ARCHIVED REPORTS XR0011739 - Laserfiche WebLink

Home Browse Search

Sue Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 27 severely affected by leakage from the maze of underground pipelines that had fed the fueling station's dispenser pumps. By superimposing the information discussed above, together with information regarding the presence of visual and olfactory indicators in soils included on the boring and well logs, onto hydrostratigraphic section B-B' (Figure 11), it is possible to develop a graphic representation of the zone of the subsurface along the section line that is affected by fuels that leaked from the maze of piping. That representation is shown on Figure 15. A shaded, lens-shaped area indicates the presence of a primary plume of diesel and gasoline in the subsurface that has a geometry typical of a distribution of hydrocarbons that is encountered at sites where leaks from sources located above the groundwater table have occurred. That zone of affected soil has a maximum thickness of some 20 ft, of which the upper approximate 5 ft is above the water table. The zone reaches a maximum depth of approximately 25 ft BGS in the area around monitoring well MW-3, which is directly down-gradient from the area of the site where fuel leaked from piping that served the former fueling station's dispenser pumps. To the west of MW-3, the thickness of affected soil in the plume reduces progressively until, at a location between PP-2 and MW-2, it pinches out entirely. The affected zone of soil within the plume is shown as having a similar lens-like geometry to the east of MW-3, but conditions in the subsurface to the east of MW-4 were not investigated, due to the prohibition imposed by the USTCF staff on the location of any wells or borings east of Chrisman Road. The assumed eastern limit of the lens of soil affected by gasoline and diesel shown on Figure 15 is based on consideration of the symmetry that would be a characteristic of plumes of hydrocarbons that through uniform, porous media. Because no borings could be drilled in that area, the hydrostratigraphy to the east of MW-4 may be different from that assumed, and the lateral extent of the zone of affected soil on the east side of the site may differ substantially from that shown on Figure 15. It is recognized that not all of the locations in the subsurface where components of fuel s, hydrocarbons were detected in samples recovered from the borings are included within the lens-shaped zone shown on Figure 15. Inspection of Tables 1 and 2 reveals that hydrocarbons in locations outside of that zone are of various compositions (see notes at foot of Table 1), are all at very low concentrations (less than 4 mg/Kg), and are at shallow depths that are generally above the water table in areas where there is no evidence of the past existence of leakage of fuel to the subsurface from a point source. ' The type of dispersed occurrences of low concentrations of fuel hydrocarbons described above can be identified over essentially all of the areas of the 7500 West Eleventh Street property and neighboring areas that were explored by the site characterization program. The presence of these arealy-distributed, low-concentration hydrocarbons is indicated on Section B-B' on Figure 15 by cross-hatching. Given their apparent ubiquitous presence in the near-surface soils and their disparate compositions, it is reasonable to assume that their source was spills, drips and leaks onto the unpaved surfaces of the 7500 West sic