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

What Streams Are Affected by Historic, Abandoned Mines?—Preliminary Interpretation of Bed-Sediment Geochemical Data, Boulder River Watershed, Montana

By D.L. Fey¹ and S.E. Church²

Trace elements associated with mine wastes and drainage typically accumulate in the bed sediments of streams downstream of abandoned mines. Data on trace-element concentrations in bed sediment can be used to define stream reaches affected by historic mining, determine sources of the contaminated material, understand the transport of dissolved and particulate trace elements, and evaluate the potential metal toxicity of bed sediment to biota.

We collected bed-sediment samples from the basins of High Ore, Cataract, and Basin Creeks, all tributaries to the Boulder River. We took 47 samples in October 1996 and 36 samples in July 1997. Sample localities were selected to verify and supplement previously collected data and to further define first- and second-order stream reaches affected by historic mining activities. We also collected sediment samples from the Boulder River at sites above and immediately below the confluences of the three impacted drainages and at sites downriver to its confluence with the Jefferson River. Sediment samples downstream of the confluences with the impacted streams were taken from both sides of the river (north and south banks) to assess differences in metals distributions caused by the inflow from the tributaries.

We used three different digestions to determine trace-element concentrations in three operationally defined phases of the bed sediment: (1) a mixed strong-acid complete digestion to determine total concentrations, (2) a warm (50 degrees Celsius) 2 molar HCl-H₂O₂ leach to determine metals associated with iron oxy/hydroxide mineral coatings and colloidal particles (the leachable phase), and (3) a mixed strong-acid complete digestion on the resulting leach residues to determine metals bound in the silicate phases. Solutions from all digestions were analyzed by inductively coupled plasma-atomic emission spectroscopy. Preliminary interpretation of data for the 1996 samples indicates the following:

1. Concentration profiles show that leachable trace-element concentrations in bed sediment are highest just downstream of the Comet Mine in High Ore Creek and in Jack Creek and Uncle Sam Gulch downstream of the Bullion-Crystal Mine lineament. The volumes of mine waste available from these two areas are also the largest in the study area. Therefore, these areas are considered the most significant sources of metal contamination in the study area.
2. Antimony, arsenic, copper, lead, and zinc are concentrated in the leachable phase and reflect sorption of these trace elements to the particulate phase. The concentration profile for each element is determined both by the rate at which the dissolved element sorbs to particulates and the rate of downstream transport of sediment.
3. The downstream concentration profiles for antimony, arsenic, and lead are similar and indicate that these elements are removed from solution by sorption within a short distance from their sources.
4. Copper and zinc are carried further downstream from source areas than antimony, arsenic, and lead. Sorption of copper and zinc depends on water chemistry; pH is probably the dominant variable. This same phenomenon has been observed in the Animas River watershed (Church and others, 1997).
5. Historic mines in the study area have affected the Boulder River. Leachable concentrations of arsenic, copper, lead, and zinc in bed sediment are elevated above crustal abundance (background values) for more than 50 kilometers from High Ore Creek downstream to the Jefferson River. Although the data are limited by analytical sensitivity, leachable concentrations of antimony in bed sediment probably decrease to crustal- abundance values upstream of the town of Boulder.

¹U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dfey@usgs.gov)

²U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (schurch@usgs.gov)

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