BULKHEAD INSTALLATION AT THE DINERO MINE TUNNEL ALTERS WATER QUALITY AND GROUNDWATER EQUILIBRIA

Dinero tunnel is an abandoned, draining mine tunnel located in the Sugar Loaf Mining District near Leadville, Colorado, and is a major contributor to zinc loads in Lake Fork Creek. Consequently, in 2009, a bulkhead was installed in Dinero tunnel to reduce drainage and improve water quality and aquatic habitat downstream in Lake Fork Creek. Monitoring during 2006 and 2010 to 2012 indicated water-quality improvement in Lake Fork Creek (zinc concentrations and loads decreased). However, water quality degraded in areas adjacent to Dinero tunnel (pH decreased and zinc concentrations increased) likely due to increased water-table elevation behind the bulkhead. In particular, flow and metal concentrations, including concentrations of zinc and iron, increased at the Nelson tunnel located near the Dinero tunnel. Oxidation and precipitation of iron oxyhydroxide minerals caused pH to decrease at a site downstream from Nelson tunnel from greater than 6 to less than 4 before and after bulkhead installation. Maps of underground mine workings show no direct connection between the Dinero and Nelson tunnels. However, a mineralized geologic structure occurs between the tunnels. Increased groundwater flow along this structure after bulkhead emplacement likely explains the increased flow and decreased water quality in the Nelson tunnel. As of 2012, the decreased water quality in Nelson tunnel had not negated the water-quality improvement noted in Lake Fork Creek downstream from Dinero tunnel, possibly due to the mitigating effects of a wetland that is located between Nelson tunnel and Lake Fork Creek. Continued monitoring will help assess if water-quality degradation continues adjacent to Dinero tunnel, and if low pH, zinc-rich water breaks through the wetland area and negates water-quality improvement in Lake Fork Creek.