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ATC Associates Inc. 1117 Lone Palm Avenue,Suite 201 Modesto,California 95351 %TC 209-579-2221 fax:209-579-2225 ASS O C I ATE S INC. and ATC will survey the occupants regarding the usage of these chemicals. ATC will request that the air conditioning and heating system be used similar to other normal business days. The purpose of the limited indoor air study will be to evaluate the potential degradation of indoor air quality. The results of the indoor air sampling will be compared to the soil gas analytical results collected in September 2010 to evaluate the presence of specific analytes of interest detected in the indoor air samples, if any. The results of the indoor air sampling will be compared to the results of the soil vapor samples to evaluate the presence of specific analytes of interest detected in the indoor air samples, if any. Ouality Assurance/Quality Control To ensure quality assurance and quality control measures have been adequately assessed, ATC will collect an ambient air sample from a location considered to be up-wind from the site. Sample containers will be labeled following sample collection and the use of permanent markers will be avoided. It should be noted that this facility is an active retail fuel station and there is a potential for gasoline vapors to be introduced to the indoor air from customers. Chemical Analyses The indoor air sample collected from inside the on-site commercial structure will be immediately transferred under chain-of-custody procedures to a State-certified laboratory. The samples will be analyzed for total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and total xylenes (BTEX), methyl tertiary butyl ether (MTBE), tertiary butyl alcohol (TBA), di-isopropyl ether (DIPE), ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), 1,2-dichloroethane (1,2-DCA), and 1,2-dibiomoethane (EDB) using EPA Method TO-15. Indoor Air Risk Assessment The data will be compared to the values listed in Table E of the Environmental Screening Levels (ESLs) for Indoor Air and Soil Gas (Vapor Intrusion Concerns) as published by the RWQCB San Francisco Bay Region in November 2007. The indoor air results will also be compared to the California Human Health Screening Levels (CHHSLs) published by the California Environmental Protection Agency (EPA) in January 2005. These comparisons will be made for additional evaluation on the impact to human health. These comparisons will be made using the values provided for a commercial exposure scenario. Discussion The impact to the environment has been evaluated through subsurface investigation and groundwater monitoring and sampling activities at the site. The impact to groundwater has reduced over time to levels that no longer exceed the maximum contaminant levels established S:\Environmenta1\25847137 TracyMepolts\Workplans IA Wkpin.doe 4 ATC Associates Inc.