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• Bicarbonate/Sulfate <br />Bicarbonate (Figure 28) and sulfate (Figure 29) show a trend of topological <br />change similar to that observed in the surface waters. The bicarbonatelsulfate <br />ratio (Figure 30) during much of the previous decade showed the alluvial waters at <br />TR-1.5 to be of a bicarbonate type while the alluvial waters at TR-3 and TR-4 were <br />of a sulfate type. However, waters at TR-1.5 have been tending to become more <br />sulfate over the period of record. During the 1990-1992 monitoring period, water at <br />TR 1.5 was commonly of the sulfate type, particularly during early spring and <br />summer. The upstream source of the sulfate discontinued production in late 1992, <br />but resumed again in mid-1994. Since mid-1994, the waters at TR-1.5 have remained a <br />sulfate type. During 1996-1998, the bicarbonate/sulfate ratio at TR-1.5 was very <br />similar to that found in WR-1 providinq further evidence that the source of sulfate <br />may be from a sulfur rich source such as leakage from an old abandoned underground <br />mine. <br />The bicarbonate value of 895 mg/1 at TR-4 during May 1990 and the sulfate <br />value of 2165 mg/1 at TR-4 in August 1992 are believed to be analytical errors as <br />the sample points before and after the sample dates are more consistent with overall <br />• trends. <br />Dissolved Iron/Manganese <br />Dissolved iron (Figure 31) and manganese (Figure 32) levels for the three <br />alluvial wells remained low during 1996 as in previous years. Manganese values at <br />WR-1 appear to reaching an equilibrium with a seasonal cycle beginning to develop. <br />• <br />37 <br />