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$460r AEGIS ENVIRONMENTAL , INC . 1050 Melody Lane, Suite 164, Roseville, CA 95678 ♦ 916. 782-2110 1916-969-21101 FAX 916.786-7830 June 25, 1992 a n Mr. Harlin Knoll RECEI Environmental Health Division J U N 2 9 1992 Public Health Services ENVIRONMENTAL HEALTH San Joaquin County PERMIT/SERVICES Post Office Box 2009 Stockton, California 95201 Subject: Reduction of Monitoring Frequency Shell Oil Company Service Station (WIC No. 204-7884-0901) 3725 Tracy Boulevard, Tracy, California Dear Mr. Knoll: Aegis Environmental, Inc. (Aegis), on behalf of Shell Oil Company, requests the frequency of groundwater monitoring at the subject site be reduced from quarterly to annually. Three wells on site have been monitored on a quarterly basis since August 1990. Laboratory analyses of groundwater samples collected from the wells to date indicate concentrations of total petroleum hydrocarbons (TPH), as gasoline, and benzene, toluene, ethylbenzene, and total xylenes (BTEX) have not been detected above practical quantitation reporting limits. Results of the most recent monitoring, performed by Aegis on February 14, 1992, are included in the attached "Quarterly Groundwater Monitoring Report." Three underground storage tanks were removed from the site on May 3 and 4, 1990. Water was encountered in the tank excavation at a depth of approximately 9 feet. A "grab" sample of the water in the tank excavation was collected by Delta Environmental, Inc. (Delta). Laboratory analysis of the water sample indicated a concentration of TPH, as gasoline, of 500 parts-per-billion (ppb) and a concentration of benzene of 25 ppb. The estimated direction of groundwater flow beneath the site has consistently been to the northeast. if petroleum hydrocarbons in groundwater had migrated from the former tank basin, detectable concentrations should have been reported in groundwater samples from the downgradient well, MW-1A. 90-089A.LTR GEOLOGISTS • ENGINEERS * GROUNDWATER SCIENTISTS