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' NOTE: Shaded cells indicate wells not used in gradient calculation. <br /> 6.0 CONCLUSIONS <br /> The followingconclusions on <br /> s and recommendations are based on the observations and <br /> analytical data collected to date. <br /> Laboratory analysis of ground water samples indicates significantly reduced levels of <br /> ' gasoline and gasoline constituents in water collected from monitoring well MW-IA. The <br /> levels reported indicate the continuation of a significant decline that began following <br /> completion of the remedial excavation. <br /> 1 A review of historical analyticalround water data for monitoring well MW-IIMW-IA <br /> g g <br /> indicates a dramatic reduction of contaminant mass over the past year. The remedial soil <br /> ' excavation took place between the June 1998 and September 1998 sampling events. <br /> Reductions in contaminant levels between 54% and 95% were observed over this period, <br /> with the exception of total xylenes which indicated a slight increase of approximately <br /> ' 11%. Since the September 1998 sampling event, contaminant levels have continued to <br /> decrease rapidly with reductions between 77% and 99% observed for all analyzed <br /> constituents. These reductions are summarized in Table 4 and are illustrated on Graph 1. <br /> Minor increases in MW-6 petroleum hydrocarbon constituent levels were observed <br /> during this sampling event. These changes are believed to be related to the subsurface <br /> disturbance caused by the remedial soil excavation. However, the apparent increase in <br /> contaminant mass observed at MW-6 is minor when compared with the contaminant <br /> i, mass decrease associated with the excavation event and subsequent reduction by natural <br /> attenuation at the source area surrounding MW-IA. <br /> 7.0 RECOMMENDATIONS <br /> Based on the results of this sampling event, in conjunction with historical analytical data, <br /> AGS recommends that full case closure be requested by the San Joaquin Public Health <br /> Services, Environmental Health Division from the Regional Water Quality Control <br /> Board. Incorporated into this document is the information required by the Regional <br /> Water Quality Control Board in order to make a determination of "no further action <br /> required" status. <br /> Following approval of the closure request by the San Joaquin Public Health Services, and <br /> the Regional Water Quality Board, the ground water monitoring wells would be <br /> scheduled for destruction in accordance with California State Water Well Standards. <br /> American Geological Service, Inc. <br /> CA95DE-029 10 <br />