USE OF PRE- AND POST-BULKHEAD WATER QUALITY TO ASSESS THE EFFECT OF BULKHEAD EMPLACEMENT IN THE DINERO TUNNEL, SUGARLOAF MINING DISTRICT, NEAR LEADVILLE, COLORADO

Mining of vein deposits, chiefly for silver, occurred in the Sugarloaf mining district, west of Leadville, Colorado, from the late 1800s to the mid 1900s. The Dinero mine tunnel is an approximately mile-long tunnel that drained the deposits and facilitated ore transport to the surface during mining. More recently, however, drainage from the Dinero tunnel was identified as a major contributor of zinc and manganese loads to the Lake Fork of the Arkansas River. Consequently the Bureau of Land Management and Colorado Division of Reclamation, Mining, and Safety remediated the tunnel by installing a structural bulkhead that was sealed in October 2009. Prior to bulkhead installation (2006), seasonal monitoring of discharge and water quality occurred at all drainage features and streams within about a 1-mile radius of the Dinero tunnel. All previously sampled locations and any new drainage features created following bulkhead closure will be sampled for 3 years (2010 – 2012) to quantify changes in discharge and water quality resulting from bulkhead closure. Comparison of pre-bulkhead water-quality data (pH, specific conductance, alkalinity, major and trace element concentrations, and stable isotopes) between sites grouped as mining features, seeps, springs, and streams indicated that stable isotope ratios (deuterium and oxygen in water) are the most useful measures for discriminating between sample groups affected by mine drainage. The Dinero tunnel water is the most isotopically depleted of all samples collected in the area and presumably represents deeper, older groundwater compared to other samples. Thus, post-bulkhead shifts to more depleted isotopic values at other sample sites may indicate migration of Dinero tunnel water to those sites. Results will help assess not only changes caused by bulkhead emplacement, but also may demonstrate the utility of detailed pre-and post-bulkhead water-quality sampling to evaluate similar sites.