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I - r 1 8 i 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. LA U SciirYNED ' In regard to other notable ground-water constituents, nitrate Ievels were elevated in both shallow and deep well samples, with concentrations in the South Tracy #1 production well fluctuating slightly above and below the drinking water maximum contaminant Ievel (MCL) of 45 mg/l I during the last. two years The lowest nitrate concentrations have typically been observed in samples from the Brown's wells (Table 2). 'These elevated nitrate concentrations appear to be indicative of historical agricultural land use in the area. I The only other constituents to exceed MCLS were iron and manganese in the Koster well (Only I once), South Tracy #1 well (only once), and Brown's wells. It is important to note that none of i these four wells is a domestic well; the Koster well is used for irrigation and the South Tracy #1 and Brown's wells are for industrial uses. The analytical results from these wells indicated that iron and manganese concentrations were typically below detection limits and were intermittently at or above the respective MCLS possibly due to higher turbidity in those samples. In the case of the Brown's #1 well, samples have exceeded the MCLS for iron and manganese more often than other wells. The Brown's #1 well is a standby well that is used very infrequently and it would be expected that samples from this well be more turbid, despite the ample amount of purging that has been performed prior to sampling the well (upwards of 10 casing volumes). Support for the assumption that the elevated iron and manganese concentrations are derived from fine sediment in 4 turbid samples is provided by field observations during sample collection. During the periods i when iron and manganese concentrations were elevated, the sample was observed to be turbid; I alternately, during the periods when these constituents were very low or non-detectable, the well was or had been pumping already and sample turbidity was very low. Total coliform bacteria were absent from the domestic wells during 1994, 1995, and early 1996 with the exception of the Faulkner well during the fall of 1994. Samples collected in October and from retesting in November of that year showed coliform bacteria were present, but none I