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September 9, 1996. <br /> Page 2 <br /> steel construction, and apparently were used to store leaded gasoline. These tanks were r. <br /> observed.in an exploratory test pit on June 26,•1995. Within the test pit, the,tank tops were <br /> %encountered at'a depth of approximately 4'feet belw <br /> o -ground surface The USTs area <br /> estimated to 4 feet'in-diameter; therefore; the UST bottoms are.located at approximately 8 . <br /> feet below grade. . <br /> UST"Removal <br /> Unocal has applied for an UST closure permit and plans to remove the two 550-gallon <br /> capacity USTs during,the third quarter of 1996: These USTs will be removed by excavation <br /> _into the sidewalk right-6f-way., The approximate a�ea of proposed excavation is-shown on <br /> Figure 1. The estimated depth of excavation willb� 9 feet, as measured from.the existing <br /> sidewalk grade. Following.'removal of the"USTs and all associated backfill material, PACIFIC <br /> will. collect soil-samples in native soils to be submitted for chemical analyses for the.presence <br /> or absence of petroleum hydrocarbons. Should field evidence, such as soil staining and/or <br /> odor;.:or chemical analysis suggest-thatpetroleum hydrocarbons are present, Unocal may <br /> perform additional (over) excavation to remove impacted soils. <br /> Over. Excavation <br /> If field evidence or chemical analyses suggest that soils beneath the,USTs are impacted by <br /> petroleum hydrocarbons, excavation of the impacted soils will be performed as a corrective <br /> action to remediate subsurface soils. Because groundwater occurs at approximately 10 feet,'it <br /> is anticipated that the maximum depth of excavation will be.-approximately.12 feet. The' <br /> resulting excavation will measure approximately I5 feet long, 10 feet wide and 12',feet deep. <br /> However, the,.fina1 extend of the excavation will be dependent upon the distribution of <br /> hydrocarbons in the subsurface. Soil samples wilt be collected at the base of the excavation..to <br /> assess soil conditions following the corrective action. If groundwater is encountered, .Y <br /> groundwater and soil samples will be collected and submitted for analysis.'. <br /> Dewatering <br /> If groundwater,is encountered, dewatering of'the excavation will be accomplished by the use <br /> of pumps provided by the Unocal's UST removal c�ntractor. The resulting volume of <br /> groundwater will be pumped directly into a vacuum"truck provided by RUST Industria of <br /> Benecia, California_ The purgewater will lie treated afthe Unocal Rodeo,Refinery through <br /> the water treatment system, and discharged to the San Pablo Bay under-Unocal's NPDES <br /> permit. <br /> Soil Disposal <br /> Based on the:estimated excavation volume, PACIF C'anticipates approximately 100 cubic <br /> yards of soil will be generated from the over-excavation activities. The soils will be <br /> 31009718/CAP <br />