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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page Ib i Esr 4.3 Sampling and Laboratory Analyses for Components of Fuel Hydrocarbons -- n5 After completing the sampling of groundwater for analyses for natural attenuation parameters and measurement of the depth to groundwater in each well, additional samples were recovered for the purpose of analyzing fuel hydrocarbons. As is required by good practice, each of the wells was purged prior to recovery of the samples used for ' analysis. �o 4.3.1 Purging of Groundwater-quality Monitoring Wells A small-diameter submersible pump was used to purge each well of stagnant water. The pumped water was discharged into 5-gallon pails, each of which was, in turn, discharged into a 55-gallon drum. The water in the drum was periodically discharged into a 1,100- gallon holding tank on the 7500 West Eleventh Street property. f :# During the purging procedure, the temperature and electrical conductivity of the stream of purge water was 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 is recorded in SJC's field notes (see Field Notes, Appendix B). 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 =j 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. �< 4.3.2 Well Recovery Time After the wells were purged, the length of time required for the ground water in each well � i to return to its original elevation was noted. This additional procedure was performed following a request by SJCEHD Case Officer. Observations of the time required for the well to recover after it has been drawn down by pumping can be a useful indicator of the hydrogeologic properties of the formation into sic