LAUSR

Abstract Water-quality effects after remediating abandoned draining mine tunnels using structural bulkheads were examined in two study areas in Colorado, USA. To improve water quality in Lake Fork Creek, a tributary to the upper Arkansas River, a bulkhead was installed in 2009 in the Dinero mine tunnel. Although bulkhead installation caused improvements in pH, and manganese and zinc concentrations and loads at the Dinero mine tunnel, water-quality degradation was observed at the nearby Nelson tunnel. Only manganese concentrations improved in Lake Fork Creek downstream from the tunnel. To improve water quality in Cement Creek, a tributary of the Animas River, multiple bulkheads were installed in mine tunnels during 1996-2003 and a water treatment plant operated from 1989-2003 to treat drainage from several draining tunnels. After bulkhead installation and cessation of active water treatment (about 2003), water quality (pH and dissolved copper, manganese, and zinc concentrations) degraded at the mouth of Cement Creek. The patterns and timing were similar to post-bulkhead increased discharge and trace-metal loads at non-bulkheaded tunnels indicating the bulkheads might have been the cause. Pre-1989 water-quality data for Cement Creek are scarce, although limited historical data indicate possible, slight improvement in only manganese concentrations after bulkhead installation. In both study areas, data potentially indicate increased, unsampled groundwater discharge after bulkhead installation that caused increased zinc loads at site LF-580 downstream from the Dinero bulkhead and decreased pH over time at Cement Creek. In these two study areas, bulkheads did not substantially improve downstream water quality.