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Geological Iiechnks/c. Page 4 Interim Remedial Action Plan Project No.723.2 March 7, 2005 groundwater to be discharged to City of Stockton public treatment works and air stripper effluent. 2.0 PURPOSE AND GOALS OF REMEDIAL ACTION The purpose of this plan is to propose an interim remedial plan to address the significant on site gasoline contamination that continues to source the groundwater plume. The site's groundwater has been monitored from October 1994 (over ten years) to the present and the only contaminant reduction that has been performed was during the July 2004 one week pilot test. There is no reason to delay cleanup of the source area while delineation of the down gradient plume takes place. Delays in gathering additional information can be expected since it is probable that a co-mingled plume is present with the Shell station across the street. The goals of this corrective action are: 1. To remove the high concentration of contaminants located in the soil and groundwater in the source area beneath the former fuel dispenser area. 2. To prevent the further migration of groundwater away from the source area by inducing a capture zone through groundwater extraction. 3. To obtain additional groundwater pumping characteristics to complete the Site Conceptual Model (SCM) and to develop a final Corrective Action Plan (CAP). 3.0 INTERIM SOIL & GROUNDWATER REMEDIATION The following sections outline GTI's proposal for achieving the above listed goals. 3.1 Source Removal In our January 21, 2004 "Additional Site Characterization Report", GTI produced cross sections utilizing the data gleaned from the installation of three new well sets. Figure 3A illustrates the geology trending from northwest to southeast side of the site and Figure 4A illustrates the geology trending from southwest to east of the site (see Figure 2 for section locations). Figures 3B & 4B illustrate the preceding cross sections with contaminant concentrations superimposed on the section. As shown in Figure 3A, the site exhibits a primarily clayey geology with clayey sands, silts and gravel layers occurring less frequently. More sand is present in the off site location of MW-7/-107/-207 but this boring was continuously cored rather than sampled at 5 foot intervals for the on site boreholes. Thus more geologic detail is known from the MW-7 series wells. Figure 3B shows cross section A — A' with soil contaminant concentrations indicated next to the borings. In this diagram, a soil contaminant plume is shown near borings SB-101, MW-101 and MW-201 (in the vicinity of the former dispenser island).