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' • 0 4 PID readings are graphed in Figure 5 to illustrate the effect of the system on soil vapor concentrations Flow rates on operating days ranged from a low of 30 cfm to a high of 46 cfm For most of the quarter, _ the unit operated at somewhat lower rates than in the past ' Figure 5 shows some interesting trends in vapor concentrations Since January of this year, there has been a marked increase in vapor concentrations in the shallow-well inlet stream Concentrations have risen from 500 ppm to more than 1,000 ppm, and have exceeded 1,500 ppm on several occasions In contrast, concentrations in the deep well dropped dramatically in November of last year and have continued to decline most of this year 'Concentrations in the combined stream have generally followed the trend in the shallow-well stream, implying that most of the vapor flow is coming from the shallow well rather than the deep well This is very probably due to the high water table, causing restriction of the air flow to the deep well However, the sharp increase in vapor concentrations in the shallow well stream is difficult to explain, and concentrations have remained above 1,000 ppm even since the sample ports were moved to bypass the vapor-contramtnated hoses and drums Perhaps the increase is partially due to volatizatton of gasoline from the groundwater, caused by air sparging in the deep well The apparent increase in vapor concentrations in the shallow well is further complicated by comparison of the laboratory result (Appendix B) of the sample collected on May 23 with the PID reading on that date When converted from parts per mullion by volume (ppmv) to parts per million by weight, the laboratory result of 490 ppm for the shallow well is only about 20% of the value of the PID reading of 2,220 ppm Since the samples and PID readings were taken from the new ports, the disparity cannot be attributed to cross-contamination from the equipment Experience has shown that PID readings can fluctuate widely during measurement, but usually decline rather steadily after reaching an initial maximum If such a decline were to continue during transport to the laboratory or during storage at the ' laboratory, laboratory measurements would undoubtedly be considerably lower than PID readings in the field This has not always been the case, however, and laboratory readings have sometimes ' exceeded field measurements 4.2 Weight of VOC's Destroyed 1 The weight of VOC's extracted during the quarter can be computed from the mean vapor ' concentration in the shallow well and the cumulative volume of extracted vapors 1 WEIGHT OF VOC'S =CF x VAPOR x 10-6 x 100 mol/wt ' 379 cf/mole =3 402 x 106 fe x 1364 x lex 100 mollwt 379 cflmole _ ' 4.640 x 10 5lbs 3 79x 102 = 1224 lbs 4