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1 <br /> 2.4 Recent Investigation <br /> During the fourth quarter of 1996, Emcon o_f Sacramento drilled two new soil borings to gather data <br /> for preparation of a Risk-based Corrective Action Plan One boring is located near MW-5, and one <br /> is located approximately 15 feet east of VE-1 (Figure 2) This boring was completed as a vapor <br /> extraction well and has replaced VE-i "deep" The new well is screened from 35 to 55 feet and is <br /> ' referred to in this report as VE-2 <br /> ' 3.0 VAPOR EXTRACTION SYSTEM OPERATION AND MONITORING <br /> 3.1 System Operation <br /> During the quarter, the system operated for a total of 2 055 hours (Appendix A) It operated for 29 <br /> days in April, 30 days in May, and 30 days in June During that period, 4,967,700 cubic feet of <br /> vapor were extracted This brings the total to approximately 66,560,000 cubic feet since July 1994 <br /> ' 3 2 Monitoring Summary r <br /> The extraction unit's on-board computer monitors several important parameters of engine <br /> ' performance, including air flow rates, well vacuum, and volume of extracted vapor These data are <br /> recorded hourly and averaged over the 24-hour day The daily averages are shown in Appendix A <br /> Hydrocarbon concentrations in the vapor stream (influent soil gas) and effluent (combustion <br /> products) were manually monitored 4 times during the quarter with a Microtip 2020 photo- <br /> ionization detector In addition, vapor samples were collected from the VE-1 "shallow" influent line <br /> ' and fom the VE-2 influent line on June 18 The samples were collected to tedlar bags and delivered <br /> to Sequoia Analytical for analysis of TPH-g and BTEX concentrations The results, reported in <br /> milligrams per cubic meter, are shown in Appendix B <br /> PID readings at the exhaust port ranged between 0 and 23 ppm during the quarter Effluent readings <br /> did not follow upward or downward trends in inlet concentrations PID readings from VE-2 were <br /> ' measured both at the well inlet and at the water dropout drum on most occasions In most instances, <br /> concentrations at the drum were between 70% and 95% of those at the well head Well-head <br /> concentrations ranged from 580 to 1,500 ppm Readings from VE-1 `Eshallow" ranged from 500 to <br /> ' 1200 ppm, and readings from the combined inlet generally fell between those of VE-1 and VE-2 <br /> Readings at all sample ports trended slightly downward during the quarter <br /> 3 , <br /> 1 <br />