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Report:Groundwater-quality Monitoring-November 28,2000: 7500 West Eleventh Street, Tracy, CA. Page 5 monitoring its temperature, electrical conductivity and pH. This was done by periodically checking those parameters using a multifunction Hydac Meter. The results for each well :! are compiled in the field notes, a copy of which is included in Appendix A. The temperatures are recorded in degrees Centigrade, the electrical conductivity in µmhos/cm, and the pH as a dimensionless number. Inspection of the temperature, conductivity and pH data in the field notes shows that all three parameters stabilize 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 such -h measurements are not technically necessary or appropriate to ensure that the shallow groundwater-quality monitoring wells at the 7500 West Eleventh Street site are adequately purged and their use unnecessarily adds to the complexity and cost of the = groundwater sampling operations. The frequently-stated objective of well purging is to remove stagnant water from a well. Elimination of this water ensures that it is not included in a groundwater sample recovered following the purging. The stagnant water's geochemistry may be different from that of the formation water due to loss of volatile compounds and a variety of other factors and, thus, influence the results of sample analyses from that well. The elimination of stagnant water can be achieved, according to popular opinion, by removing a volume of water from the well equal to the "well volume," which is defined as the volume of the submerged casing. In practice, extracting that volume of purge water has, in some cases, been insufficient to enable recovery of a representative sample of the formation water. �..� That problem lead to the development of several "rules of thumb," which call for a <� certain number of"well volumes" of water-typically three or five -to be purged before a groundwater sample is recovered. Even so, studies have shown that there is often little 1 correlation between the number of well volumes purged and the measured concentrations of analytes of concern in groundwater samples recovered from the wells; such concentrations may increase, decrease or remain unchanged as the volume of purge water j is increased. (Rehm et al 1989) Another issue of major importance to groundwater sampling at contaminated sites in general is the high cost of management and disposal of purge water that contains analytes of concern. Excessive purging of deep wells can yield large volumes of water that must be disposed off-site at a permitted facility. To address this issue, which can.be of vital importance to the economic practicability of groundwater-quality monitoring sampling r- using deep wells, measurements of indicator parameters such as temperature and electrical conductivity may be used to estimate the point at which the water being extracted from the well during purging is representative of the formation water. Thus, over-purging of those deep wells may be avoided and the amount of contaminated water generated during groundwater-quality monitoring kept to a practical minimum. In some cases, particularly in deep wells where water is extracted from a relatively short screened zone in an otherwise watertight casing by a pump set at a depth below the water sic -rl