SITE-SPECIFIC GROUNDWATER STANDARDS FOR THE VAPOR INTRUSION PATHWAY

For the first time in Colorado, the Colorado Water Quality Control Commission (WQCC) established site-specific groundwater standards for a man-made pollutant, trichloroethene (TCE), protective of the vapor intrusion (VI) pathway in residential areas overlying a shallow TCE plume adjacent to the former Lowry Air Force Base (LAFB). The groundwater standards were developed due to technical impracticability of further remediation, and considered multiple rounds of empirical groundwater, soil vapor, sub-slab vapor, and indoor air data. VI risks to residences overlying the off-site TCE plume were estimated using the EPA spreadsheet version of the Johnson & Ettinger (J&E) model. Paired groundwater, soil vapor, sub-slab vapor and indoor air sampling over the highest concentration portion of the off-site TCE plume, in multiple winter-heating season events over a ten year time frame, allowed for “calibration” of the J&E Model to site-specific conditions. Agreement between measured and modeled results was very good (within a factor of approximately five). In order to extrapolate the results from the sampled area to the remainder of the residential area overlying the mile-long off-site plume where no indoor air data were available, detailed lithology for 16 off-site exposure units was compiled from boring logs and used as input to the J&E model. The most recent toxicology data for TCE was used to estimate VI risks. All calculated risks were shown to be below the Colorado Department of Public Health and Environment (CDPHE) 10^-5 (or HI=1) action level for indoor air, but were above a 10^-6 risk level. In accord with Colorado groundwater policy, the Water Quality Control Division of the CDPHE recommended to the WQCC that site-specific groundwater standards for TCE be set equivalent to a 10^-6 risk level for residential vapor intrusion, which corresponds to a groundwater TCE concentration of 12 ug/L in this instance. Previously, the CDPHE had been unwilling to accept modeling for the VI pathway and always required empirical data on indoor air collected during the winter heating season.