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2009 Quik Stop Station No. 76 March.1.8, Stockton,California Soil Gas Investi ation Work Plan 4 The EHD, in consultation with the RWQCB, reviewed data from this site and the neighboring site at 4075 E. Main Street and concluded in the 18 February 2009 letter that, at the .Quik Stop, site, soil impacts have been minimal and that concentrations of total petroleum hydrocarbons as gasoline-(TPHg) in groundwater have decreased ,significantly. On the basis of the review,the EHD requested a work plan for a soil gas investigation. Proposed Soil Gas Investigation Gasoline hydrocarbon vapors (soil gas) that may emanate from residual impacted groundwater or soil have the potential to intrude into building enclosed spaces and complete the-inhalation exposure pathways. A soil gas investigation (SGI) will provide data on which t to -evaluate the potential'for soil gas that may_exist in the subsurface to intrude into the station building. This..work plan -was developed in general accordance with soil gas investigation guidance..contained with the California Department of Toxic Substances Control (DTSC)'Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to-Indoor Air-Interim Final dated 15 December 2004-(revised 7 February 2005), and the California Environmental Protection Agency (Cal EFA) Advisory—Active Soil Gas Investigations dated 28 January 2003. w Five soil gas-sampling points (SG-1 through SG-5) are proposed at the locations shown on Figure 3. The soil gas sampling points were selected on the basis of the position of the site building, and accessibility to areas between the site building and existing and former UST/pipeline ' systems while avoiding known underground installations. Further consideration was given to known .soil and groundwater conditions at the neighboring site at 4.015 E. Main-Street. Three soil gas sampling points are located along the eastern property boundary north and south'of the site building. Two sampling points are located between the former and existing UST basins and the fuel .dispensers west of the- station building. If conflicts with underground utilities occur, actual sampling locations may vary. To increase the likelihood of adequate soil permeability, the soil gas investigation will not be conducted during or within one day.after a significant rain event (e.g., 0.5 inch or greater). Soil gas collection probes shall be advanced utilizing a direct-push rig. If advancement of the soil.probes to the target depth is unsuccessful, the sample probe shall be re-advanced 3 to 5 feet from the initial .location. Advancement of soil probes to target depth shall be attempted up to three times prior to abandoning the location or, depending on the refusal depth, the sample may be collected at.a shalom er de th. :Soil gas samples shall be analyzed onsite immediately after collectiop. utilizing a rn bile-laboratory. Installation -of soil gas collection probes, sample collection and sample analyses shall be conducted by the soil gasinvestigation contractor in general, accordance with the DTSC and Cal EPA guidelines, and in accordance with the drilling contractor protocol. Fieldwork conducted by Horizon shall be in accordance with, where applicable, the attached Horizon Methods and.Procedures. Important data, events and times shall be recorded on daily logs during the fieldwork. The proposed soil gas investigation shall consist of the following tasks: SGI WP Mar09 Project No.7076.15 2 HORIZON ENVIRONMENTAL INC.