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i Site Characterization Report: 7500 West Eleventh Street. Tracy, CA. Page 9 when temporary borings were closed by grouting and when soil and groundwater samples were recovered. 3.4 Subsurface Exploration Using Push-probe Borings On April 24, 2000, Spectrum Exploration, Inc., which holds a C57 license issued by the California Contractors State License Board, mobilized a truck-mounted drilling rig to the site. The drilling rig was used to advance small-diameter push-probe borings into the subsurface to explore the lateral and vertical extent to which the soil and groundwater beneath the site has been affected by releases of petroleum hydrocarbons. A total of 14 push-probe holes were drilled over a period of two days, extending from April 24 to April 25, 2000. The push-probe boring locations are shown on Figure 8. 3.4.1 Recovery of Soil Samples from Push-probe Borings Soil samples were recovered from the probe holes at discrete depths. To recover the samples, a 4-ft long, split-spoon sampling tool, with a %-inch diameter Teflon liner was introduced into the boring and driven downward from the bottom of the hole at each 4-ft. depth interval. The boring was continuously logged by a California-licensed geotechnical engineer. The boring logs are presented in Appendix A. The total depth of each push- probe hole was selected by the engineer based on knowledge of subsurface conditions F � that had been progressively obtained from the elements of the site characterization program that had been completed up to the time that a given boring was drilled and by visual and olfactory indications of the presence or absence of petroleum hydrocarbons in the soil recovered in the Teflon split-spoon liner. In general, soil samples were recovered from each boring at depth intervals of 4 ft. 1 However, where appropriate, the engineer selected specific sampling depths from other than the 4-ft. intervals, based on features of the stratigraphy and indications of the presence of petroleum hydrocarbons in the soil. There was significant loss of recovery in some soil zones. At each location where recovery was lost, it was caused by one or the other of the following two phenomena: 1) in some zones, the soil was a very wet and loose fine sand or. silty sand that flowed out of the sample liners before the split-spoon reached the surface; and 2) in other zones, pieces of gravel that are occasionally found in the clays, silts and sands beneath the site blocked the bottom end of the 1/2-inch diameter split-spoon. When that occurred, soil did not enter the split-spoon liner from the depth at which the gravel was encountered to the bottom of the four-foot sampling depth interval. Soil samplings from the borings were identified by the nominal depth from which they f were recovered, as measured by their position in the split-spoon liner from which they were cut. However, in cases where only partial recovery was made due to gravel blocking the entrance to the split-spoon, it was possible to adjust that measured depth so as to record the actual depth below the ground surface (BGS) from which the sample was raised. sic