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Report:Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page25 `'_ 7.0 CONCLUSIONS Based on the data obtained from the groundwater-quality monitoring round conducted at 16' the 7500 West Eleventh Street site in March 2006 and its integration with the data $4 gathered cumulatively over the seven-year period since December 1998 when investigations were initiated at this site, SJC concludes the following. 7.1 Groundwater Quality ` 1. The plumes of groundwater affected by fuel hydrocarbons that were released at the former fueling station at 7500 West Eleventh Street remain stable. 2. A groundwater mound centered near Monitoring Well MW-16 that was L first observed in February 2005 continued to be present. During the December 2005 sampling round, it was determined that Monitoring Well MW-16 contained a heavy concentration of sewage. The same situation LO existed in March 2006. Due to the biohazard posed by that material, that well has not been sampled since October 20, 2005. The presence of the sewage supports SJC's earlier conclusion that the groundwater mound resulted from the failure of the septic tank leach field that serves the Capitol Furniture building at 7501 West Eleventh Street. 3. Concentrations of components of fuel hydrocarbons within the plumes of affected groundwater are continuing to oscillate around a downward trend. 4. The temporally highest concentrations of analytes of concern in groundwater occur when the elevation of the groundwater table it at its annual maximum. Due to the dominant influence of agricultural irrigation on the groundwater regime in the shallow aquifer beneath the site, these conditions typically occur in July of each year i.. 5. Less than a 0.01 ft. thickness of floating product (LNAPL) was present on the groundwater in Monitoring Well MW-7 in March 2006. 7.2 Natural Attenuation Natural processes of attenuation of analytes of concern in groundwater that were *- comprehensively evaluated based on field and laboratory data obtained in March 2005 (The San Joaquin Company Inc. 2006d) continue to be vigorously active at the 7500 West Eleventh Street Site. 7.3 Optimal Groundwater Monitoring Frequency The groundwater-quality monitoring program optimization analyses that were first ...� conducted using data compiled through March 2005 (The San Joaquin Company Inc. sic