DIURNAL TRENDS IN DISSOLVED REE CONCENTRATIONS IN A MINERALIZED WATERSHED, MONTEZUMA, COLORADO

The environmental impact of acid rock drainage (ARD) is a problem facing many waterways across the Rocky Mountains and throughout the world, particularly in areas of historic mining. Here we examine ARD 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 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, and has enhanced dissolution of metals from the Silver Plume Granite plus schist and gneiss of the Idaho Springs Formation. Of particular interest is the presence of REE, found in unusually high concentration in tributaries to the Upper Snake. The tributary studied in this project has REE concentrations that are an order-of-magnitude higher than adjacent streams. Iron is the predominant metal present in the Upper Snake River. Acidic water with pH 3.3 to 3.8 maintains a significant dissolved iron concentration, but mixing with higher pH soil and groundwater inflows facilitates the precipitation of iron hydroxides. Photochemistry also plays a large role in the sorption of metals to and from these precipitates. 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 presents data from a 24-hour sampling event in August 2013 during low flow conditions, in which the fate of REE are discussed as a function of iron speciation, dissolved organic matter composition, and concentration.