Overview of Rare Earth Element Investigations in Acid Waters of U. S. Geological Survey Abandoned Mine Lands Watersheds

The geochemistry of rare earth element (REE) variations in acid waters is being studied as part of the U. S. Geological Survey Abandoned Mine Lands Initiative in two pilot watersheds, upper Animas, Colorado and Boulder, Montana. The following objectives are under investigation: (1) comparison of acid mine waters and naturally acidic springs, (2) determination of whether the dominant control on REEs in acid waters is source-related or post-dissolution process-related, (3) determination of the role of iron and aluminum colloid formation on the REE patterns, (4) address the utility of REE geochemistry in acid waters as an analogue for the actinides, and (5) produce a Standard Reference Water Sample for REEs. Results demonstrate that the REE concentrations in acid waters increase with decreasing pH but tend to be two to three orders of magnitude lower than ore elements such as Cu and Zn. REE patterns are generally convex-up for waters in the upper Animas, and they are nearly flat with a negative europium anomalies for waters in the Boulder basin. These results reflect predominantly source-related signatures. Natural acid springs are frequently, but not consistently, characterized by a negative Ce anomaly that may be process-related. Field and laboratory experiments indicate that dissolved REEs are affected by iron and aluminum colloid formation but sorption or coprecipitation with aluminum at pH values greater than 4.5 is stronger than with iron. Uranium and thorium, however, show a tendency to be removed from solution more strongly at lower pH (3-4) values, consistent with expected differences in oxidation state and a stronger affinity for iron precipitation.