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i~ <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 />' 1.5 have been [ending [o become more sulfate over the period of record. During [he 1990-1992 monitoring period, water at <br />TR-1.5 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, [he waters a[ TR-1.5 have remained a <br />sulfate type. During 1996-2002, the bicarbonate/sulfate ratio at TR-1.5 was very similar to that found in WR-1 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 />' Dissolvedlron/Manganese <br />Overall, dissolved iron (Figure 31) and manganese (Figure 32) levels for the three alluvial wells remained low during <br />' 2002 as in previous years. However, in August 2002, [he iron concentrations in TR-1.5 and WR-1 spiked. The values <br />returned to historical levels in October 2002. In July 2001, [he manganese level at WR-1 elevated in a similar manner to the <br />' 1995 through 1998 data. This trend was missing in the 1999 through 2000 data. <br />37 <br />