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(:VATC A S S O C I A T E S I N C -60 detected in well MW2. Well MW was installed with the top of screen below the groundwater table. During the last monitoring event, only MTBE was detected in well MW 1. To perform mass balance calculations, ATC averaged the concentrations of petroleum hydrocarbons detected in well MW 1 from all five monitoring events in addition to the concentrations detected in nearby soil probe boring SP-l. The total mass of dissolved TPHg present in the saturated zone is estimated to be less than one pound. The total mass of BTEX present in the groundwater is estimated to be less than one pound. The total mass of MTBE present in the groundwater is estimated to be less than one pound. Mass balance calculations are included in Appendix D. 5.0 GROUNDWATER RECEPTOR SURVEY ATC reviewed Department of Water Resources (DWR) groundwater well construction logs, and conducted a site walk in the vicinity of the site to identify any groundwater wells located within 2,000-foot radius of the site. The description information provided on the well construction logs was used to locate the wells on a vicinity map according to "well location". Based on the DWR records, no wells were located within approximately 2,000 feet of the site. A site map showing the survey area is presented on Figure 7. Available well construction logs are contained in Appendix C. During a visual inspection of the survey area by ATC, a well was located adjacent and west of the property at 1230 Jackson Street, approximately 1,600 feet northwest of the site. The well is owned by California Water Service Co. No log for this well was found during the DWR record search. The well location is shown on Figure 7. L 6.0 ASTM TIER 2 RBCA EVALUATION A human health risk assessment (HHRA) study was performed to address potential significant `-' health risks associated with petroleum hydrocarbons at the subject site. An American Standard of Testing Materials (ASTM) standard RBCA Tier 2 evaluation was performed following ASTM Standard E 1739-95 and the Tier 1 and Tier II Guidance Manuals for Risk Based Corrective Action published by Groundwater Services, Inc. (GSI) of Houston, Texas (GSI, 1995). 6.1 Exposure Pathway Evaluation Evaluation of the exposure pathways is the first step in the RBCA analysis. There are four steps used to determine which exposure pathways are valid for the site. These steps are described below. Secondary Sources The first step is to identify the sources of contamination. The primary source of the contamination at the site is the former gasoline UST. There are five potential secondary sources that must be evaluated when performing a"CA analysis: w-.\62574.05\rcporu\rlos=-FS3.doc 5