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Bicarbonate/Sulfate Bicarbonate (Figure 28) and sulfate (Figure 29) show a trend of topological change similar to that observed in the surface waters. The bicarbonate/sulfate ratio (Figure 30) during much of the previous decade showed the alluvial waters at TR-1.5 to be of a bicarbonate type while the alluvial waters at TR-3 and TR-9 were of a sulfate type. However, waters at TR-1.5 have been tending to become more sulfate over the period of record. During the 1990-1992 monitoring period, water at TR 1.5 was commonly of the sulfate type, particularly during early spring and summer. The upstream source of the sulfate discontinued production in late 1992, but resumed again in mid-1994. Since mid-1999, the waters at TR-1.5 have remained a sulfate type. During 1996-1997, the bicarbonate/sulfate ratio at TR-1.5 was very similar to that found in WR-1 providing further evidence that the source of sulfate may be from a sulfur rich source such as leakage from an old abandoned underground mine. The 'oicarbonate value of B95 mg/i at TR-9 during May 1990 and the sulfate value of 2165 mg/1 at TR-9 in August 1992 are believed to be analytical errors as the sample points before and after the sample dates are more consistent with overall trends. Dissolved Iron/Manganese Dissolved iron (Figure 31) and manganese (Figure 32) levels for the three alluvial wells remained low during 1997 as in previous years. Manganese values at WR-1 appear to reaching an equilibrium with a seasonal cycle beginning to develop. A similar cycle for both parameters appears to be developing at TR-4. Since 1988, an elevated manganese concentration has been noticed during the first portion of the annual monitoring period at WR-1 with a significant decrease occurring by the end of the year. Over time the initial concentration has decreased suggesting that the sources of leachate enriched with manganese are being depleted. • 37