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Quarterly Groundwater Monitoring,Results of Verification ' Sod Borings,and Closure Request Report Moore Petroleum, 1399 E Yosenute,Ave,Manteca,CA July 1, 1996 ' Page 6 5.0 CONCLUSIONS ' In summary, it was estimated that approximately 590 pounds of TPH, as gasoline, was initially present in the soil underlying the subject site It was also estimated that the vapor extraction system extracted approximately 211 pounds of TPH, as gasoline It should be noted that ' approximately 380 pounds of TPK as gasoline are unaccounted for There are several reasons for this discrepancy The first is that most likely biodegradation has been occurring at the subject site It is not unreasonable for this quantity of TPH, as gasoline,to be btodegradated over a period of eight years Secondly,vapor samples used for calculation of extraction rates for the vapor extraction system were collected on a monthly basis and does not represent the actual daily ' quantity of TPH, as gasoline, extracted Another possible reason for this discrepancy is that all of the soil samples used in the estimation of the initial TPH, as gasoline, were not collected at the same time, some of the soil samples were collected five years apart When considering closure at the subject site, an mdnndual must look at the facts • A vapor extraction system operated at the subject site and had a radius of influence encompassing the entire TPH, as gasoline,plume ' The vapor extraction system was operated until soil vapors fell below laboratory detection limits ' Groundwater at the site has been clean for four consecutive quarters • Two verification soil borings were installed at the subject site in the vicinity of the former UST locations which verified the absence of gasoline constituents in the soils surrounding the former tank basin Considering these facts, Apex believes petroleum constituents at the subject site are no longer present in soil or groundwater underlying the subject site As a result of this conclusion, Apex believes this site should receive closure and be classified as`no further action "