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The six proposed new wells are described as follows: New Well Adjacent to Proposed Identification Well No. Screen Interval* MW -1B MW -1 -27 to -57 MW -2B MW -2A -35 to -55 MW -3B MW -3 -27 to -57 MW -5B MW -5A -27 to -47 MW -12B MW -12 -27 to -47 MW -13B MW -13 -35 to -55 A fence diagram showing the locations, depths, and screened intervals of the existing wells along the north side of the landfill is shown on Figure 3. Figure 3 also shows the elevation of ground water in March and August 1990, as well as the locations of the wells with respect to generalized lithology. Four of the six proposed wells are also shown on Figure 3 to demonstrate the relationship of the proposed new wells to the a) existing wells, b) the ground water table, and c) the general lithology. The screen -intakes of the proposed wells are designed to extend from just above the bottom of the adjacent well to the top of the screens in the deep wells (MW -8, MW -9, and MW -10). Consequently, the screen length of two of the wells, MW -1B and MW -3B, will be 30 feet. While this screen length is longer than most monitoring wells, the longer screens in these two wells will have the following distinct advantages: 0. they will bring the bottoms of all six proposed new wells to approximately the same common elevation and/or lithologic datum, they will provide consistency in monitoring, given the extreme water table fluctuations documented at Forward Landfill, ► they will completely span the gap between the bottoms of the existing shallow wells and the tops of the existing deep wells. Therefore, any future "deep" wells are likely to be complemented by these wells. Langer well screens are appropriate in some situations. According to the "RCRA Ground - Water Monitoring Technical Enforcement Guidance Document", U.S.E.P.A., September 1986, "Certain hydrogeologic settings necessitate the use of longer well screens for detection monitoring. Hydrogeologic settings with widely fluctuating potentiometric surfaces are better monitored with longer screens that continuously intercept the water surface and provide monitoring for the presence of contaminants less dense than water." The Guidance Document goes on to list "Homogeneous uppermost aquifer; simple geology" as one of the factors that justify a single well per sampling location. Based on the lithology (see Figure 3), it does not appear that an aquitard exists in the upper 100 feet of soil that underlays the landfill. Consequently, we are approaching the monitoring with the assumption that the upper 100 to 110 feet is one continuous water -bearing zone. New wells are not proposed adjacent to two of the existing water table wells at this time (MW -14 and MW -15). To date, these wells have not contained constituents of concern (see Attachment A). Since water levels in these wells are likely to rise within the next two to three months, we propose to evaluate whether installation of these adjacent wells is necessary, pending further analysis of the potential effects of future drought or overdraft. * Referenced to mean sea level (msl) Cr3-90-35 2 <'-E:\ �___ER :1-0= 9usiness'aric ^rive. Sacramento. (:A 0-8117 1016) 366-1701