourage regulators to insist on the use of the upper end of the cancer slope factor range. In fact, there is an indication that regulators in Connecticut, U.S. EPA Region 1, and U.S. EPA Region 9 have recently taken this position.

TCE – Implications of Proposed Toxicity Value Changes on Remedial Levels
If the proposed toxicity values in the Synthesis Document are adopted, remedial requirements for TCE sites may become significantly more stringent. For example:

• TCE concentrations in indoor air that are considered “unacceptable” could be reduced substantially, perhaps as much as 20-fold. In many cases, a risk-based level that is considered to be “acceptable” may be lower than anthropogenic background levels in urban environments, resulting in remedial levels that are established at background concentrations.
• Groundwater cleanup standards that are based on migration of TCE vapor from groundwater to indoor air could be lowered by as much as 20-fold. This would hold true where the standards are solely risk-based, and do not consider background indoor air levels.
• Because the MCL is based on a PQL, it is less likely that the drinking water MCL would be revised in the near future in response to the proposed changes to toxicity values.
• Soil cleanup standards based on direct contact exposures may also be reduced substantially, perhaps as much as 20-fold.
• In addition, Superfund-type risk assessments that evaluate remedial necessities based on calculated health risks will be more likely to show TCE as a risk driver or chemical of concern (COC) in environmental media and land use exposure scenarios where it historically would not have been identified as such.

1,1-DCE – Recent Changes to Toxicity Values
In August 2002, U.S. EPA published a toxicity reassessment for 1,1-DCE on the IRIS data base.  The reassessment for 1,1-DCE involved reclassification of 1,1-DCE carcinogenicity and removal of the cancer dose-response values, development of a new oral RfD, and derivation of an inhalation RfC (previously unavailable).  The reassessment on IRIS indicates that the weight-of-evidence for human carcinogenicity is not sufficient to justify derivation of quantitative cancer slope factor or unit risk values for 1,1-DCE;  the IRIS file for 1,1-DCE no longer publishes CSF and UR values.  The changes and additions with respect to 1,1-DCE toxicity values presented in the IRIS file are summarized in bulleted form below.

• Oral CSF and inhalation UR values are no longer published for 1,1-DCE; the cancer risk endpoint need not be quantitatively evaluated.
• An Oral RfD of 0.05 mg/kg/day is published, representing a 5-fold increase over the previous RfD.
• An Inhalation RfC of 200 ug/m3 is published.

As a result of this change, human health risk assessments need not calculate cancer risks for 1,1-DCE, and media protection standards need not be derived in consideration of the cancer risk endpoint.

1,1-DCE – Implications of Toxicity Value Changes on Remedial Levels
Historically, 1,1-DCE has been a substantial risk driver in human health risk assessment due to the carcinogenic potency reflected in the previously-published CSF and UR values.  With the re-classification of 1,1-DCE carcinogenicity and removal of quantitative dose-response values for carcinogenicity, the influence of 1,1-DCE on overall site risk estimates will be reduced.  For example:

• 1,1-DCE concentrations in indoor air that are considered “unacceptable” could be increased substantially, perhaps as much as 4 orders of magnitude. In many cases, remedial standards that were previously based on a PQL or background value may now be increased and based on a risk-based level.
• Groundwater cleanup standards that are based on migration of 1,1-DCE from groundwater to indoor air could be increased by as much as 4 orders of magnitude.
• The MCL should be revised (increased) in response to the proposed changes to toxicity values. However, U.S. EPA has not yet established a timeline for revising the MCL for 1,1-DCE.
• Soil cleanup standards based on direct contact exposures may also be increased substantially, perhaps as much as 2 orders of magnitude.
• In addition, Superfund-type risk assessments that evaluate remedial necessities based on calculated health risks will be less likely to show 1,1-DCE as a risk driver or COC in environmental media and land use exposure scenarios where it historically would have been identified as such.

Summary
The proposed changes to the TCE toxicity values and the recent changes to the 1,1-DCE toxicity values will potentially result in much more stringent standards and cleanup criteria for TCE, and significantly less stringent standards and cleanup criteria for 1,1-DCE. Although most regulatory risk-based criteria will not be updated right away to reflect these changes, there are usually options for implementing technical updates and advances in the science in the interim.  For example, many state programs that rely on published “generic” criteria provide options for calculating alternative or site-specific criteria that can incorporate the updated toxicity values.

• The potential implications of these changes on site-assessment, risk management, and remedial objectives and decisions are summarized in Table I. In summary, parties that are currently managing remedial sites that involve TCE or 1,1-DCE in soil, groundwater, or indoor air, need to carefully track potential changes to remedial requirements that may result from changes to U.S. EPA’s toxicity values. As U.S. EPA makes changes to the toxicity values for these compounds, target cleanup levels for TCE may become more stringent, and target cleanup levels for 1,1-DCE may become less stringent, under federal or states’ regulatory programs. Responsible parties also should ask their risk assessment consultants to provide:
• information concerning the basis of the cleanup standards for 1,1-DCE and TCE for a particular site;
• information concerning any on-going activities by state or federal agencies that might revise the cleanup requirements (soil, groundwater, indoor air) in response to U.S. EPA’s recent and planned updates of the toxicity values (the nature of the changes and the schedule for changes are important pieces of information for site-specific strategic decision-making);
• information about potential duration and financial ramifications of achieving closure of a site under current cleanup requirements, as compared to site closure under future, potentially more or less stringent cleanup requirements;
• information about how to provide input to state or federal agencies that might be revising 1,1-DCE and TCE cleanup standards that could affect the remedy at a particular site;
• confirmation that the characterization of nature and extent of TCE under current standards is valid for future potential standards;
• confirmation that the remedy selection planned or chosen under current cleanup requirements would still be the remedy selected under future, more or less stringent, requirements;
• frequent updates to the above information as U.S. EPA and state agencies proceed with changes to 1,1-DCE and TCE cleanup requirements.