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1 Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA, Page 29 7.0 DISTRIBUTION OF HYDROCARBONS IN THE SUBSURFACE The several stages of tank removal, excavation and removal of contaminated soil and groundwater, site characterization and groundwater quality monitoring that have been conducted at the 7500 West Eleventh Street site since December 1998 have permitted synthesis of historic, geologic, hydrostratigraphic and geo-chemical data. That synthesis has resulted in a tri-fold interpretation of the distribution of hydrocarbons in the subsurface. The site is affected by a primary plume of diesel and gasoline that has _ affected both soil and groundwater. In addition, a secondary plume of diesel and gasoline affects the soil on the subject property and in some nearby areas. Finally, there are dispersed, low concentrations of a variety of hydrocarbons affecting wide areas of the shallow subsurface beneath the 7500 Eleventh Street property and nearby land that have sources different from those of the primary and secondary plumes of diesel and gasoline. 7.1 Areas where Shallow Soil is Affected by Hydrocarbons The areas where shallow soil is affected by low concentrations of hydrocarbons are r shown on Figure 12. In addition to the area where the dispenser pumps were formerly located on the subject property, they include an extended area of that site along its West Eleventh Street frontage and other areas along both sides of West Eleventh Street and along Chrisman Road. The hydrocarbons detected in the latter areas can be attributed to minor leaks and spills that had resulted from vehicles using unpaved areas of the former service station and parking on the shoulders of the streets over the past decades (The San Joaquin Company 2001e). Information obtained from exploratory borings and from samples recovered when groundwater-quality monitoring wells were installed also indicates that fill used to bring West Eleventh Street, Chrisman Road and some of the adjoining sites to their present grade included some debris from bituminous macadam -:r paving. For example, when the boring for Monitoring Well MW-8 was drilled, small fragments of bituminous macadam were seen in a sample of obvious fill material recovered from a depth of 5 ft. 7.2 Releases of Hydrocarbons from Underground Tanks on Nearby Sites Y� `= Figure 12 shows six sites in the neighborhood of the 7500 West Eleventh Street property where underground fuel storage tanks were formerly located. 7.2.1 23801 and 23901 Chrisman Road Investigations conducted following removal of tanks from the ADS site at 23801 Chrisman Road in 1989 (WaterWork 1989) and from the Suburban Propane site at 23901 Chrisman Road in 1994 (Professional Service Industries, Inc. 1996) showed that, except for minor amounts of soil in the pit bottoms, the soil and groundwater at those sites were unaffected by components of fuel hydrocarbons. r sic