ARCHIVED REPORTS_TIER I RISK BASED CORRECTIVE ACTION EVALUATION

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ARCADIS BBL Mr. Brian Taylor July 27, 2007 concentrations were collected from MW-69UA on December 5,2006.Xylenes were the only COPC detected in WSW-1 at a maximum concentration of 6 pg/L on March 6,2007. A scientific model for evaluating the migration of volatile chemicals from groundwater to indoor air,GW-SCREEN,(Version 3.0), developed by the USEPA(2003)and modified by the Cal-EPA DTSC was used to predict concentrations of the volatile chemicals in indoor air based on their maximum concentrations in groundwater. The GW-SCREEN model was last modified on January 21,2005 and is based on the Johnson and Ettinger(1991)indoor air vapor intrusion model, often referred to as the J&E Model. In addition to maximum chemical concentrations in groundwater and the slab-on- grade building foundation, site-specific characteristics of soils at the property were used in the GW-SCREEN model,when available, and were assumed to represent the characteristics of soils beneath the office building. Table 2 presents the input parameters used to estimate indoor air levels of the COPCs.The average soil/groundwater temperature at the site, 62 OF, is based on an isothermal contour line from DTSC(2004)guidance. The soil type,clay loam,was obtained from the United States Department of Agriculture (USDA,2007)Soil Conservation Survey for San Joaquin County. Cal-EPA DTSC default assumptions were used for all other soil properties.The average vadose zone soil dry bulk density is 1.48 g/cm3, the average vadose zone total porosity is 44.2%, and the water-filled porosity is 16.8%. The average depth below grade to the water table is approximately 5.4 feet based on the depths to groundwater from the samples collected at MW-69UA from the last four sampling events. The indoor air concentrations of the COPCs predicted by the GW-SCREEN model were used to estimate potential health risks and hazards to onsite office workers. The exposure parameters used to estimate potential exposures to workers are summarized in Table 3. Standard default parameters,as recommended by DTSC (1992)and USEPA(1989),were used. Inhalation Toxicity Criteria The USEPA(2007) Integrated Risk Information System (IRIS)and Cal-EPA OEHHA (2007)have established specific toxicity criteria for BTEX and MTBE for use in risk evaluations. Following standard practice for sites located in California,if available, the most recent OEHHA inhalation toxicity criteria were used. If unavailable,then toxicity criteria established by the USEPA were used. Table 4 presents the inhalation toxicity criteria used in this evaluation. Page: G iao 11�AIWImtow71I10Li IRea3/7