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Former Countryside Market,Stockton,California Feasibility Study and Interim Remedial Action Plan May 20,2013 3.0 PRELIMINARY CONCEPTUAL SITE MODEL The Conceptual Site Model (CSM) provides the basis for a comprehensive evaluation of the potential risks to human health by identifying the mechanisms through which relceptors may be exposed to chemicals of potential concern (COPCs). The CSM traces the COPCs identified at the Site in a logical migration from the former source area through various release mechanisms and exposure routes to potentially affected receptors. The CSM also identifies the exposure routes that are potentially complete under the current and anticipated future land us (s). Additionally, the CSM facilitates the analysis and screening of exposure pathways not likely top se significant risks. The following sections describe the COPC sources, and how the associated chemicals migrate from the original source(s) through various environmental media to the exposure media, as well as how human populations living or working at the Site and surrounding area m y be exposed to the COPCs as chemicals migrate from the original source through environmental me ia. 3.1 Sources and Transport Mechanisms Primary, secondary and tertiary sources of contaminants are listed in the CSM (Figure 3). Sources include environmental matrices (e.g., soil, water and air) where the COPCs can be found. COPCs may migrate from their primary source to secondary and tertiary sources. Based on prior environmental investigations, the primary source of the COPCs is associated with spills or similar releases from the former underground storage tanks (two 1,000 gallon leaded-gasoline USTs), and former dispenser island that were removed from the property in March 1993. As detailed previously, the former UST pit was excavated to depths ra ging from 11 to 13 feet bgs during October 1995 (approximately 40 cubic yards of hydrocarbon-imp cted soil was removed), and the excavation was backfilled with clean stockpiled soil and pea gravel (UEC, 1995). The primary COPCs at the Site include: 1,2-Dichloroethane (1,2-DCA, also known asi ethylene dichloride or EDC), benzene, toluene, ethylbenzene and xylenes (BTEX compounds). From this primary source, chemicals have been detected in soil (secondary source) from which COPCs have transported to groundwater (tertiary source), and potentially indoor and/or outdoor air. These potential sources of COPC exposure are discussed further in the following sections. 3.1.1 Surface and Subsurface Soil Surface and subsurface soils are potential sources for direct exposu a (i.e., contact with soil via ingestion and/or dermal contact). Additionally, compounds adsorbEd to soil particles may be entrained into the air as dust and inhaled and/or volatiles in subsurface soil may migrate into indoor air. As indicated above, detections of residual contamination in soil is limited to the relatively small former UST release area with secondary soil impacts outside of this zone resulting from natural groundwater fluctuations (i.e., chemicals migrating from the primary zone of impacted soil to Feasibility SNdy a Feasibility SWy&IRP_TeM FinW.dw 3-1 The Source Group,Inc.