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1 Work Plan for Additional Assessment S E C O R ARCO Service Station 2130 Stockton, California Page 2 of 15 (cfm) and a vacuum of 70 inches of water, the consultant estimated the radius of influence to be 70 feet. A wellhead flow rate of 30 cfm and a vacuum of 25 inches of water produced an estimated radius of influence of 25 feet . In August 1992, eight soil borings (B- 9 through 13- 16) were drilled and sampled for TPHg and BTEX to assess soil contamination at the site. Soil contamination was limited to the UST complex. Borings B-9 and B- 16 were converted to monitoring wells MW4 and MW-5, respectively. Vapor extraction wells , VW-5 through VW-10, were installed in borings B- 10 through B- 15. One air sparge well (AS -1 ) was installed in boring 13- 17. An air sparge (AS) pilot test was conducted in October 1992. The test report recommended a limited number of AS wells be installed where the majority of the contamination exists. In November 1992, two off-site borings (B-21 and 13-22) were drilled and converted to monitoring wells MW-6 and MW-7. Petroleum hydrocarbons were not detected in soil samples collected from these borings. In August and September 1993, four USTs and associated piping were replaced with the four existing 10,000 gallon USTs. Soil samples collected beneath the former USTs and product piping indicated that soil was impacted with TPHg and BTEX. Three additional off-site borings (B-23 through B-25) were drilled in November 1993 and monitoring wells MW-8 through MW- 10 were installed . Petroleum hydrocarbons were not detected in soil samples collected from these wells. In August 1995, Borings B-26 through 13-29 were drilled in and around the UST complex. Borings 13-26 and 13-27 were converted to air sparge wells , AS-2 and AS-3. Both wells are located within the UST complex. Borings B-28 and B-29 were converted to vapor extraction wells VW- 11 and VW- 12. Monitoring well MW-8, MW-9, and MW- 10 have been destroyed and are no longer included in the monitoring well network. An SVE system utilizing 12 SVE wells began operation in August 1995 and was operated continuously through November 1997. EMCON performed monitoring, operation , and maintenance of the system . In June 1998 the system was shutdown due to low influent Ir concentrations. EMCON concluded that insufficient screen was available to effectively operate the SVE system. During operation , the SVE system removed approximately 9, 100 pounds of TPHg. Remediation of groundwater continued by injecting ambient air at low flow rates into selected wells to enhance biodegradation (URS, 2004) . Currently, air is being injected into VW- 10. Rising groundwater levels have caused the screens of the site monitoring wells and vapor extraction wells to become partially or fully submerged. Three new monitoring wells were installed on- site in July 2006 to monitor concentrations at first encountered groundwater. Groundwater monitoring has been performed at the site since March 1992. Tables 1 and 2 present the current and historical analytical data . Groundwater flow has generally been to the north- northeast at a hydraulic gradient of 0.003 foot per foot (ft/ft) (Table 3) . Well construction details are presented in Table 4.