INNOVATIVE USE OF NATURAL TRACERS IN MOUNTAIN HYDROLOGY: LEADVILLE CO AND VALLES CALDERA, NM

Understanding of sources and pathways of groundwater is vital for mountain hydrology. Hydrologic models are usually impractical in modeling the flowpaths because of difficulties in parameterizing the mountain catchments with complex terrains and highly fractured bedrocks. Here we report on innovative use of isotopic and geochemical tracers in understanding flowpaths in a draining tunnel of mine working waters in Leadville, CO and in natural streams in Valles Caldera, NW. Understanding flowpaths and source waters in Leadville is problematic because of the occurrence of thousands of mine shafts, stopes, and adits. In Leadville, mixing spaces were determined by the eigenvectors of Principal Component Analysis (PCA) based on compositions of 11 isotopic and geochemical tracers in the tunnel outflow. O-18 and tritium were used as benchmarks to determine the number of end-members and conservative geochemical tracers. Two mixing spaces were determined to be appropriate for O-18 and tritium, indicating that three end-members contribute to the tunnel outflow. Geochemical tracers, Ca²⁺, Mg²⁺, Na⁺, K⁺, Si, ANC and conductance, behaved similarly as isotopes and thus believed to be conservative. Three end-members contributing to the tunnel outflow were finally identified and quantified by all conservative tracers using end-member mixing analysis (EMMA): mine pool water (42%), shallow groundwater (35%) and deep groundwater (23%); individual contributions varied seasonally. The selected end-members and their contributions were validated by predicting isotopic and chemical compositions in the tunnel outflow using the EMMA results. Source waters and flowpaths from Valles Caldera will also be presented following the same procedure.