WATER TRACING AT SCALES OF HOURS TO DECADES AS AN AID TO ESTIMATING HYDRAULIC CHARACTERISTICS OF THE LEADVILLE MINE DRAINAGE TUNNEL

The Leadville Mine Drainage Tunnel (LMDT) is a 3.3 kilometer structure that was constructed in the complicated geology of the Leadville mine district in the 1940’s. Discharge from the LMDT is impacted by heavy metals and is treated at a plant built in 1992 operated by the United States Bureau of Reclamation. On the surface waste rock and other remnants of the mining operations litter the landscape and this material is exposed to precipitation. As a result of contact with this material, surface water often has pH of less than 3 and its containment and disposal is necessary before it impacts surface drainage and the nearby Arkansas River. Using a borehole drilled into the mine workings the U.S. EPA has devised a plan in which the impacted water is contained on the surface which then can be discharged into the mine workings to discharge from the LMDT and be treated. The percentage of water discharging from the mining district along the drainage tunnel is unknown, and since there is no access, information about the condition of the tunnel with regards to blockages is also relatively obscure. Application of quantitative water tracing using fluorescent dyes was used to model the flow parameters at the scale of hours in the tunnel and evaluate the likelihood of blockages. Because the tunnel has intersected several lithologies and faults, other locations such as discharging shafts, adits and surface streams that could be hydraulically connected to the LMDT were also monitored. An initial tracer experiment was done using an instantaneous injection, which was followed by additional injections of water. Another tracer injection was done when there was a continuous flow of impacted water into the workings. Analysis of the tracer concentration responses at water-filled shafts and at the portal were used to model the flow along the tunnel and estimate several hydraulic parameters. Waters in these settings are mixtures of components with different residence times, so, qualitative tritium data were used to evaluate residence times of decades. The combined injected tracer and tritium data as well as other geochemical data were used to infer the nature of flow and recharge into the tunnel.