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i <br /> r i <br /> 7 <br /> E 790 to 1,100 umho/em with the highest EC values in samples from the shallow wells (Figure 11). <br /> i <br /> Samples from all wells were of a similar sodium/calcium quality, but the proportions of major <br /> anions varied with aquifer depth. The shallow ground water appeared to be of a <br /> bicarbonate/chloride quality, such as shown by the Hall well analyses (Figure 12), while deep <br /> k <br /> ground water appeared to be of a bicarbonate/sulfate quality as shown by the Brown's New well <br /> analyses (Collins, 1928; Piper, 1953; Hem, 1959) (Figure 13). i <br /> As has been described in the previous two annual reports (LSCE, 1996, 1997), a notable apparent <br /> increase (25 to 35 percent) in bicarbonate (and alkalinity) concentration was observed in samples <br /> from all the network wells during the October 1995 and January 1996 monitoring periods (see <br /> i <br /> Figures 12 and 13). These concentrations subsequently returned to the lower concentrations <br /> observed during the initial year and one-half of monitoring and the apparent increase was largely <br /> due to laboratory error, with some small portion resulting from actual seasonal or annual <br /> fluctuations of ground-water general mineral quality. <br /> j <br /> In regard to other notable ground-water constituents, nitrate levels were elevated in both shallow <br /> and deep well samples, with concentrations in the South Tracy #1 production well fluctuating <br /> slightly above and below the drinking water maximum contaminant level (MCL) of 45 mg/1 <br /> during the first three years of monitoring. Nitrate concentrations declined during early 1997 in <br /> several of the shallow and deep wells (Figure 14). The lowest nitrate concentrations have <br /> typically been observed in samples from the Brown's wells. It should be noted that a gradual <br /> increase in nitrate concentration that had been observed in the Hall well, from 15 to 31 mg/1 over <br /> the first three years of monitoring, stabilized during 1997 at approximately 28 mg/1. These <br />` elevated nitrate concentrations appear to be indicative of historical agricultural land use in the <br /> area and cannot be due to aggregate mining by Teichert Aggregates. The planned aggregate <br /> extraction has not yet commenced, with only a very limited removal of surficial earth materials <br /> conducted under emergency conditions in January 1997 (L. Noble, Teichert Aggregates, personal <br /> communication, March 11, 1998). <br /> I <br /> i <br /> LUHDORFF L <br /> 6 SCAMANEN? <br /> c C N <br /> 2 V 'i I ra G F N c <br />