RARE EARTH ELEMENTS AND ACID ROCK DRAINAGE 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. The focus of this research examines the Snake River watershed, located near the former mining boomtown of Montezuma, Colorado, which is experiencing decreasing water quality due to metal-rich, low pH inflows from abandoned mines and natural weathering of sulfide minerals present in this region.

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 2-3 weeks earlier. A related 30-year water chemistry record from the upper Snake River also highlight that metal concentrations have risen 100 to 400% over this time during summer, baseflow conditions. This advancement of peak spring snowmelt, increased drought frequency, warmer summer air temperature, reduced groundwater recharge and lower flows are cumulatively the influence of climate change and are coupled to these rising concentrations of metals through their enhancement of geochemical weathering rates.

The increasing generation of acids from these reactions are likewise responsible for the dissolution of secondary and accessory mineral stocks containing rare earth elements (REE), which are then dissolved and mobilized by these low pH waters. This study hypothesizes that headwater loading of REEs are historically increasing at rates observed with other conservative solutes such as zinc. Similarly suggested and also related to alterations of the hydrologic regime, are the enhanced enrichment of REEs by periods of extreme flow in mobilizing freshly weathered materials accumulated from the steep, unvegetated areas above the catchment.

Due of its confluence with Dillon Reservoir, decreasing water quality in the Snake River watershed has secondary implications; the City of Denver relies on this as a major source of drinking water. And although enhanced enrichment and transport of heavy metals into this lake have potential to increase water treatment costs as well as further stress the aquatic ecosystem, additional questions loom regarding the effects of rare earth elements for which no toxicity standards exist.