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r- • 7 k anions varied with aquifer depth. The shallow ground water appeared to be of a bicarbonate/chloride quality, such as shown by the Hall well analyses (Figure 13), while deep ground water appeared to be of a bicarbonate/sulfate quality as shown by the Brown's New well analyses (Collins, 1928; Piper, 1953; Hem, 1959) (Figure 14). I , 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 13 and 14). Upon discussion with laboratory staff, (K. Wilkinson, Fruit Growers Lab, personal communication, March 1996), it was their opinion that a calibration problem accounted for a portion of this apparent increase. As can be seen in Figures 13 and 14 and in Table 2, the bicarbonate (and alkalinity) concentration from each subsequent sampling period has "returned" to the lower concentration observed during the initial year and one-half of monitoring. The results from May 1996 were confirmed by an independent laboratory (Table 2). Thus, it appears that the apparent increase in bicarbonate concentration, during October 1995 and January 1996 was largely due to laboratory error, with some small portion resulting from actual seasonal or annual fluctuations of ground-water general mineral quality. To date, Teichert Aggregates has not yet commenced aggregate mining, thus eliminating the possibility'that an actual increase occurred due to Teichert's mining activities (L. Noble, Teichert Aggregates, personal communication, February 7, 1997). A more detailed analysis of the apparent increase has been provided previously (LSCE, 1996). -water constituents, nitrate levels were elevated in both shallow In regard to other notable ground and deep well samples; with concentrations in the South Tracy 41 production well fluctuating slightly above and below the drinking water maximum contaminant level (MCL) of 45 mg/l during the last three years. The lowest nitrate concentrations have typically been observed in samples from the Brown's wells. It should be noted that a gradual increase in nitrate concentration has been observed in the Hall well, from 15 to 31 mg/l over the past three years (Table 2) (Figure 15). These elevated nitrate concentrations appear to be indicative of historical agricultural land use in the area and, as mentioned above, cannot be due to aggregate mining. E © LUHDUFiFF & SCALMANINI CONS L3 LTIN G GN GI NFF RE jC'