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QUARTERLY MONITORING RESULTS <br /> Summary <br /> f <br /> } Based on the water level gauging and groundwater sampling conducted during August <br /> 2996,the following summary points are provided below: <br /> i • Water levels are approximately 60 feet bgs which is approximately 17 feet higher than <br /> I <br /> when groundwater monitoring was initiated at the Disco property in 1991. The current <br /> water levels are approximately 6 to 10 feet above the monitoring well well-screens. <br /> f • The AS/SVE system operated from February 1995 through February 1996 removing <br /> approximately 9,000 pounds of TPH as gasoline from the source area. However,due to <br /> the rise in water levels above the SVE well screens,the AS/SVE system is currently <br /> inoperable. <br /> 0 BTEX and TPH as gasoline concentrations continue to remain stable in the plume area <br /> and are more than an order of magnitude below the concentrations that existed prior to <br /> operation of the AS/SVE system. <br /> • BTEX and TPH as gasoline concentrations continue to remain non-detectable in the <br /> monitoring areas near the fringe of the plume. <br /> • MTBE was detected(4.3µg/L) slightly above the detection limit in monitoring well <br /> MW-2;the concentration detected is well below the proposed drinking water standards <br /> for MTBE (20 to 200 gg/L). <br /> • The ITB demonstration continues to indicate the presence of gasoline reducing <br /> constituents in the groundwater including dissolved oxygen, sulfate,nitrate,and <br /> ferrous iron. Based on simple calculations,it appears that ITB is capable of stabilizing or <br /> reducing the plume. This finding is supported by the Lawrence Livermore National <br /> Laboratory study(Recommendations To Improve the Cleanup Process for California's Leaking <br /> underground Fuel Tanks, October 16, 1995)which states that plume concentrations greater <br /> than 10µg/L benzene extend no more than about 250 feet in 90%of the leaking <br /> underground tank cases.If ITB is selected as the final remedy for the Disco property,it <br /> is recommended that groundwater monitoring be conducted for an extended period of <br /> time to confirm plume stability and/or reduction. <br /> A more detailed assessment of ITB or natural attenuation will be made following the fourth <br /> ITB monitoring event which is currently scheduled for December 1996. <br /> l <br /> i - <br /> SFOIG.(PROJECTStELMONTEIPLANT_33\OTFt_REP.DCC 17 <br />