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" Former ARCO Service Station No. 51" E C ® R -- Work Plan for Well Destruction M March 12, 2008 Page 2 i In October 1991 , four on-site groundwater monitoring wells (MW-1 through MW-4) , and three soil vapor extraction (SVE) wells (VWA through VW-3) were installed . Soil samples results contained maximum TPHg and benzene concentrations of 340 mg/kg and 0. 031 mg/kg, respectively. These results also indicated that petroleum hydrocarbon impacted soil was limited to the area of the UST complex. In 1992, RESNA of Sacramento, California conducted an SVE test at the site. The test concluded that the site soil could be mitigated by SVE , with each well having a radius of influence of 30 ft. i In 1993, two additional monitoring wells (MW-5 and MW-6) and one SVE well (VW-4) were installed. Soil samples did not contain detectable amounts of TPHg . One sample from B-15 (near the dispensers) had concentrations of benzene reported at 0.024 mg/kg . During the drilling of boring B-18 , a product line was accidentally fractured , resulting in the release of product into the soil . VW4 was installed adjacent to B- 18 for future mitigation of the released product. In 1994, EMCON Associates of Sacramento, California (EMCON) conducted a site evaluation for prioritization of remediation efforts at ARCO sites in San Joaquin County. EMCON calculated approximately 383 pounds (lbs) of total petroleum hydrocarbons remained at the site (URS , 2005a) . In February 1998 , four gasoline LISTS of various capacities and their associated piping were removed . Soil samples indicated that soil was impacted with TPHg and benzene in the area of the USTs. In October 1998, a dual completion SVE well (VW-5S and VW-5D) and three air sparge (AS) wells (AS- 1 through AS-3) were installed . Soil samples collected at this time confirmed the presence of hydrocarbon-impacted soils in the vicinity of the USTs. Maximum concentrations of TPHg and benzene were 7 , 830 mg/kg and 11 . 5 mg/kg , respectively. An SVE/AS system utilizing six SVE wells (VW-1 through VW-4 , VW-5S, and VW-5D) and three AS wells (ASA through AS-3) began operation in September 2000. The system was operated nearly continuously until June 2004. Low influent concentrations prompted the shutdown of the SVE system in June 2004. The SVE system removed approximately 2 , 100 lbs of petroleum hydrocarbons from the subsurface. In addition , seven gallons (43. 17 lbs) of free product were removed from site wells during this time. In December 2004 , two groundwater monitoring wells (MW-7 and MW-8) were installed to monitor the lateral and vertical extent of groundwater impacts at the site. MW-7 is located off-site and down gradient of the former LISTS and MW-8 was installed in the vicinity of the former USTs to monitor deep groundwater. TPHg , benzene, toluene, ethylbenzene, and xylenes were reported in soil samples from MW-8. No detections were found in MW-7. In August 2007, Stratus Environmental , Inc. of Cameron Park, California (Stratus) installed a new blower and the AS system resumed operation. From September 21 through September 25, 2007 Stratus field staff observed Woodward Drilling Company of Rio Vista, California , (Woodward) air knife and then install two on-site groundwater monitoring wells (MW- 11 and MW-12) and one off-site groundwater monitoring well (MWA3) . On January 7, 2008 , Stratus field staff observed Woodward air knife and then install one off-site 548 Work Plan for Well Destruction.doc SECOR International Incorporated