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Evaluation of Natural Attenuation: 7500 West Eleventh Street,Tracy, CA. Page 50 9.0 CONCLUSIONS Based on the data obtained from the groundwater-quality monitoring round and natural attenuation investigation conducted at the 7500 West Eleventh Street site in February and March 2005 and their integration with the data gathered cumulatively over the seven-year i period since December 1998 when investigations were initiated at this site, SJC concludes the following. -' • Between October 27, 2004, when depths to groundwater were last measured at the subject site, and February 24, 2005, a groundwater mound with an amplitude of some 5 ft. developed in the area around Monitoring Well MW-16 (see Figure 2 for location). SJC attributes the existence of the groundwater mound to failure of a septic tank leach field that serves Capitol Furniture at 7501 West { Eleventh Street. • With one minor exception, the geometry of the principal plume of affected groundwater emanating from the 7500 West Eleventh - Street property that was observed during the March 2005 monitoring round was the same as that observed in October 2004. ! The exception was a minor westward shift in the periphery of the plume in the area around Monitoring Well MW-17 where a trace of MTBE at a concentration of 0.54 F.gIL was detected in the groundwater on March 16, 2005. SJC believes that the shift was caused by the development of the groundwater mound described above, which mound produced local changes in the groundwater flow direction that are reflected in the groundwater contours shown on Figure 2. That change caused MTBE to migrate northwesterly from the previous location of the plume boundary into the area { around Monitoring Well MW-17. F "=! • Between October 27, 2004, when Monitoring Well MW-7 was r'1 previously purged, and February 24, 2005, 0.02 ft, of floating product (LNAPL) had accumulated on the surface of the groundwater in that well. It was removed on February 24, 2005. • Neither of the lead scavengers ethylene dichloride or ethylene dibrornide is present in groundwater at the Navarra Site. • There is no ethanol or methanol present in groundwater beneath the Navarra Site. • Vigorous natural attenuation processes are actively reducing the i; load of contaminants in groundwater at the 7500 West Eleventh Street site. In the secondary plume and over the greater part of the primary plume those processes are aerobic in nature but in the area sic