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10 INTRODUCTION <br /> Mittelhauser Corporation (Mittelhauser) has prepared this report for Unocal <br /> Corporation (Unocal) to document the remediation activities that have occurred at <br /> former Service Station #6348 located at 3788 Tracy Boulevard in Tracy, California <br /> (see the Site Location Map, Figure 1 ) The successful intent of this work was to <br /> identify and remove sols containing hydrocarbons by removing the underground <br /> storage tanks (USTs) and preforming overexcavation in the affected areas <br /> The subject site was in operation from 1972 until March 1992 for the retail sale of <br /> unleaded gasoline and maintenance of automobiles Mittelhauser performed an initial <br /> series of exploratory drilling beginning in July, 1991 (see the Predemolition Site Plan, <br /> Figure 2) <br /> 20 PREVIOUS INVESTIGATIONS AND REMEDIATION ACTIVITIES <br /> In June 1990 an underground waste-oil storage tank located near the northeast corner <br /> of the station building was removed because of suspected leaks During the removal, <br /> the pit was over-excavated to remove hydrocarbon-affected soil <br /> In July 1991 , four monitoring wells (MW-1 through MW-4) were installed and <br /> additional soil borings were drilled to assess the level and extent of hydrocarbons in <br /> .a soil and groundwater Total petroleum hydrocarbons as gasoline (TPH-G) were <br /> identified in soil near the fill spout of the southern 10,000 gallon UST The results of <br /> the investigation were presented in the Preliminary Investigation and Evaluation Report <br /> by Mittelhauser dated January 31 , 1992 (Appendix A) <br /> Quarterly groundwater sampling results indicated persistent levels of dissolved <br /> hydrocarbons in well MW-3 located adjacent to the USTs Dissolved hydrocarbons <br /> have generally not been detected in the site perimeter monitoring wells (MW-1 , MW-2, <br /> i <br /> /"ittelhauser <br /> C 0 c P 0 a + f I O <br /> P2I88R008 wp1031e41 <br />