GENERAL SECTION

Updated Vapor Intrusion Model Lowers Cleanup Goals

Many consultants and regulators use the Johnson and Ettinger (1991) model (J&E) to evaluate the indoor inhalation risk posed vapor intrusion from soil and ground water. Model calculations using an acceptable risk level can establish site-specific remediation goals. A recent update to the J&E model to incorporate default values recommended by US EPA can increase risk levels and lower cleanup goals by as much as ten fold.

SIGNIFICANT CHANGES
The update changed both chemical toxicity values and vapor exposure parameters. Adjustments have been made in the applicable chemical list based on volatility and toxicity. Default soil parameters and default building parameters also have been updated. Among the most significant changes are the changes to chemical toxicity and the building ventilation rate.

Many sites with chlorinated solvent contamination have a suite of tetrachloroethene (PCE), trichloroethene (TCE) and their breakdown products dichloroethene (DCE) and vinyl chloride. In many cases the risk driver on these sites is the vapor intrusion hazard posed by vinyl chloride. The updated J&E model has doubled the toxicity term (called the Unit Risk Factor) for vinyl chloride from 4.4E-6 to 8.8E-6. This increase has a significant impact on the resulting calculated risk.

The toxicity of TCE has undergone and continues to receive significant review. The IRIS database withdrew the TCE toxicity value in 1994, but the Superfund Technical Support Center recommended the continued use of the old value during the review process. The J&E model uses the most recent slope factor from the 2001 US EPA draft, but advises that the value is still under review and should be considered a “provisional” value.

Because the default values for building characteristics were changed in the update, the building ventilation rate was decreased by more than half. The effect is fewer vapors escaping, which increases the potential exposure to contamination and increases the risk significantly.

RECOMMENDATIONS
As with any model, there is a danger in using default values. The default values are usually very conservative and may be “worst-case” values. The model default values should be compared to actual site-specific conditions, if available, to select the most appropriate values.

Toxicity values for chemicals are under constant scientific review. You should be certain that the values that you are using will be accepted by your regulators.

The Payne Firm, Inc. provides risk assessment assistance and can assist you with the evaluation of risk using the Johnson and Ettinger model. For more information contact Mike Saul or Mark Berkich at mts@paynefirm.com or meb@paynefirm.com.