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1.0 INTRODUCTION This Workplan was prepared by Winzler& Kelly on behalf of the State of California Department of General Services, Real Estate Services Division(DGS-RESD), under Contract No. 3152182. This scope of work was requested by the Central Valley Regional Water Quality Control Board (CVRWQCB) for the Former Blue Star Gasoline Station(site) located at 4040 East Main, Stockton, California(Figure 1). This Workplan has been prepared to initiate a subsurface investigation to determine if the soil and groundwater beneath the site have been impacted from the former underground storage tanks (USTs). The scope of work includes the advancement of one exploratory soil boring, installation of two groundwater monitoring wells, and two soil gas monitoring points (see Site Map, Figure 2)to determine the extent of on-site petroleum hydrocarbon impacts. A 2,000-foot sensitive receptor survey will be performed and the new monitoring wells will be used to implement a quarterly monitoring and sampling program. 1.1 OBJECTIVE The objective of the proposed work is to determine the vertical and lateral extent of hydrocarbon impacted soil and groundwater in order to assist with moving the site towards environmental case closure. Based on previous investigations conducted at the site, hydrocarbon impacted soil is present beneath the former USTs and residual groundwater contamination is present in the on-site monitoring wells. 1.2 SITE DESCRIPTION The topography at the site is relatively flat and the property is surrounded by mixed commercial and residential land uses. The three former USTs were located on the eastern side of the property, near Oro Street. Two former dispenser islands were located to the west of the northernmost former UST. Groundwater was measured at approximately 57 feet below ground surface (bgs)and flows in an easterly direction, as reported in historical quarterly groundwater monitoring reports (Appendix A). 1.3 SITE HISTORY Winzler&Kelly has reviewed the available project files and reports; however,we have not verified the information, and therefore, Winzler&Kelly does not take responsibility for the accuracy of the data reported herein. Based on the file review of the available data,the site contains residual soil impact from gasoline and diesel components,primarily beneath and to the south of the former USTs. Three former USTs were reportedly removed from the site in 2002, including one 6,000-gallon gasoline UST, one 12,000-gallon gasoline UST, and one 4,000-gallon diesel UST. Soil samples collected within the immediate vicinity of the former USTs have been reported to contain: total petroleum hydrocarbons as gasoline (TPH-G) up to 24 milligrams per kilogram (mg/kg), benzene,toluene, ethyl benzene, and total xylenes (BTEX)constituents up to 13.6 mg/kg,and total petroleum hydrocarbons as diesel (TPH-D)up to 0.015 mg/kg. Soil data from a subsurface investigation performed in 2003 indicated the presence of elevated concentrations of TPH-G(up to 1,800 mg/kg), BTEX (up to 170 mg/kg)and TPH-D (up to 580 mg/kg). Although 200 tons of soil was reported to be excavated during UST removal in 2002,the soil was placed back into the excavation due to the proximity of the adjacent public street. Three groundwater monitoring wells were installed in 2005, although two of the wells ended up in the upgradient groundwater flow direction. Groundwater samples collected during periodic sampling events conducted in 2007 reported the presence of TPH-G up to 300 micrograms per liter(µg2),methyl tertiary butyl ether(MTBE)up to 120 gg/L,and the lead scavengers 1,2-dichloroethane(1,2-DCA)and ethylene 03009-10016-37020 2 WINZLER&KELLY August 2011