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Phase VI Report Memorandum -2- 14 May 1991 Nestle Foods Corporation in determining past historical pumping patterns and potential areas of the plume that may need adjustments to the treatment system. c. Head difference maps should be constructed if there are an adequate number of cluster sites. The vertical head difference will help to indicate whether there is a positive or negative vertical flow component. If the maps are constructed using data from different times of the year, seasonal differences due to pumping influences may be revealed. 3. The construction summary table should be updated to include all monitor wells, supply wells and extraction wells. This table would be a valuable reference. 4. The rationale for selecting the March 1990 ground water level measurements and the October 1990 water quality sampling round was not presented. It would be preferable if the months selected for data analysis were changed for the following reasons. a. The month of ground water level measurements selected for analysis should be either at the height of the wet season (usually January or February) or at the height of the dry season (usually July or August) rather than selecting seasonal transition months. The wet season may be preferable for data analysis because this season would be anticipated to be the most hydraulically quiescent. Agricultural pumpage which can cause the formation of regional ground water cones of depression, would be at a minimum during the wet season. b. The month selected for ground water level analysis should be the same as the water quality sampling quarter selected for analysis (preferably January measurements) because the hydraulics and chemical components of the system will be interpreted together. 5. The text indicates (page 3-5) that there are no seasonal trends in the vertical gradient. However, the ground water level maps for March 1990 and August 1990 show that the gradient increases significantly in all of the zones between March and August. Therefore, these maps indicate that there may be seasonal changes in the vertical gradient. 6. The ground water level maps in the Report (Figures 3-2 to 3-7) show the flow direction to be away from the Stanislaus River. In March 1990, the flow direction was generally westerly to northerly in all of the zones and in August 1990, the flow direction is northerly. However, the concentration contour maps in the Report (Figures 3-8 to 3-13) show the general plume movement to be toward the south. The discrepancy between the 1990 flow directions and the plume movement was not discussed in the Report. 7. The Report indicated that the Stanislaus River appeared to be recharging the aquifer during August 1990. However, the Report did not indicate whether the River was gaining or loosing during March 1990. The Stanislaus River may be an additional factor, in addition to ground water pumping at Simpson to the south and regional agricultural pumpage, affecting ground water flow direction and plume movement.