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28 April 1995 AGE-NC-95-0103 Page 7 of I 1 I5 1 4 MONITORING ACTIVITIES I Monitoring of vapor extraction systems involves weekly measurement of Vapor concentrations at the inlet to the destruction unit and periodic service and repair Systems can be equipped with automatic telecommunication system to alert the operator of system malfunction or failure Vapor samples should be collected monthly to monitor the efficiency of the remediation program When the concentration of extracted vapors decreases and stabilizes after se,,eral months of operation, borings are drilled to collect soil samples for quantitatv,e analysis in the laboratory 51 5 FEASIBILITY Generally, a vacuum of l-inch of Dater is required to successful]) Nolatillze subsurface hydrocarbons The nlaxnnum vacuum measured at NMI-I and NIW-3 (at 1800 rpm) Naas plotted ' versus the distance of each Nell from the extraction x\ell Figure 4 is a graph of distance \s vaCUL1111. for 11ff-1 and MW-3 Drawing a best-fit line throuoh these data points will deteiimne the effective radius of influence at this site r I Frons Figui e 4, the effective radius of influence at the site appears to be approximately 29 feet Figure 4 predicts a vacuuni of less than our detection le\el in NIW-2, corresponding to field data A vacuum of 33 inches is predicted at M-1, corresponding to t,,pical \acuum generated by a VES The screened inter\al of the monitoring wells, \Oic11 ,\ere used as observation ��ells for this test, is at and belox� the screened interval of VW-1 therefore, observations form these hells should be indicati\e of conditions at the site Peti oleum hydrocarbons N\ere detected in soil samples ti oin V\\-I at depths of 25, 35 and 45 feet Most of the petroleum h)di ocai bon impacted soil is �N ithin 15 feet of the bottom of the former UST cavity HoN�e\er, small concentrations of toluene and xylenes are present at a depth of 45 feet Plugging or filling the bottom 25 feet (72 to 47 feet depth) of VW-1 will facilitate a more effective iemediation rate, targeting the impacted zone Petroleum hydiocarbons were not detected in soil samples from X IW-3 or 11911'-4 However, Iow concentrations of petroleum hydiocaibons weie detected in Nff-1 at a depth of 45 feet Therefore, the impacted soil appears to be limited to the area immediately around the former UST Iexcavation, apparently within a 20-foot radius I A drawback of vapor extraction is that the i ernediation rates will vary depending on the characteristics of soil layers For instance, sandy soil Nvill cleanup more quickly than more fine-grained soil, such as silt or clay This method is not usually effective rn soils Nvith 111911 clay content I