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1.0 INTRODUCTION This Work Plan outlines Site Groundwater Assessment activities using step-out sampling techniques to be conducted at the Site known as Joe's Travel Plaza, located at 15600 Harlan Road, Lathrop, California(Figure 1). The site is identified by the local oversight program (LOP)#231585. Based on the results of recent soil and groundwater sampling data collected from the Site on June 26 and 27,2006(by others),the complete lateral and vertical extent of Methyl tertiary butyl ether(MtBE) impact to the groundwater has not been determined. Additional sampling is necessary to identify the extent of the impact to the north/northwest,west,and southeast of the Site. The purpose and objective of this Work Plan to further delineate MtBE groundwater concentrations both laterally and vertically. This Work Plan describes proposed efforts to further delineate the extent of groundwater impact using a step-out sampling technique. 2.0 SITE SETTING Joe's Travel Plaza is an operating service station and is located at 15600 Harlan Road in Lathrop, California. The service station consists of four automobile fuel dispenser islands, seven fuel dispenser islands for large trucks, four 10,000-gallon diesel fuel aboveground storage tanks(ASTs), one 4,000-gallon diesel fuel underground storage tank(UST),and three USTs holding 12,000-, 10,000-, and 4,000-gallons of gasoline. The contaminants of concern(COCs)for this site are the result of petroleum hydrocarbons in soil and groundwater from a fuel release. MTBE is the primary chemical of concern in groundwater at the Site. Figure 2 depicts the Site Layout. 2.1 Background It has been reported a release of gasoline occurred at the Site but the date is not known as well as the amount. Since the release numerous investigations and site assessment activities have been conducted resulting in the installation of numerous groundwater monitoring wells(Secor Int. Inc. June 2006). Currently 15 groundwater monitoring wells are monitored and show evidence of groundwater impact approximately 350 feet down gradient from the current USTs. Figure 2 depicts the monitoring well locations. Distribution of petroleum hydrocarbons as diesel in groundwater appears to be concentrated in the vicinity of the former USTs pit in the southeast comer of the site (monitoring well MW-4),the dispenser island and current USTs(MW-1), and north (MWO-1X)and northeast(MW-8)of the current USTs. The extent of diesel impact to groundwater has not been delineated to the west. Methyl tert-butyl ether(MTBE) has been detected in wells and is migrating along the gradient to the north/northeast. The extent of MTBE impact not been delineated to the west,northwest,and east. MTBE concentrations in groundwater extend in a down gradient direction in excess of 300 feet from the Site to the vicinity of MWO-3, as reported in the Quarterly Environmental Remediation Group Page 1 Site Assessment Work Plan Project No.0406 Joe's Travel Plaza