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Work Plan for SVE/AS Feasibility Testing. S E C 0 R ARCO Service Station 548 Stockton, California Page 2 of 9 r 40 feet, respectively. The investigation concluded that hydrocarbon impacted soil was limited to the proximity of the UST complex. In October 1991 , four on-site groundwater monitoring wells (MW- 1 through MW4), and three soil vapor extraction (SVE) wells (VW-1 , VW-2, and VW-3) were installed . Soil samples collected identified 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 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 feet. In 1993, two additional monitoring wells (MW-5 and MW-6) and one SVE well (VW-4) were installed . Soil samples did not contain detectible 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 conducted a site evaluation for prioritization of remediation efforts at ARCO sites in San Joaquin County. EMCON calculated approximately 383 pounds (Ibs) of total petroleum hydrocarbons at the site (URS , 2005a) . i t In February 1998, four gasoline USTs 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 , AS-2, and 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. Alf SVE/AS system utilizing six SVE wells (VW- 1 through VW-4, VW-5S and VW-5D) and three air sparge wells (AS-1 through AS- 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 USTs 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. Currently all groundwater monitoring , vapor, and air sparge wells are sampled quarterly. Well construction details for all site wells are presented in Table 1 .