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Report: Groundwater-quality Monitoring—September 11, 1001: 7500 West Eleventh Street, Tracy, CA. Page 4 In the period between May 30, 2001 and September 11, 2001, the groundwater table had fallen about 0.4 feet. Since May 2000, when groundwater monitoring began at the site and groundwater elevations were at their highest, the range of groundwater elevation change has been about one foot. The groundwater elevation data presented in Table 1 was used to construct the water table contours for September 11, 2001 shown on Figure 2. When last computed for conditions on May 30, 2001, the general direction of flow through the subsurface where the fueling station infrastructure was formerly located was to the north-northeast. ` However, as can be can be seen on Figure 2, the groundwater contours derived from the depth to groundwater measurements made on September 11, 2001 indicate that over a =� large area of the site the direction of groundwater flow has shifted to a more northerly direction. In the northeastern area, around monitoring well MW-7, the flow direction remains to the north-northeast. Similar realignment of groundwater flow directions associated with seasonaland water table elevation changes have been observed by SJC on several other sites in the Tracy area that have similar hydrogeologic settings. 2.2 Purging of Monitoring Wells After the depths to groundwater were measured on September 11, 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 54-gallon drums remain staged on the 7500 West Eleventh Street property. f 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 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 4` temperature, pH and electrical conductivity meter manufactured by Oak-ton. The results for each well were compiled in field notes, a copy of which is 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 criterion is not sufficient to ensure adequate purging of the shallow groundwater-quality J monitoring wells at the 7500 West Eleventh Street site. The submerged casing volume of each well is approximately 2.45 gallons and, if a filter pack porosity of 33% is assumed, the actual volume of stagnant water in the 8-in. diameter well borings in which the wells were constructed is 14.56 gallons. sic