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tw Report.-Groundwater-quality Monitoring—January 26,2004: 7500 West Eleventh Street, Tracy, CA. Page 7 - magnitude, differences between the elevations of the groundwater in Wells MW-3, MW- 3A and MW-3B and in MW-12 and MW-12A are sufficiently great to indicate that the • groundwater monitored by the shallow well at the locations of each of those well clusters has a different piezometric pressure from that of the deeper wells in the same cluster. It must therefore be assumed, at least locally, that the shallow wells monitor aquifers different from those monitored by the deeper wells. During the January 26, 2004 sampling round, the groundwater level in Monitoring Well MW-3B, which is screened in an aquifer the top of which is some 37 ft. BGS, was 0.31 ft. higher than that in Monitoring Well MW-3, which is screened in the shallow, near-surface aquifer, while the level in Monitoring Well MW-3A was 0.01 f1. lower than that in Monitoring Well MW-3. On January 26, 2004, the mean groundwater gradient in the shallow, near-surface aquifer y� beneath the site was approximately 0.003 f1/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 to be sampled of stagnant water. A.- The pumped water was discharged into 5-gallon pails, each of which was, in turn, discharged into a 55-gallon drum. #-V 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 SJC's 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. sic