POTENTIAL EFFECTS OF CLIMATE CHANGE ON WATER QUALITY IN MINERALIZED WATERSHEDS

A unique long-term water chemistry dataset has been compiled for Upper Snake Creek, a stream draining a mineralized alpine watershed in the Front Range of Colorado, USA. Trace metal (mainly zinc) and sulfate concentrations in stream water at baseflow have increased by a factor of two to three over the past three decades. Although small mines are located in the watershed, they are abandoned and no mining or mine-remediation activities have occurred during this period. A similar increase in sulfate concentration has been observed in another part of the Colorado Rockies in lakes within mineralized watersheds also free of recent mining activity. These trends are concerning because mineralized watersheds are common throughout the mountains of the western US, and further water quality degradation could negatively impact downstream drinking water resources and aquatic ecosystems. Climate change is a potential cause, but specific links between climate and the natural production of low-pH, metal-rich ground and surface water (acid-rock drainage) in mineralized watersheds is poorly understood. In this study, we employ the code TOUGHREACT to perform schematic numerical flow and reactive transport simulations to evaluate the relative importance of different climatic and hydrologic variables in the oxidation of sulfides in the subsurface in unmined settings. Inverse modeling techniques are used to quantify the sensitivity of acid-rock drainage production rates to various parameters. Preliminary results suggest that water-table depth, recharge rate, and the magnitude of seasonal water-table fluctuations are of primary importance. Climate change scenarios involving increasing temperature and decreasing snowpack, as predicted for much of the western US, may thus portend further water quality degradation in many mineralized watersheds due to decreasing groundwater recharge and falling water tables.