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Report:Groundwater-quality Monitoring-November 28, 2000: 7500 West Eleventh street, Tracy, CA. Page 4 <br /> incipient curvature of the isopiestic lines constructed from the measurements made on <br /> that earlier date in the area of monitoring well MW-3. (The San Joaquin Company Inc. <br /> 2001.) This feature of the groundwater flow in the area can be attributed to the presence <br /> of the clays and silty clays in the area around MW-3, that, as is shown in cross-section A- <br /> A' on Figure 4, separates zones of sand through which groundwater flows at a relatively <br /> i higher velocity than is possible through the intervening clayey facies. <br /> The more pronounced curvature of the isopiestic lines constructed using the depth to <br /> 'i groundwater data measured in November 2000, compared to those constructed from the <br /> measurements made in May 2000 can be accounted for by the approximate 1-ft. change <br /> in the mean elevation of the groundwater table that occurred between those dates; the <br /> strata through which groundwater flowed were significantly different on the separate <br /> occasions on which groundwater measurements were made. <br /> We note that a groundwater flow direction of north, 13 degrees west has been computed <br /> from measurements of the depth to groundwater made_ in monitoring wells at C&B <br /> Equipment, located across Chrisman Road to the east of the subject site (Geological <br /> Technics Inc. 1998). On casual inspection, that direction of groundwater flow might <br /> appear to be incompatible with the flow direction of north, 20 degrees east beneath the <br /> former fueling station site at 7500 West Eleventh Street. However, an examination of the <br /> directional trend of the groundwater contours on the western, southern and eastern fringes <br /> of the contoured area shown on Figure 2 indicates that those contours might be <br /> extendable so as to merge into those compatible with a regional pattern having a more <br /> northerly, or even slightly northwesterly, regional groundwater flow direction that is <br /> locally perturbed by the details of the hydrostratigraphy beneath the former fueling <br /> station site. Unfortunately, limitations on the scope of the site characterization program <br /> imposed by the California Underground Storage Tank Cleanup Fund (USTCF) <br /> (California Underground Storage Tank Cleanup Fund 2000) prevented installation of <br /> groundwater-quality monitoring wells to the east of Chrisman Road that would have <br /> resolved the apparent paradox between groundwater flow directions computed for the <br /> 7500 West Eleventh Street site compared to those computed for the C&B Equipment site. <br /> 2.2 Purging of Monitoring Wells <br /> After the depths to groundwater had been measured on November 28, 2000, a small- <br /> diameter, submersible pump was used to purge each groundwater-quality monitoring well <br /> of stagnant water. The pumped water was discharged into five-gallon pails. As each pail <br /> was filled, it was, in turn, discharged into an open-topped, 50-gallon drum equipped with <br /> a securable lid, The 50-gallon drums were staged on the 7500 West Eleventh Street <br /> property. <br /> It is SJC's normal practice when purging the shallow monitoring wells at the 7500 West <br /> Eleventh Street site to extract some 15 gallons of water from each well before a sample is <br /> recovered from it. However, to comply with a guidance letter dated September 19, 2000 <br /> issued by the SJCPHS (San Joaquin County Public Health Services 2000), an attempt was <br /> made to determine the required volume of groundwater to be purged from each well by <br /> sic <br />