Environ. Sci. Technol. 1999, 33, 2626-2632
Assessing the Efficacy of Lime Amendment To Geochemically Stabilize Mine Tailings A N D Y D A V I S , * ,† L . E . E A R Y , ‡ A N D S. HELGEN† GEOMEGA, 2995 Baseline Road, Suite 202, Boulder, Colorado 80303, and Shepherd Miller, 3801 Automation Way, Suite 100, Fort Collins, Colorado 80525
Historical mining practices in Butte, MT, resulted in deposition of tailings as overbank deposits in the Clark Fork River. This study used pore water chemistry, electron microprobe analysis, and geochemical modeling to compare the chemistry of these acid-generating deposits 5 years after liming with naturally revegetated and barren tailings. The sequence of weathering reactions is predictable, covering a continuum from acidic soils through the lime-amended soils to the naturally revegetated overbank tailings deposits. For example, sulfides and sulfates predominated in the untreated tailings mineral assemblage, while liming facilitated alteration of pyrite to ferrihydrite that sequestered weight percent concentrations of As, Cu, Pb, and Zn. Collocated pore waters collected using suction lysimeters installed in the surficial treated material (10 cm, the lime amendment was tilled into the soils by deep plowing (0.3-1 m) while soils with 7) were oversaturated with respect to calcite (Figure 6a), demonstrating that the amendment is still continuing to produce alkalinity 4-5 years after addition to the tailings. The future of the GP area can also be assessed by comparison to naturally revegetated tailings, where the development of an A-horizon with associated plant litter and corresponding increased water retention capacity results in positive feedback that facilitates further colonization of tailings. The reduction in acidity and metal solubility by the formation of secondary solids results in a habitat more conducive to seed germination and plant growth. The sequence of weathering reactions covers a continuum from acidic soils (i.e., the BT and NRV soils) through the amended, near-neutral soils of the GP site to the naturally revegetated RTC overbank tailings deposits. There is a logical alteration sequence, i.e., unreacted sulfides f precipitation of ferrihydrite f sorption of metals to ferrihydrite with increasing pH. In turn, these in situ reactions result in generally decreasing pore water metal concentrations as the pore water pH becomes more neutral (Table 2). Our field results are in distinct contrast to those of Jones et al. (8), who found that laboratory columns filled with limeamended tailings leached As at 10-4 M as compared to ∼10-6.3 M in our study. The discrepancy may be due to the fact that these workers limed their tailings immediately prior to their column experiments. In contrast, we investigated tailings VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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that had 4-5 years to react in situ with the applied lime, probably resulting in more complete incorporation of As within the matrix of the secondary precipitates. This comparison clearly identifies the importance of collecting sitespecific data to interpret the efficacy of any process rather than relying solely on bench-scale experiments. Our site data in conjunction with humidity cell tests designed to assess the acid generating properties of mine waste (see Supporting Information) demonstrate that lime amendment in the GP area has effectively sequestered metals in tailings and hence represents a viable long-term alternative to mitigate a historical problem.
Acknowledgments The authors appreciate the help of Larry Petersen and staff at Exponent, Boulder, CO, for assistance with lysimeter installation and sampling and two anonymous reviews that resulted in a more comprehensible manuscript.
Supporting Information Available Text, 8 figures, and 1 table (15 pages) provided. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited (1) Noble, A. C. Report to the Anaconda Reduction Works Superintendent on repairing tailings dyke located on the Henry Williams Estate Ranch and attached map. Montana Historical Society, MC-169, Box 248, Folder 6, Butte, MT, 1911. (2) Essig, D. A.; Moore, J. N. Clark Fork damage assessment, river bed sediment sampling and chemical analysis report. Prepared for Clark Fork Natural Resource Damage Program, University of Montana, Missoula, MT, 1992. (3) Geomorphology, Flood-Plain Tailings, and Metal Transport in the Upper Clark Fork Valley, Montana; Water-Resources Investigations Report 98-4170; U.S. Geological Survey: Helena, MT, 1998. (4) Nimick, D. A.; Moore, J. N. Appl. Geochem. 1991, 6, 635-646.
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(5) Phillips, G.; Lipton, J. Can. J. Fish. Aquat. Sci. 1995, 82, 19901993. (6) Schafer and Associates. Clark Fork River Demonstration Monitoring (1993-1995): Final Report, Bozeman, MT, 1996. (7) Nimick, D. A.; Moore, J. N. Stratigraphy and chemistry of sulfidic flood-plain sediments in the Upper Clark Fork Valley, Montana. In Environmental Geochemistry of Sulfide Oxidation; American Chemical Society: Washington, DC, 1994; pp 276-288. (8) Jones, C. A.; Inskeep, W. P.; Neumann, D. R. J. Environ. Qual. 1997, 26, 433-439. (9) Chappell, R. W.; Davis, A.; Olsen, R. L. Portable X-ray fluorescence as a screening tool for analysis of heavy metals in soils and mine wastes. In Proceedings of the 7th National Conference on Management of Controlled Hazardous Waste Sites, Washington, DC, 1986. (10) Govindaraju, K. Geostand. Newsl. 1989, 13. (11) Davis, A.; Drexler, J. W.; Ruby, M. V.; Nicholson, A. Environ. Sci. Technol. 1993, 27, 1415-1425. (12) Hach DR/2000 Spectrophotometer Procedures Manual; Hach Co.: Loveland, CO, 1994. (13) Nordstrom D. K. Aqueous pyrite oxidation and the consequent formation of secondary iron minerals. In Acid Sulfate Weathering; Soil Science Society of America: Madison, WI, 1982; pp 37-62. (14) Karanthansis, A. D.; Evangelou, V. P.; Thompson, Y. L. J. Environ. Qual. 1988, 17, 534-543. (15) Ruby, M. V.; Davis, A.; Nicholson, A. Environ. Sci. Technol. 1994, 28, 646-654. (16) Eary, L. E.; Jenne, E. A. Version 4.00 of the MINTEQ geochemical code; PNL-8190/UC-204; Pacific Northwest Laboratory: Richland, WA, 1992. (17) Runnells, D. D.; Skoda, R. E. Redox modeling of arsenic in the presence of iron: Applications to equilibrium computer modeling. In Environmental Research Conference on Groundwater Quality and Waste Disposal; EN-6749; Electric Power Research Institute: Palo Alto, CA, 1990.
Received for review September 29, 1998. Revised manuscript received March 22, 1999. Accepted May 11, 1999. ES9810078
