The May Day Mine dump is located on a mountain side above Cement Creek, north of Silverton,<br>Colorado. As Cement Creek flows past the May Day Mine, dissolved metal loads increase in the<br>stream, although there are no tributaries entering the stream reach. It had been thought that these metals<br>were being leached from the dump by snowmelt and rainwater. Induced polarization studies show local<br>pockets in the upper bench of the May Day Mine dump with acidic pore waters. The pore water appears<br>to get less acidic as it migrates through the dump. Nevertheless, water samples from a well located at<br>the toe of the dump are highly acidic, and they contain large concentrations of dissolved metals. An<br>airborne EM survey was recently flown over the area, which shows a linear conducting feature that<br>enters Cement Creek just south of the May Day Mine dump. The airborne survey was followed up with<br>ground geoelectrical surveys, which confirm the existence of the conducting feature and suggest that it<br>dips steeply and extends to a depth of at least a few tens of meters. We speculate that this conductor<br>may reflect a fissure zone that carries water. Such a fissure zone could be the source of some of the<br>water that enters Cement Creek near the May Day Mine. The metals dissolved in that water might<br>originate through natural processes, rather than human-caused ones.


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