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

Quantification of Metal Loading by Tracer-Injection Methods in Cataract Creek, Boulder River Watershed, Montana: Study Design

By Tom E. Cleasby,¹ David A. Nimick,² and B.A. Kimball³

Tracer-injection methods can be used to determine streamflow at numerous points more accurately and quickly than is possible with traditional current-meter measurements. Consequently, tracer-injection methods combined with sampling for metals can be useful for locating and quantifying individual and diffuse sources of metal loading to streams in basins affected by abandoned mine lands and for studying instream geochemical processes that affect metal transport. The highest concentration of inactive metal mines in the Deerlodge National Forest are in the Cataract Creek watershed, and Cataract Creek is a major contributor of metals to the Boulder River. Cataract Creek was selected for this tracer-injection study because many abandoned mines in the basin are adjacent to Cataract Creek, making it possible to quantify both surface and subsurface inflows. In addition, geochemical processes affecting metal transport downstream of Uncle Sam Gulch, the largest metal contributor to Cataract Creek, could be studied.

During a 3-day field reconnaissance in July 1997, the length of the stream was measured, and tributaries and inflows from adits, tailings, and waste-rock piles were identified. The study reach is 7.7 miles long and extends from a point upstream of most mine workings, where effects of abandoned mines were thought to be minimal, downstream to the confluence with the Boulder River. Four tributaries, 21 minor inflows, and 47 instream sites were chosen for sampling during the tracer study on the basis of their location upstream or downstream from inflows or suspected metal sources. Water samples collected during the reconnaissance indicated that pH values at all sites were higher than 6.8 and that ambient chloride concentrations were <0.35 milligrams per liter.

During a 3-day period in August 1997, a tracer solution containing 219 grams per liter NaCl was injected continuously at the most upstream site at a rate calculated to increase the chloride concentration to about 3 milligrams per liter at the mouth of Cataract Creek, where streamflow was about 11 cubic feet per second. After the tracer concentration reached a constant level, samples were collected within 12 hours from the previously identified sites and analyzed for chloride and for total recoverable and dissolved (using 0.1-micrometer filtration) metals. When available, the analytical data will allow calculation of metal loads at each of the 72 sites to determine the location of important sources.

In past use of tracer-injection methods for determining metal loading in abandoned-mine-land areas, study streams typically have been acidic with high metal concentrations. In contrast, Cataract Creek has lower metal concentrations and neutral to slightly alkaline pH. Results of this study can be used to assess the utility and sensitivity of tracer-injection methods in streams with more moderate mine-drainage effects.

¹U.S. Geological Survey, Drawer 10076, Federal Building, Helena, MT 59626 (tcleasby@usgs.gov)

²U.S. Geological Survey, Drawer 10076, Federal Building, Helena, MT 59626 (dnimick@usgs.gov)

³U.S. Geological Survey, 1745 West 1700 South, Salt Lake City, UT 84104 (bkimball@usgs.gov)

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