Surface water and groundwater interactions in natural and mining impacted mountain catchments
PhD: University of Colorado Boulder, 2014.
Understanding source waters, flow paths, and residence times of water in mountain ecosystems is important when considering critical issues including the sustainability of downstream use, contaminant transport, and the predictive capabilities of hydrologic modeling. Critical Zone development, climatic conditions, and ecosystem characteristics all influence water movement between surface and sub-surface environments in mountain catchments. Additionally, past and present impacts from hardrock mining have significantly influenced the hydrology and geochemistry of many mountain catchments. An improved understanding of surface water and groundwater interaction in natural- and human-influenced mountain environments is therefore critical to adequately manage water resources derived from the mountains.
This work uses hydrologic measurements and mixing models to address surface water and groundwater interactions in three headwater catchments along an elevational gradient (2446 m to 4084 m) within the Boulder Creek Watershed. Isotopic (δ18O and δ2H) and geochemical (Na+, K+, Si, Ca2+, Mg2+, Cl-, SO42- ANC) tracers are analyzed with End Member Mixing Analysis (EMMA) to identify temporal changes in the relative contributions of source waters to streamflow generation. Results show that streamflow is derived from groundwater, rain, and snow in the montane and sub-alpine catchments, and from groundwater, talus water, and snow in the alpine catchment. Results also demonstrate that total precipitation, the fraction occurring as snow, and runoff efficiency increased with elevation. In contrast, the relative contributions of groundwater to streamflow decreased with elevation.
This dissertation also includes two investigations of water sources and pathways leading to the generation of Acid Mine Drainage (AMD) at abandoned hardrock mines. At the Argentine mine in Rico, CO hydrologic connections were identified using natural and applied tracers including isotopes, ionic tracers, and fluorescent dyes. Stable water isotopes (δ18O/δD) identified a well-mixed hydrological system, while tritium levels in mine waters supported a fast flow-through system with mean residence times of months to years. At the Nelson Tunnel in Creede, CO a combined suite of physical hydrologic parameters, hydrogeologic information, solute chemistry, applied tracers, and isotopic tracers (3H, δ18O/δD, 87Sr/86Sr, and δ14C of DIC/DOC) were used to determine the sources, pathways, and sub-surface residence times of water contributing to AMD. Results indicated a well-mixed hydrological system where mine waters were not receiving significant direct meteoric inputs or flow from locally recharged groundwater. Instead the most significant contributions to mine discharge were from a deep groundwater flow system with apparent mean residence times of 5,000 to 10,000 years. Results from the mine studies will aid in developing targeted remediation strategies to reduce the impact of AMD to nearby surface waters.