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1n 2101 Webster Street S 12th Floor Oakland, CA 94612 (510)'663-4100 - FAX (51 O) 663-4141 GEOIKIATRIX March 22, 2000 i Project 6115.008 Ms. Katie Hower Chevron Environmental Management Company �'� P.O. Box 6012 San Ramon, California 94583 Subject: Work Plan for Off-Site Investigation Pombo Property(San Joaquin County APN 232-170-07) Tracy, California Dear Ms. Hower: Geomatrix Consultants, Inc. (Geomatrix),has prepared this work plan for additional investi- gation of petroleum hydrocarbons in the subsurface near the Pombo Property(Figure 1). Soil and groundwater investigations were conducted at the Pombo property.by Earth Systems Consultants:and Geomatrix in July and September 1999, respectively. The,.results of these investigations indicat&the presence of petroleum hydrocarbons in the subsurface (Figure 2). The off-site lateral extent of petroleum hydrocarbon-affected soil and groundwater quality downgradient of the petroleum hydrocarbon-affected soil has not been assessed. The vertical extent of the petroleum hydrocarbon-affected soil has been assessed. Geomatrix proposes advancing one off-site boring upgradient of and up to five off-site bor- ings downgradient of the petroleum hydrocarbon-affected soil on the Pombo property (Figure 3). Two soil borings will be advanced to an approximate depth of 15 feet below ground surface(bgs) in the West Side.Irrigation District (WSID) easement north of the site, and if necessary, three additional(step-out)borings will be advanced in the residential neigh- borhood north of the easement. Per City of Tracy requirements, Geomatrix will remove and replace sections of sidewalk at step-out boring locations. Prior to drilling,Underground Service Alert(USA)will be notified, the soil boring locations will be cleared by a private underground utility locator;and appropriate permits will be ob- tained. Borings will be advanced using a hydraulically driven, direct-push core barrel and drive sampler. Soil recovered from continuous_cores will be examined and logged in accor- dance with the Unified Soil Classification System(USCS) as,presented in the American Soci- ety of Testing and Materials Standard D2488-90. One soil sample will be collected for chemical analyses from each boring at either the area of soil core most visibly affected with petroleum hydrocarbons, or from just above the groundwater table if petroleum hydrocarbons are not identified visually or through screening using an organic vapor meter. Soil samples will be collected in brass or stainless steel liners, sealed with Teflon sheets, plastic end caps, and tape, labeled, and stored in an ice-cooled chest. Geomatrix Consultants, Inc. Engineers, Geologists, and Environmental Scientists