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�- cwtogual recknics lnc. Page 3 Interim Remedial Action Plan Project No.622.2 March 28,2005 trends and after the interim remedial action has been in operation for a year to determine it's effectiveness at mitigating this plume. Once these tasks have been accomplished a full scale CAP can be developed if needed, in accordance with County mandates and USTCFP requirements for three remedial alternatives. In our February 20, 2004 "Additional Site Characterization" report GTI presented a cross section illustrating the site's Iithology with contaminant concentrations superimposed on the section. Figure 3: Cross Section B — B' with Contaminant Concentrations shows that a "core" of high gasoline concentration is present beneath the former UST excavation (see Figure 2 for cross section location). Based on the lithology and contaminant data GTI ` divided the aquifer into shallow, intermediate and deep zones to determine contaminant mass. These zones are shown in Figure 4: Cross Section B — B' with Aquifer Zones. The cross section has been updated to further refine the Site Conceptual Model (SCM). Figure 5 `" was developed using the previous B — B' cross sections with the inclusion of the two new monitoring wells MW-5 and MW-6. The groundwater analytical data from the February 17, 2005 event is transposed on section adjacent to the respective wells. Discrete groundwater sample data from the December 2003 CPT investigation is also plotted to define the plume margins. The plume is shown as having a contaminated core beneath the former UST excavation. A lobe extends from the core to the right (northeast) that is the predominant down gradient direction. The depth of the lobe occurs in a sand layer of varying thickness that has been recorded in the core location and the down gradient location of wells MW-5 and MW-6. The two new monitoring wells MW-5 and MW-6 installed on October 26 - 27, 2004 were La sampled for the first time on February 27, 2005. The presence of MTBE in these wells- 12 and 2.2 ug/l, respectively, further refined the site conceptual model in regards to the down gradient lateral extent of the MTBE plume. L. It is noted that the core contaminant concentrations have decreased significantly since Figure 3 was drawn in 2004. The extraction of 32,250 gallons of groundwater during the dual phase extraction pilot test completed in December 2004 accounted for some source removal. The wet winter months could also account for dilution of the core from fresh water infiltration. But these conditions aside, the MTBE core concentrations are high enough to warrant remedial action. In the following sections GTI has proposed a work plan to perform the interim remedial actions listed in item#1 above.