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Extended Site Characterization Report; 7500 West Eleventh Street, Tracy, CR. Page 57 underlying sands. In fact, Monitoring Well MW-3B penetrates two clay aquitards. If any fuel or otherspillage were to enter it at the surface groundwater in sand a uifers as dee � g q p as 50 ft. beneath the site would become contaminated. == In SJC's professional opinion, installing groundwater-quality monitoring wells that penetrate aquitards protecting groundwater at depth from chemicals of concern in shallow aquifers (unless there is compelling evidence that they are essential to adequately characterize a site) or allowing them to remain open when they no longer serve a vital function violates the standards of good engineering and environmental practice. -' That the deep wells currently open on the 7500 West Eleventh Street property pose more -- than a theoretical risk to the quality of groundwater at depth beneath the site is well illustrated by an incident that occurred on August 3, 1999. On that date, a tractor-trailer rig stopping at the Casa Mendoza restaurant raptured its fuel tank as it entered the western portion of the site. Some 150 gallons of diesel spilled onto the ground surface E. � and penetrated into the soil of the unpaved lot. That incident required an emergency response by the SJCEHD and Dietz Irrigation to contain the spill and excavate affected soil to remediate the area (Dietz 1999, Sassoon 1999). Fortunately, that spill did not [ j expand far enough to enter the subsurface housing of a groundwater-quality monitoring well. Nevertheless, the threat that, on the next occasion that such an incident or similar event occurs, fuel or some other spillage may be lost into a well casing and through that casing contaminate the currently unaffected groundwater at depth beneath the site cannot be dismissed. For the reasons stated above, SJC recommends that, immediately following completion of the three additional rounds of quarterly groundwater-quality monitoring recommended `- in Section 10.2, Monitoring Wells MW-3, MW-3B and MW-12A be closed by grouting with Portland cement under the permit and oversight of the SJCEHD. At the same time that the deep monitoring wells are closed, it would also be programmatically cost effective to close Monitoring Well MW-8. That well was installed to investigate the maximum extent of the plume of affected groundwater emanating from -_ the 7500 West Eleventh Street property and to check for any hydrocarbons that might have migrated across West Eleventh Street from the Ieaking underground fuel storage tanks formerly located at 7474 West Eleventh Street.Groundwater-quality monitoring to detect any eastern expansion of the primary plume of affected water can be performed by �. recovering samples from Monitoring Well MW-9. 10.4 Removal of Floating Product from Groundwater As has been noted elsewhere in this report, floating product that flowed into the pit opened to permit exhumation of underground tanks on the 7500 West Eleventh Street property in December 1998 was successfully and efficiently removed by vacuuming it from the surface of the groundwater into a tank truck. That vacuuming operation continued until the flow of floating product into the pit from the subsurface to the west ceased. An additional volume of affected groundwater in the tank pit was also pumped ._i SJC