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Report:Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page 9 <br /> 2.0 MARCH 2006 GROUNDWATER-QUALITY MONITORING ROUND <br /> On March 29-31, 2006, SJC conducted the twenty-first round of groundwater-quality <br /> monitoring at the 7500 West Eleventh Street Site. <br /> The scope of groundwater sampling conducted in March 2006 was designed to <br /> incorporate both the findings from the natural attenuation studies that had been conducted <br /> on the site in March 2005 and the results of a Monitoring and Remediation Optimization <br /> System (MAROS) study, which was also conducted at that time in the manner advocated <br /> by the United States Environmental Protection Agency (US-EPA) and the California <br /> Environmental Protection Agency (Cal-EPA) (The San Joaquin Company 2006d). <br /> Although the results of the MAROS and natural attenuations studies called for quarterly <br /> recovery of groundwater samples from only Monitoring Wells MW-3, MW-7 and MW- <br /> 13, SJC, to ensure an abundance of conservatism, recommended that, for a period of one <br /> year following the March 2005 sampling round, additional wells located along the long <br /> axis of the primary plume, at key locations on its western and eastern periphery, and in <br /> the secondary plume of affected groundwater that is present on the site should be <br /> sampled. Accordingly, the array of wells sampled in the March 2006 monitoring round <br /> included Monitoring Wells MW-2, MW-3, MW-4, MW-5, MW-7, MW-10, MW-11, <br /> MW-12, MW-13, MW-17, MW-18, MW-19, MWFP-1, MWFP-2, MWFP-4 and MWFP- <br /> 5 (see Figure 3 for locations). <br /> Note: Although it was planned to purge and recover a sample from <br /> Monitoring Well MW-16 during the December 2005 and March 2006 <br /> monitoring rounds, that was not done because it was clear to the sampling <br /> team that the water in that well was heavily.contaminated by sewage and <br /> it was judged to be a significant biohazard. <br /> 2.1 Groundwater Elevations and Flow Direction <br /> To initiate the March 2006 sampling program, the depth to groundwater in each of the 27 <br /> monitoring wells was measured using a conductivity probe on March 29, 2006. The water <br /> table elevations were computed relative to mean sea level (MSL). <br /> In the period from December 20, 2005, the date on which the depths to groundwater in <br /> the monitoring wells were last measured, to March 29, 2006, the groundwater table in the <br /> majority of wells that are screened in the near-surface aquifer beneath the site rose no <br /> more than 0.25 ft., but in a few wells, such as Monitoring Wells MW-2 and MW-5, the <br /> groundwater actually fell by a similar amount. In the monitoring wells that are screened <br /> in the first aquifer that is located deeper than the near-surface aquifer (i.e., Monitoring <br /> Wells MW-3A and 12A), the piezometric head rose 0.24 ft. In the deepest aquifer that is <br /> monitored at the site (by Monitoring Well MW-3B), the piezometric head fell by 0.14 ft. <br /> between December 2005 and March 2006. <br /> The groundwater elevations presented in Table 1 were used to generate the groundwater <br /> contours shown on Figure 2. <br /> SJC <br />