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Mr Kyle Christie Project 0805-114 04 August 7, 1995 ' Page 2 additional remedial wells were required and should include additional air sparge wells, a monitoring air sparge well, and vapor extraction wells Residual hydrocarbons were detected in soil to groundwater (approximately 60 feet below ground surface [bgs]) near the existing tanks, ' but only relatively low concentrations were detected in soil samples collected from the other on-site borings Gasoline hydrocarbons were not detected in the off-site soil borings Dissolved hydrocarbons have been detected in all wells Total petroleum hydrocarbons as gasoline ' (TPHG) concentrations have ranged from less than 10 parts per billion (ppb)to 18,000 ppb, and benzene concentrations have ranged from less than 0 3 ppb to 2,500 ppb. ' Based on the FRP, RESNA proposed the installation of a combined S VE and AS system to remove gasoline hydrocarbons from soil and groundwater beneath the site The system consists of a 100-standard-cubic-foot-per-minute catalytic oxidizer equipped with a 7 5-horsepower vapor extraction blower ' In September 1994, EMCON observed the final installation of the remediation system The system consists of an air sparging-enhanced SVE system with catalytic oxidation treatment (i.e., AIREX KATOX 100). In November 1994, the system was started up The system was shut down in February 1995 due to the catalyst malfunction The catalyst was replaced and the system was restarted in April 1995 ' Based on a letter from San Joaquin County Public Health Services (SJCPHS) dated August 29, 1994, the County requested an addendum to the PAR be submitted outlining remedial alternatives and further assessment activities necessary to delineate hydrocarbons north and south of the site. EMCON submitted three access agreement forms to the property owner north of the site and did not receive a response However, based on a mseetmg conducted on May 16, 1995, between SJCPHS, ARCO, and EMCON, SJCPHS concluded that the additional ' monitoring well north of the site was not necessary at this time Because relatively low concentrations of benzene have been detected in water samples collected from monitoring well MW-10 during the last four quarters (highest concentration of 0 9 ppb, and the hydrauhc gradient has been consistently toward the east to southeast, it is EMCON's opinion that a monitoring well south of monitoring well MW-10 (laterally cross-gradient) is not warranted at this time During the second quarter 1995, EMCON began preparation of a workplan to install additional remediation wells After approval of the workplan from SJCPHS, the wells will ibe installed during the third quarter 1995 SAC/N\pj0Q805N08051140 4cs-95 pas 1 i