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PRO Report:Groundwater-quality Monitoring-November 28,2000: 7500 West Eleventh Street, Tracy, CA. Page G table, the total volume of water that must be pumped to purge all stagnant water from the well is affected by the proportion of the pumped flow that originated in the casing and the amount that is drawn into the pump from the formation. If the pump is set in such a well at the depth of the screened interval, a large proportion of the purge water that is pumped may be drawn directly from the formation instead of from the well itself. Several well volumes would then need to be pumped before all of the stagnant water is actually purged from the well. That disproportionate extraction of formation water in the early stages of purging can be remedied by setting the purge pump at a shallow depth below the water level in the well and lowering it as the water level falls rather than initially setting the pump deeper or at the level of the screened interval. In such wells, monitoring of parameters of the purge water flow such as temperature and conductivity can play an important role in limiting the quantity of potentially contaminated purge water generated. -- However, in properly designed, shallow groundwater-quality wells where the screened interval is constructed from a level above the water table down to the bottom of the casing, the early phase flow of purge water contains little formation water, particularly if the pump is set initially just below the water level in the well and then lowered as the water table falls. From the above discussion it can be seen that the total volume of water that must be extracted from a groundwater-quality monitoring well to purge it adequately of stagnant { water, while at the same time avoiding unnecessary and costly over-purging depends on the depth of the well, the screened interval and other well-design parameters, many of .which are affected by site specific hydrostratigraphy and contaminant chemistry. A consideration that is, unfortunately, infrequently addressed in the literature is the ! definition of the actual volume of water in a monitoring well that may become "stagnant" and thus must removed by purging before a representative sample of the formation water :f can be recovered. Again, the answer to this question depends on the details of the well design and the site-specific geotechnical conditions. Because fuel hydrocarbons are.less dense than water, they float on the surface and usually affect shallow groundwater more than water at great depths below the water table. For this reason, most groundwater— quality monitoring wells at sites affected by hydrocarbons have a screened interval protected by permeable filter that extends from above the water table to the bottom of the well, which is at some relatively shallow depth below the water table. In such situations, soils in the walls of the well boring that are above the elevation of the current water table may contain components of fuel hydrocarbons deposited in the vadose zone when the groundwater level was higher. During those periods, affected vadose-zone water, or under some conditions non-aqueous phase liquids, can seep from the walls of the well E boring, flow down through the filter material around the screened interval and affect the geochemistry of the water in the well volume, which, in a well of that type, should be considered to be the submerged volume within the well boring, not simply that within the well casing. In the case of the groundwater-quality monitoring wells at the 7500 West Eleventh Street site, the submerged casing volume of each well is approximately 2.45 gallons, while, ! ;> assuming a filter pack porosity of 33%, the actual volume of stagnant water in the 8-in. sic