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<br />' Bicarbonate/Sulfate <br />' Bicarbonate (Figure 28) and sulfate (Figure 29) show a trend of topological change similar to that observed in the <br />surface waters. The bicarbonate/sulfate ratio (Figure 30) during much of the previous decade showed the alluvial waters at <br />TR-I.5 to be of a bicarbonate type while the alluvial waters at TR-3 and TR-4 were of a sulfate type. However, waters at TR- <br />' I.5 have been tending to become more sulfate over the period of record. During the 1990-1992 monitoring period, water at <br />TR I.5 was commonly of the sulfate type, particularly during early spring and summer. The upstream source of the sulfate <br />' discontinued production in late 1992, but resumed again in mid-1994. Since mid-1994, the waters at TR-I.5 have remained a <br />sulfate type. During 1997-1999, the bicarbonate/sulfate ratio at TR-I.5 was very similar to that found in WR-I providing <br />' further evidence that the source of sulfate may be from a sulfur rich source such as leakage from an old abandoned <br />underground mine. <br />' The bicarbonate value of 895 mg/I at TR-4 during May 1990 and the sulfate value of 2165 mg/I at TR-4 in August <br />1992 are believed to be analytical errors as the sample points before and after the sample dates are more consistent with <br />overall trends. <br />Dissolved Iron/Manganese <br />' Dissolved iron (Figure 31) and manganese (Figure 32) levels for the three alluvial wells remained low during 1999 <br />as in previous years. Manganese values at WR-I appear to reaching an equilibrium with a seasonal cycle beginning to <br />' develop. <br /> <br />1 <br />1 <br /> <br />1 <br />1 <br /> <br /> <br />37 <br />