ARCHIVED REPORTS XR0003037 - Laserfiche WebLink

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• TIER 2 EVALUATION In the Tier 2 assessment, site-specific information or data is used in equations or models which are the basis for the Tier 1 RBCS. For the RWQCB RBSLs, the Johnson and Ettinger model is used This model is available from USEPA's web site SECOR downloaded this model from the USEPA web site, then substituted site-specific input values for default values as appropriate and available Two parameters or parameter -types were substituted • depth below grade to water table/thickness of soil stratum A, and • soil type For the first parameter, a value of 1,678 cm was used, corresponding to a depth to first - encountered groundwater of 55 feet bgs Finally, a silty soil type was used in the model, consistent with what is known about the nature of the vadose zone at the site The Tier 2 Johnson and Ettinger model runs are included at the end of the PEA All runs were conducted for a subsurface, basement -type structure, representing both the current site (i e , lube bay), as well as the highest possible indoor air exposures Ethanol, although formally a Tier 2 chemical, was not run, as the groundwater concentrations are significantly below the levels individuals routinely ingest in alcoholic beverages For TBA, the physical and toxicological properties of n -butyl alcohol, contained in the downloaded model, were used, as properties for TBA were not readily available For EDB, the physical properties of 1,2- dichloroethane, contained in the downloaded model, were used with the toxicological properties of EDB. Both of these two "substitutions" are appropriate and conservative. In general, physical and toxicological properties were obtained from standard sources, and can be provided on request. Results of the Tier 2 computations, that is hazard quotients for noncarcinogemc chemicals (methanol and TBA) and cancer risks for carcinogens (EDB), are within values considered acceptable by the RWQCB and CalEPA for both onsite commercial/industrial and residential use SUMMARY AND CONCLUSIONS A conservative, tiered risk-based assessment was conducted to evaluate on-site exposures from site VOCs via soil and groundwater volatilization into indoor air The majority of VOCs (including benzene, toluene, ethyl -benzene, xylenes, MtBE, DIPE, EtBE, Tame, 1,2 - DCA, TCE, P-IPMB, and PCE) detected in site soil and groundwater were eliminated from further evaluation in Tier 1 Based on the Tier 2 analysis, which included site specific fate and transport modeling and evaluation of the impact to indoor air, the four chemicals evaluated in Tier 2 (including methanol, ethanol, TBA, and EDB) were also found to not pose a significant risk