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31 January 2013 AGE-NC Project No. 02-0926 Page 9 of 21 6.0. SENSITIVE RECEPTOR SURVEY The purpose of the sensitive receptor survey is to locate vertical conduits within a 2,000 foot radius of the site, such as municipal, domestic, irrigation or groundwater monitoring wells, which could serve as potential pathways for contaminant migration to groundwater Currently, a formal sensitive receptor survey has not been performed or submitted for the site. However, it is AGE's understanding that drinking water near the site is currently supplied by the private groundwater well system, maintained by California Water Service Company (Cal-Water). 7.0. SITE CONCEPTUAL MODEL Based on the data collected to date from soil borings and groundwater monitoring wells, AGE has developed a site conceptual model for the release, migration and distribution of the contaminants in the subsurface as described below: Release(s) of petroleum hydrocarbons occurred from beneath the UST area. Once released into the subsurface, petroleum contaminants migrated vertically through the uppermost fine grained lithology units between surface grade and approximately 40 feet bsg, then migrated vertically and laterally in coarser grained lithologies located between 40 and 55 feet bsg. Based on the columnar nature of the impact, it is likely that groundwater was significantly lower than current depths (30 feet bsg) during the time hydrocarbons were released from the formerly installed UST system. Since groundwater was likely lower than currently measured levels, significant saturated soil impact is located at the site between 30 feet bsg to 55 feet bsg. The adsorbed and dissolved hydrocarbon impact at the site is generally columnar in shape and lateral impact is highly limited. Vertically, hydrocarbons have been detected intermittently in deeper monitoring zones located at 60 feet bsg, but it appears that the hydrocarbon plume is vertically defined by these low to non-detections. Laterally, the shallow groundwater plume is largely defined by the well network currently established at the site. However, the plume is somewhat undefined to the east of monitoring well MW-5. Soil and groundwater at the site is generally impacted by TPH-g, TPH-d, BTEX, EDB, 1,2-DCA and TAME. The bulk of the petroleum contaminants above the current water table (30 feet bsg) have been remediated by soil vapor extraction activities and a small portion of the saturated zone soil impact and shallow groundwater impact have been remediated by an in-situ air sparging (IAS) pilot test. Historical remedial action performed at the site to date is described in detail in Section 8.0 below. Advanced GeoEnvironmental, Inc.