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Former Countryside Market,Stockton, California May 20,2013 Feasibility Study and Interim Remedial Action Plan y Francisco Bay Region), the volatilization and inhalation of COPCs from 'Ithis domestic water supply source is considered to be an insignificant exposure pathway. 3.1.4 Groundwater Because groundwater at the Site is encountered at depths of approximately 90 feet bgs, incidental contact by workers is considered to be an incomplete exposure pathway. However, since potable water for the Site and surrounding residences is supplied by domestic WE IIs, the ingestion of COPCs in groundwater is the only complete exposure pathway. This has been confirmed via the detection of 1,2-DCA above its California Maximum Contaminant Level (MCL) in t e domestic well located at 10967 Copperopolis Road (Figure 2) during March 2012 sampling activities. Per SGI's February 8, 2013 Site Characterization/Fourth Quarter 2012 Groundwater Monitoring and Sampling Report, domestic well 10967 has been equipped with a treatment system that began operating during January 2013 in accordance with SGI's February 22, 2013 Wellhead Treatment Memorandum. Groundwater monitoring conducted since January 2012 shows a generally northeast gradient, indicating that this well is directly downgradient from the former UST area. 3.2 Summary of Potential Exposure Pathways The USEPA defines an exposure pathway as "the course a chemical or pollutant takes from the source to the organism exposed" and an exposure route as "the way a chemical or pollutant enters an organism after contact" (USEPA 1989). A complete exposure pathway requires four key elements: on-Site chemical sources, migration routes (i.e., environmental transport); potentially exposed human receptors; and routes of exposure to impacted media (e.g., ingestion of c iennicals in domestic water). All four factors are required for a complete exposure pathway; if anN one factor is missing, the pathway is considered incomplete. Current use, and anticipated future use, of the Site and surrounding ar Da is residential/agricultural. Domestic water supply wells are present both on-Site and off-site. T ierefore, all individuals that occupy the various residences are the primary receptors of concern, wit i ingestion of COPCs in the groundwater the relevant exposure pathway. To date, one COPC (1,2-DCA) has been detected above its MCL in a regional domestic well with no other detections besides benzene at a laboratory estimated/below MCL value. The well (10967) is located approximately 300 feet down-gradient from the former UST area. Based on the available groundwater data from the on-Site and off-site monitoring wells that have been installed to delineate the plume, the maximum concentration of 1,2-DCA that has been detected in proximity to the former UST area is 570 lug/L in well MW-2 (Figure 2). The most recent concentration of 1,2-DCA detected in domestic well 10967 (prior to treatment) was 25 µg/L. Thus, while natural attenuation process are effectively serving to reduce the concentrations of COPCs along the flo path of the plume, remedial actions are necessary to ensure full future compliance with cleanup level (Section 8.0). Feasibility 5Wy and Feasiblity SNdy B llA _Te#Final.dw 3-3 ime SOUMO GfOUR IRL