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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Rare Earth Element Geochemistry of Acid Waters: Preliminary Results Identifying Source Signatures and Instream Processes

By Philip L. Verplanck,¹ Winfield G. Wright,² M. Alisa Mast,³ and Howard E. Taylor⁴

The objective of this study is to determine the major processes governing the fractionation of rare earth elements (REEs) during weathering of mined and unmined mineralized rock. REE geochemistry is a powerful tool for constraining geochemical processes. This has been demonstrated in many petrologic studies but is just beginning to be applied to aqueous systems.

The Boulder River watershed, Montana, and upper Animas River watershed, Colorado, sites are ideally suited for this study because of the multidisciplinary approach that is being used at these abandoned-mine-land sites to identify processes that potentially control the occurrence and transport of contaminants. REE geochemistry is being used with other geochemical indicators to identify source signatures of acidic waters, as well as to constrain processes controlling the fate and transport of metals upon entering the fluvial system. Preliminary data from studies of subbasins within the upper Animas River watershed show subtle variations in the REE patterns of acid waters derived from various sources. These variations are currently being investigated.

A 2-kilometer long stream reach in the South Fork Cement Creek basin was sampled during low flow in October 1996 to investigate processes affecting REE geochemistry during transport. Downstream, the pH decreases, and the loads of the REEs and major and trace elements increase. Comparing the measured load at the lowermost site to the sum of the input loads, 77-93 percent of the REEs are accounted for; in contrast measured loads of calcium, cobalt, strontium, sulfate, and zinc averaged 117 ± 3 percent of the summed input loads. The percentages less than 100 percent suggest that some REE removal occurs in this acidic, alpine stream. Iron and aluminum colloids are actively precipitating, and this process may remove REEs and other metals from the water column. Field and laboratory studies are underway to investigate the role colloids play in the removal of REEs and other metals from acid waters. The results from the first year of this study show that REE variations of acid waters may help meet the AML Initiative objective of identifying processes that control the occurrence and transportation of contaminants in alpine watersheds.

¹U.S. Geological Survey, 3215 Marine, Boulder, CO 80303-1066 (plv@usgs.gov)

²U.S. Geological Survey, 3215Marine Street, Boulder, CO 80303 (dkn@usgs.gov)

³U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (wgwright@usgs.gov)

⁴U.S. Geological Survey, 3215 Marine, Boulder, CO 80303-1066 (hetaylor@usgs.gov)

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