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URS Mr Mike Infurna Page 3 • San Joaquin County Environmental Health Department January 5, 2004 vacuum of 25 inches of water are estimated to produce an effective radius of influence of 25 feet Given these results, vapor extraction was deemed a feasible method of soil remediation at the site In August 1992,nine so.l borings were drilled and sampled for TPHg and BTEX to determine the extent of soil contamination at the site Soil sampling results identified the soil contamination was limited to the UST complex and immediate vicinity Contaminants were identified at the following maximum concentrations TPHg, 520 mglkg, benzene, 3 4 mg/kg, toluene, 23 mglkg, ethylbenzene, 14 mg/kg, and total xylenes, 69 mg/kg Vapor extraction wells were installed in borings B-9 through B-15 One additional on-site well (MW-4)and one air sparge well (AS-1) were installed in borings B-16 and B-17 In October 1992,an air sparge pilot test was performed at the site Based on the test results and the excessive costs to install sparge wells for all impacted areas, the test report recommended installation of a limited number of sparge wells where the majority of the contamination exists �f In November 1992,two off-site borings(B-21 and B-22)were drilled and soil sampled collected Petroleum hydrocarbons were not detected in the soil samples from these borings Monitoring wells MW-6 and MW-7 were installed in these borings In August and September 1993,four former gasoline USTs of various capacities were replaced with the four existing 10,000-gallon USTs The product piping was also replaced at this time Soil samples collected beneath the former USTs product piping indicated that soil was impacted with TPHg and BTEX in these areas up to 4,000 mg/kg TPHg,2 7 mg/kg benzene, 120 mg/kg toluene, 100 mg/kg ethylbenzene, and 620 mg/kg total xylenes In November 1993,three additional off-site borings(B-23 through B-25)were drilled and monitoring wells MW-8 through MW-10 were installed in these borings Soil samples collected from these borings did not identify petroleum hydrocarbons An SVE system that utilized 12 SVE wells began operating in August 1995 and was operated continuously through November 1997 EMCON performed monitoring,operation,and maintenance of the system During } the operation of the system,EMCON reported that extraction rates for TPHg and benzene decreased by 78% and 99 9%,respectively As of June 1998,EMCON reported that the system was inoperable Remediation of groundwater continued by injecting ambient air at low flow rates into selected wells to enhance biodegradation Monitoring wells MW-8 and MW-9 were destroyed and are no longer included in the monitoring program Rising groundwater levels have caused the screens of the site monitoring wells and vapor extraction wells to become partially or fully submerged As of September 2000,EMCON concluded that insufficient screen is available to effectively operate the SVE system Groundwater elevations remain elevated and ARCO does not intend to use the SVE system in the future unless site conditions change i K 1Wprocessl15 BP ARM02130\Sitc Conceptual Model ReportM30 SCM Rev 2 doc