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I SITE CONCEPTUAL MODEL 1.0 BACKGROUND 1.1 Site Location and Existing Conditions The Caltrans Stockton Yard Maintenance Station (the Site) is located at 1604 B Street in Stockton, California. The approximate site location is depicted on the attached Vicinity.Map, Figure 1. The Site contains various maintenances,buildings, warehouses, several office buildings, two aboveground fuel storage tanks and a gas house. The Site is bounded to the north by Clark Well and Equipment Co., to the east by vacant land, to the northeast by a mobile home park, to the south by residential development and to the west by B Street and the Caltrans Shop 10 Maintenance Station. The existing improvements are depicted on the Site Layout,Figure 2. 1.2 Background In December 1989, one 5,000-gallon diesel underground storage tank (UST) and one 10,000-gallon gasoline UST, two pump islands and associated productpiping were removed from the western portion of the Site. Analytical'results of soil' samples collected from the UST excavation reported total petroleum hydrocarbons as gasoline and diesel (TPHg and TPHd) concentrations up to 2,000 and 58 milligrams per kilogram (mg/kg), respectively, from the east and west ends of the former gasoline UST, respectively. In September 1993, one 115-gallon backup diesel generator UST was removed from the eastern portion of the Site. Soil discoloration and odors were reportedly not observed during the 1993 UST removal activities. TPHd, benzene, toluene, ethylbenzene and total xylenes (BTEX) were not reported for the soil samples collected from the 1993 UST excavation and stockpile. The former UST locations and soil samples location for the former gasoline UST are presented on LFigure 2. The exact locations of the soil samples collected from the former diesel UST and former diesel generator UST are not known. Twelve groundwater monitoring wells (MW-1 through MW-12) and two vapor extraction wells (VW-1 and VW-2) were installed at the Site between 1992 and 1996. Boring logs are presented in Appendix A. Results of soil samples collected from the borings are presented on Tables 1 through 3. Well construction details are provided on Table 4. With the exception of soil samples from depths of 46 and 51 feet in boring VW-2, elevated petroleum hydrocarbon concentrations were not reported for each of the soil samples for the borings/wells. The estimated limits of hydrocarbons in soil are depicted on the Site Plan,Figure 3. Groundwater monitoring has been conducted at the Site since April 1992 for wells MW-1, MW-2 and MW-3, since May 1995 for wells MW-4 through MW-8 and VW-1, and since February 1996 for wells MW-9 through MW-12 and VW-2. TPHg, TPHd and benzene concentrations up to 54,000, 11,000 and 13,700 micrograms per liter (µg/1), respectively, have been reported for the groundwater samples. Project No.Stockton Yard,Task Order No.45 -1- Caltrans Contract 03A1368,EA No.10-ON8300 Project No.S9300-06-45 January 28,2609