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r Report: Groundwater-quality Monitoring—May 30, 2001: 7500 West Eleventh Street, Tracy, CA, Page 4 zones of sand through which groundwater flows at a relatively higher velocity than is ' possible through the intervening clayey facies. We note that a groundwater flow direction of north, 13 degrees west has been computed from measurements of the depth to groundwater made in monitoring wells at C&B Equipment, located across Chrisman Road to the east of the subject site (Geological Technics Inc. 1998). On casual inspection, that direction of groundwater flow might appear to be incompatible with the flow direction of north, 20 degrees east beneath the former fueling station site at 7500 West Eleventh Street. However, an examination of the t:. directional trend of the groundwater contours on the western, southern and eastern fringes of the contoured area shown on Figure 2 indicates that those contours might be extendable so as to merge into those compatible with a regional pattern having a more northerly, or even slightly northwesterly, regional groundwater flow direction that is locally perturbed by the details of the hydrostratigraphy beneath the former fueling G i station site. Unfortunately, limitations on the scope of the site characterization program imposed by the California Underground Storage Tank Cleanup Fund (USTCF) (California Underground Storage Tank Cleanup Fund 2000) prevented installation of groundwater-quality monitoring wells to the east of Chrisman Road that would have resolved the apparent paradox between groundwater flow directions computed for the 7500 West Eleventh Street site compared to those computed for the C&B Equipment site. 2.2 Purging of Monitoring Wells >3 After the depths to groundwater had been measured on May 30, 2001, a small-diameter, submersible pump was used to purge each groundwater-quality monitoring well of stagnant water. The pumped water was discharged into five-gallon pails. As each pail was filled, it was, in turn, discharged into an open-topped, 50-gallon drum equipped with a securable lid. The 50-gallon drums were staged on the 7500 West Eleventh Street property. As is SJC's normal practice when purging the shallow monitoring wells at the 7500 West Eleventh Street site, 15 gallons of water from each well was extracted before a sample 1' f was recovered. However, to comply with a guidance letter dated September 19, 2000 issued by the SJCPHS (San Joaquin County Public Health Services 2000), during the purging procedure, the temperature, electrical conductivity and pH of the stream of purge water was monitored by periodically checking those parameters using a multi-function Hydac Meter. The results for each well are compiled in the field notes, a copy of which is r included in Appendix A. Inspection of the temperature, conductivity and pH data in the field notes shows that all three parameters stabilized to within plus or minus 10% after the first few measurements were made on the purge water discharged from each well. This parametric stability is sufficient to demonstrate adequate well purging according to the criteria suggested by the SJCPHS. However, it is the professional opinion of the author of this report that that criteria is not sufficient to ensure, adequate purging of the shallow groundwater-quality monitoring wells at the 7500 West Eleventh Street site. sic i