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Chevron Products Company NW9/ 2 Chc%Ton Service Station#9-0557, 139 South Center Street,Stockton,California 6/01/01 south of the site across Washington Street. A summary of previous investigations is presented as Attachment B. QUARTERLY MONITORING Quarterly monitoring has been ongoing at the site since March 1991. Historical groundwater gradient has been generally consistent to the south-southeast. Water levels have fluctuated in the wells from approximately 27 feet bgs (recently) to 47 feet bgs in 1992. Generally, groundwater levels have risen at the site since 1994. Historically petroleum hydrocarbons have been reported in wells MW-1, MW-2, MW-6, MW-7, MW-8, and MW-9, with maximum concentrations of total petroleum hydrocarbons as gasoline (TPHg) and benzene reported as 120,000 ug/1 (MW-9, November 24, 1997) and 14,000 ug/l (MW-9, November 24, 1997), respectively. Historically, concentrations of methyl tertiary butyl ether have not been detected at the site. During the most recent sampling event(First Quarter 2001) separate phase hydrocarbons (SPH) were reported in on-site well MW-9 (located in the vicinity of the former USTs). Petroleum hydrocarbons were reported in off-site well MW-7 with concentrations of TPHg and benzene as 1,000 ug/1 and 30 ug/l, respectively. Petroleum hydrocarbons were not detected in on-site wells MW-1 through MW-4 and MW-6 and off-site wells MW-5 and MW-8. PROPOSED SCOPE OF WORK The scope of work proposed for this investigation will include the drilling and installation of five(5) feasibility testing/groundwater monitoring wells and the performance of both a dual phase extraction(DPE) pilot test and an aquifer test. The specific scope of work is discussed below: Well Installation • Borings. Five(5)proposed soil borings(MW-9A, MW-10, MW-11, and MW-12, MW-13) will be advanced by hollow-stem auger at the general locations shown on Figure 2. All of the borings will be completed as wells as described below. • Well Installation. Based on recent site groundwater data, the depth of four(4) of the borings will be advanced to approximately 32 feet bgs and completed with a screen interval of approximately 15 feet, from 17 to 32 feet bgs. The completed depth of MW-9A will be 57 feet bgs and completed similarly to MW-9, with 5 feet of screen extending from 52 to 57 feet bgs. • Permitting. The applicable permits for groundwater monitoring wells will be obtained from the SJCEHD prior to drilling. I:\Chevron\90557\1ZEPORTS\Feasibility Study\workplan.doc