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• f Report: Groundwater-quality Monitoring—July 30,2003: 7500 West Eleventh Street, Tracy, CA. Page 8 1 � On July 30, 2003, the mean groundwater gradient in the shallow, near-surface aquifer beneath the site was approximately 0.003 ft./ft. Although there were some minor changes in the details of the groundwater elevation contours, the direction of groundwater flow remained substantially unchanged from the north-northeasterly direction that has prevailed since depths to groundwater were first measured in May 2000. 2.2 Purging of Monitoring Wells After the depths to groundwater were measured, a small-diameter, submersible pump was used to purge each groundwater-quality monitoring well of stagnant water. The pumped water was discharged into 5-gallon pails, each of which was, in turn, discharged into an open-topped, 55-gallon drum. ' During the purging procedure, the temperature and electrical conductivity of the stream of purge water were monitored by checking those parameters periodically using a multi- function electronic meter. Purging continued until both parameters stabilized (i.e., variations between measurements were less than 10%), or, in the case of wells screened above the water table, until a minimum of 15 gallons of groundwater had been removed, whichever was greater. The array of parametric results for each well was recorded in field notes. (See Field Notes, Appendix A). Inspection of the temperature and conductivity data in the field notes shows that both 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 criteria suggested by the SJCEHD (San Joaquin County Public Health Services 2000). However, it is SJC's standard practice when purging shallow monitoring wells that have casings slotted in the zone above the water table at sites where groundwater is affected by analytes of concern that are lighter than water, regardless of the temperature, conductivity and pH data obtained, to purge a minimum quantity of groundwater equal to the casing volume plus the void space in the annular filter pack between the casing and the borehole wall. That volume (in this case, 15 gallons) is much greater than the three to five casing volumes that is often erroneously considered to be sufficient to purge adequately a well of that type and it is almost always greater than the volume that would be permissible based on the physical properties of the purge water alone. 2.2.1 Disposal of Purge Water During earlier sampling rounds, purge water temporarily held in the 55-gallon drum used to manage it at the well locations was normally pumped into a 1,000-gal. storage tank located on the 7500 West Eleventh Street property. It was planned to ship that water to a permitted recycling facility for disposal when sufficient quantity had accumulated for that to be done economically following future rounds of groundwater sampling and analysis. However, some time between the round of groundwater-quality monitoring that was conducted on the site on April 15, 2003 and the round reported herein that was conducted E:. 1f sic