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When constructing groundwater monitoring wells in confined aquifer systems, the placing of the well screen is different than in an unconfined system. In an unconfined groundwater system, the well screen is normally placed so that the mid portion of the screen intersects the groundwater surface, which in an unconfined system is at atmospheric pressure. This allows for the greatest amount of annual fluctuation in groundwater. In a confined aquifer system, the screen is normally placed so that the top of the screen does not extend above the zone of saturation, or above the base of the confining layer. There are several reasons for this. If the screen were to be placed at a level above the zone of saturation or the base of the confining layer, any dry unsaturated material above could become saturated. Any contaminants that may have been present in the unsaturated material, but had not migrated to the zone of saturation, or through the confining layer, could potentially mobilize, resulting in lateral spreading of the contaminants in the previously dry unsaturated zone. In addition, the potential for vertical migration of soluble constituents with specific gravities greater than water (PNA's specifically) is now enhanced, thereby increasing the probability of contaminating groundwater in the confined system. This would result in "false positives" manifesting in the analyses of groundwater quality samples collected from the confined aquifer, thereby triggering an unjustified corrective action. During your 22 August 1989 phone conversation with AEMC personnel, you dismissed out of hand any chance that either tidal influences or a nearby slough could have any bearing on groundwater flow direction in the vicinity of the 5,500-barrel #6 fuel tank. Yet it was the CVRWQCB that originally suggested to AEMC that groundwater gradients be measured during high and low tide cycles to measure the effects. „ "„ Page 7