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I GROUND ZERO ANALYSIS, INC. 1714 Main Street Escalon, California 95320-1927 Telephone: (209) 838-9888 Facsimile: (209) 838-9883 August 8, 2002 Mr. Harlin Knoll San Joaquin County Environmental Health Department 304 East Weber Avenue, Third Floor Stockton, CA 95202 Subject: Workplan for Additional Subsurface In estigation 8960 W. Walnut Grove Road, Thornton, A Dear Mr. Knoll: This Workplan is submitted on behalf of Barnyard Coip. in response to a San Joaquin County Environmental Health Department (EHD) directive. This Workplan describes the proposed methodology to install and sample three additional groundwater monitoring wells at the subject property. SITE BACKGROUND J F Site Description and Previous Investigations k The site is located at 8960 W. Walnut Grove Road in Thornton, California. The site is bounded on the north by Walnut Grove Road, on the west by Highway 1-5 and its off-ram p on the south by undeveloped land and on the east by low-income housing. The site location is shown on Figure 1. The site was formerly operated as a convenience store and retail gasoline station, which utilized one 10,000-gallon undergrounds rage tank (UST) to store diesel fuel and two 10,000-gallon USTs to store unleaded gasoline. A limited subsurface assessment conducted by Ground Zero in December 1996 provided evidence that petroleum hydrocarbons had impacted oil and groundwater beneath the site. The three USTs and associated piping were removed under permit from EHD on January 21, 1999. Analytical results of side wall soil samples and a grow dwater sample.collected from the UST excavation indicated that elevated levels of petroleum hydrocarbons were present in the soil and groundwater. Additional investigation was required by E14D. G:\Data1GROUND2.E1$amyazd-ManserlReportslWKPLAN_MW5-MW7.doc L