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

Acid-Neutralizing Potential of Igneous Bedrock in Basin and Cataract Creeks, Boulder River Watershed, Montana

By G.A. Desborough¹ and P.H. Briggs²

This study shows that the igneous bedrock in the Basin and Cataract Creek basins has a high acid-neutralizing potential. Our studies of acid-generating properties of 19 mining-related waste piles of 500 tons or more in this area showed that leachates of samples from 15 piles had a final mean pH of 3.3, whereas samples from only three piles had a final leachate pH of between 5.9-6.8. However, the pH of stream water is routinely higher than 7.0 at all but one monitoring site in the Basin and Cataract Creek basins (Nimick and Cleasby, 1998, this volume). Results of these two studies show that if significant amounts of acid are generated from the waste piles, any drainage must be buffered by bedrock, alluvium, soil, or stream water. To test the importance of geologic materials, we designed laboratory tests that would determine the acid-neutralizing potential of representative bedrock and soils through the use of solutions that simulate water in contact with mine wastes.

In a previous study, 100-gram (g) samples of <2.0-millimeter (mm) mine waste were exposed to 2 liters (L) of deionized water at rest. In the present study, the acidic solution (pH = 2.85) generated from the previous study was mixed with crushed, <0.090-mm fractions of fresh and altered bedrock and a <2.0-mm fraction of the soils. The ratio of rock and soil exposed to the acidic solution for 24 hours at rest was 1:20. After exposure of 11 samples of granite bedrock to the acidic solution, the mean final pH of the solutions was 6.72. Interaction of four fresh samples of Elkhorn volcanic rocks with the acidic solution yielded a mean final pH of 7.00. The reaction of seven samples of silicified altered rocks from Jack Mountain with the solution yielded a mean final pH of only 3.90, and 13 soil samples provided a mean final pH of 4.20.

Igneous bedrock of the area contains minor, but important, amounts of calcite. Calcite was not detected in any of these igneous rocks using three different X-ray diffractometer units. Similarly, prior petrographic studies of about 250 thin sections of the igneous intrusive rocks from the Basin 15-minute quadrangle reported no carbonate minerals. However, using a chemical stain (alizarin red) on large 10 · 10 centimeter sawed rock slabs, we found dispersed calcite in all but one fresh igneous sample. Small amounts of calcite appear to be partly responsible for the acid-neutralizing capacity of the fresh igneous rock. Minor amounts of other carbonates that we did not detect also may contribute to the acid-neutralizing potential of these rocks. The rapid neutralization of acidic solutions observed for the igneous rocks (5-30 minutes) is consistent with carbonate dissolution. Although intermediate plagioclase (andesine) in these rocks may buffer acidic solutions, it reacts much more slowly (several to many days) than calcite.

The high acid-neutralizing potential of igneous bedrock, which is widespread in the Basin and Cataract Creek basins, has favorable implications for the siting of a repository for acid-generating waste because the bedrock can be expected to neutralize some or perhaps all of the acid generated by waste in the repository.

¹U.S. Geological Survey, MS 905, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (gdesboro@usgs.gov)

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

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