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KEI-P91-0202 .QR5 October 27 , 1993 Page 3 DISCUSSION AND RECOMMENDATIONS Based on the analytical results for the ground water samples collected and evaluated to date, and no evidence of free product or sheen in any of the wells, KEI recommends the continuation of the current ground water monitoring and sampling program. The wells are currently monitored quarterly and sampled semi-annually. The results of the monitoring and sampling program will be documented and evaluated after each monitoring and sampling event, and recommendations for altering or terminating the program will be made as warranted. On May 27, 1993 , a representative of KEI reviewed the file for the ad3acent Arco station at the offices of the Regional Water Quality Control Board (RWQCB) , Central Valley Region, in Sacramento, California. The Arco station (#2076) is located at 800 East Kettleman Lane, approximately 200 feet southeast of the Unocal site. The purpose of the file review was to investigate the reported shallower ground water level at the Arco site, and to determine the current status of investigation and any remediation. A preliminary Investigation and Evaluation Report dated April 1, 1993 , by RESNA was contained in the file. Based on the information contained in this report, five soil borings were completed at the Arco site in October of 1990. Ground water was encountered at a depth of 30 feet below grade. Analysis of soil samples from the borings yielded non-detectable results for TPH as gasoline and BTEX. In July of 1991, three vapor extraction wells were installed at the Arco site. Analytical results of soil samples collected from the borings indicated non-detectable results for TPH as gasoline and BTEX. A vapor extraction test was subsequently conducted on these wells. In November of 1991, five underground storage tanks and product lines were removed from the Arco site and the new tanks were installed in a separate location. Soil samples obtained from beneath the tanks at a depth of 14 feet below grade indicated concentrations of TPH as gasoline up to 17, 000 ppm. Soil samples collected from the former product line trenches indicated concen- trations of TPH as gasoline up to 28 , 000 ppb. The former tank pit was not overexcavated and the tank pit and product line trenches were refilled with the native soil, based on the planned remedi- ation by vapor extraction. In October of 1992 , four vapor extraction wells and four monitoring wells were installed at the Arco site. Ground water was encoun- tered at a depth of 54 feet below grade. On October 23 , 1992 , the