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1 1 9 n the first three years of monitoring. Nitrate concentrations declined during early 1997 through 1998 in several of the shallow and deep wells (Figure 14), including the South Tracy#1 well; these concentrations have remained lower since then. The lowest nitrate concentrations have typically been observed in samples from the Brown's wells. It should be noted that a progressive increase in nitrate concentration that had been observed in the Hall well, from 15 to 31 mgll over the first three years of monitoring, stabilized during 1997, and has subsequently declined to approximately 20 mg/l. A similar decline in nitrate concentration has been observed in the Faulkner well. The earlier elevated nitrate concentrations appear to be indicative of historical agricultural land use in the area prior to the commencement of aggregate mining by Teichert Aggregates. Teichert's aggregate extraction did not commence until 1998, with only a very limited removal of surficial earth materials n conducted under emergency conditions in January 1997 (L. Noble, Teichert Aggregates, personal communication, March 11, 1998). The only other constituents to exceed MCLS were iron and manganese in the Koster well (only once), South Tracy#1 well (twice), and the two Brown's wells. It is important to note that none of 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. S 0% AE N P In the case of the Brown's wells, samples have exceeded the MCLs for iron and manganese more often than other wells, and this has continued to be the case during 1999. 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). The Brown's New well had in the past been used less frequently due to operational problems and had shown similar elevated iron and manganese concentrations. Support for the assumption that the elevated iron and manganese concentrations are derived from fine sediment in turbid samples is provided by field observations during sample collection. During the periods when iron and manganese concentrations were elevated, the sample was observed to be © LOHOORFF 6 SCALMANINI r) C O N S U L T I N G ENGINEEPS