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A notable apparent increase (25 to 35 percent) in bicarbonate (and alkalinity) concentration was observed in samples from all the network wells during the October 1995 and January 1996 monitoring periods (see Figures I I and 12). Upon review of all existing g monitorin data it appeared that these results were due to laboratory error for the following reasons. First, the ? bicarbonate concentrations increased during the sante monitoring periods in all network wells, which are located across a large area and are of varying depths. It was thought that an actual increase, whether due to natural changes or human. impacts, would show some variation in time, r� location, and depth of occurrence. Secondly, the concentrations of bicarbonate (and alkalinity, I which is derived from the same titration procedure in the laboratory) increased without any corresponding water duality changes, such as increased EC, TDS, or cation concentrations or pH changes. Further, total cations and anions were not balanced, with the total anions exceeding total cations by 10 to 18 percent during the winter 1996 period. This exceeds the limit of permissible error in a natural water as specified by the US Geological Survey (2 percent difference). Lastly, it was confirmed with Teichert Aggregates that aggregate mining has not commenced at the Vernalis plant site to date, thus eliminating the possibility that an actual increase occurred due to Teichert's mining activities (L. Noble, Teichert Aggregates, personal communication, April 1996). These points were discussed with laboratory staff who, upon review of the analytical QA/QC data from the last two years, identified a recent change in the calibration results; specifically, the control standards for alkalinity and bicarbonate were high during recent calibrations (K. Wilkinson, Fruit Growers Lab, personal communication, March 1996). It was their opinion that this recent calibration problem could have accounted for a portion of the apparent increase in i alkalinity and bicarbonate concentration. As a result, during the May 1996 monitoring period, sample analyses for alkalinity and bicarbonate were run by the laboratory with tighter calibration procedures, and duplicate samples were collected from selected wells for submittal to a separate j independent laboratory, thus providing a check of the first laboratory's results. As can be seen in Figures 11 and 12 and in Table 2, the bicarbonate (and alkalinity) concentration. from the May i , 1996 period "returned" to the lower concentration observed during the initial year and one-half of monitoring, and these latest results were confirmed by the independent laboratory's results. Thus, 1 © 1.....lf"li.,k(..) ir': ` A ' J