APPLYING NUMERICAL MODELING TO EVALUATE THE SIGNIFICANCE OF CLIMATE AND HYDROLOGY IN THE FORMATION OF NATURAL ACID-ROCK DRAINAGE IN MINERALIZED WATERSHEDS

The importance of mineralogy in the natural production of low-pH, metal-rich groundwater and surface water (acid-rock drainage, or ARD) in mineralized watersheds is well established, but the role played by climatic and hydrologic factors remains poorly understood. An understanding of ARD is important for establishing pre-mining baseline conditions in mined locations and for evaluating the potential environmental impacts of future mining in unmined areas. Climatic and hydrologic factors such as shallow-ground temperature, amount of snow versus rain, recharge rate, water-table depth, and seasonal water-table dynamics could have a nontrivial affect on ARD generation because of their potential influence on sulfide oxidation reaction kinetics and oxygen transport into the subsurface. We employ the code TOUGHREACT to perform simple but representative 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. These models include groundwater flow and oxygen transport by advection and diffusion within the unsaturated zone. Inverse techniques are used to quantify the sensitivity of ARD production rates to various parameters. Preliminary results suggest that water-table depth and the magnitude of seasonal water-table fluctuations (largely a function of snowpack depth) are of primary importance. In addition to providing a better general understanding of processes controlling ARD, results of this study should assist in predicting possible future changes in ARD generation under anticipated climate change scenarios. This in turn should address the question of whether baseline conditions and related clean-up standards may be a “moving target” in some mined watersheds in the coming decades.