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, , G <br /> , R604:UPDATE 11-JULY, 1992 <br /> August 6 , 1992 , 3 pounds per day of hydrocarbons are being removed <br /> from the subsurface via vapor extraction. <br /> ' As , can be seen when comparing the three per day effluent samples <br /> as shown on Table 3B (March 31 through April 24 , 1992) , a slight <br /> bleedthrough (channeling) is occurring from the last carbon, even <br /> during the first minutes of operation. This channeling occurs as <br /> {r - specific molecules of the contaminant in the vapor stream absorb <br /> � r3 i and "desorb" from the carbon surfaces and are not trapped in the <br /> micro or macro pores of the carbon. This absorption-desorption <br /> along with the low concentration of the influent stream ( 3 .12 to <br /> 1 .56 lbs/day) , which allows the hydrocarbons to remain in <br /> equilibrium in the vapor stream, does not allow an effective <br /> absorption (greater than 90% ) to occur . <br /> CONCLUSIONS AND DISCUSSION <br /> E Vapor extraction has successfully removed the lighter fractions <br /> (lighter than ethyl benzene) of the gasoline range petroleum <br /> ' hydrocarbons from the soil beneath this site and has removed all <br /> ` of the gasoline range hydrocarbons from the ground water <br /> associated with this site . The vapor recovery system, operating <br /> under the Permit to Construct, Permit # AP 91-106 has removed <br /> �lrtr ,� 13 ,924 .5 pounds of gasoline range hydrocarbons , see Table 3 . <br /> Vapor extraction has greatly reduced the dissolved contaminant <br /> ' found in the ground water . The March 5 , 1992 sampling occurred 80 <br /> 'I' days after the vapor recovery system was shut down and showed that <br /> since vapor recovery has been used at this site, MW1 has been <br /> f' 1 reduced from 2 mg/L TPH to <0 . 05 mg/L TPH. MWS , likewise has been <br /> F3 it reduced from 31 mg/L TPH to 0 . 08 mg/L TPH. The BTEX's have also <br /> r4 been removed from the ground water beneath the site to below the <br /> ; detection limits . Vapor extraction was resumed on March 31 , 1992 <br /> land has operated 1507 hours prior to sampling on June 2 , 1992 . <br /> ass The sample analysis for the June 2, 1992 sampling is below <br /> �r detection limits for all constituents analyzed for (TPH-g, BTEX) , <br /> see Table 2A. <br /> Ground water gradients , corrected for vacuum influence, have shown <br /> > > potentiometrzc lows at MW1 and MW5 , which are being used for vapor <br /> 1 , recovery . Once the vapor recovery is shut off , normal flow <br /> directions again take place to the east-southeast; with the <br /> ! exception of the March 5, 1992 gradient, which flowed to the north <br /> #� northwest and may represent a recharge and/or a decline in water <br /> usage of the municipal wells to the east of the site. <br /> Fecal coloform has been analyzed for monitoring wells MW3 , MW4 , <br /> MW5 , and MW6 on a monthly basis from December , 1991 through <br /> February , 1992 . Only trace amounts of fecal coloform have been <br /> a occasionally detected in any of the wells during those 3 months . <br /> ! <br /> This analysis will occur on a quarterly basis starting on the <br /> June, 1992 around water sampling visit, see Table 2B. <br /> page 7 <br />