Geologic Misfortune: The Source, Fate, and Transport of Rare Earth Elements in the Snake River Watershed, Montezuma, Colorado.

The environmental impacts of acid rock drainage (ARD) and acid mine drainage (AMD) are a problem facing many waterways across the Rocky Mountains and throughout the world, particularly in areas of historic mining. Here we examine ARD/AMD enriched in rare earth elements (REE) in the Snake River watershed, located near the former mining boomtown of Montezuma, Colorado. Long-term data sets of precipitation, temperature, river discharge at many sites throughout the region show decreasing trends in summer flows from 1980 to 2010 which correlate to a snowpack melting occurring 2-3 weeks earlier. A 30-year water chemistry data set from the Upper Snake River further shows that metal concentrations are 100 to 400% higher than baseline concentrations during low-flow months. In addition, a low water table and decreased snow cover have increased the area of exposed sulfide minerals and the production of ARD, enhancing dissolution of metals from the disseminate pyrite present in the country rock and other secondary minerals present. The concentration of REEs present in tributaries of ARD-impacted Upper Snake and AMD-effected Peru Creek are three orders-of-magnitude higher than rivers worldwide and offer the unique opportunity to contrast their behavior against other dominant aspects of water chemistry. Iron is the predominant metal present both in the Upper Snake River and Peru Creek. In these headwaters, acidic water with pH 3.3 to 3.8 maintains a significant fraction of iron and aluminum dissolved, but mixing with shallow groundwater and pristine, circumneutral surface inflows downstream facilitate oxide precipitation. Of additional influence are diurnal changes in the valence state of dissolved iron coupled with the re dissolution of precipitates owing to competing microbial and photochemical reactions. This study hypothesizes that headwaters loading of REEs are historically increasing at rates observed with other conservative solutes such as zinc. Similarly suggested and also related to alterations of hydrologic regime, are the enhanced enrichment of REEs by periods of extreme flow. In support, investigations for this thesis present data collected over the last 3 years regarding the sourcing, fate, and transport of rare earth elements as well as in relation to other solutes in the Snake River Watershed