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TECHNICAL REPORTS

Heavy Metals in the Environment

Accelerated Weathering of Biosolid-Amended Copper Mine Tailings

^a Department of Soil, Water, and Environmental Science, University of Arizona, 429 Shantz Building #38, Tucson, AZ 85721
^b Geosystems Analysis, 2015 North Forbes, Suite 205, Tucson, AZ 85745

^* Corresponding author (thompson{at}ag.arizona.edu)

Received for publication November 2, 2004. Application of municipal biosolids to mine tailings can enhance revegetation success, but may cause adverse environmental impacts, such as increased leaching of NO^–₃ and metals to ground water. Kinetic weathering cells were used to simulate geochemical weathering to determine the effects of biosolid amendment on (i) pH of leachate and tailings, (ii) leaching of NO^–₃ and SO₄^2–, (iii) leaching and bioavailability (DTPA-extractable) of selected metals, and (iv) changes in tailing mineralogy. Four Cu mine tailings from southern Arizona differing in initial pH (3.3–7.3) and degree of weathering were packed into triplicate weathering cells and were unamended and amended with two rates (equivalent to 134 and 200 Mg dry matter ha^–1) of biosolids. Biosolid application to acid (pH 3.3) tailings resulted in pH values as high as 6.3 and leachate pH as high as 5.7, and biosolids applied to circumneutral tailings resulted in no change in tailing or leachate pH. Concentrations of NO^–₃–N of up to 23 mg L^–1 occurred in leachates from circumneutral tailings. The low pH of the acidic tailing apparently inhibited nitrification, resulting in leachate NO^–₃–N of <5 mg L^–1. Less SO₄^2––S was leached in biosolid-amended versus unamended acid tailings (final rate of 0.04 compared with 0.11 g SO₄^2––S wk^–1). Copper concentrations in leachates from acidic tailings were reduced from 53 to 27 mg L^–1 with biosolid amendment. Copper and As concentrations increased slightly in leachates from biosolid-amended circumneutral tailings. Small increases in DTPA-extractable Cu, Ni, and Zn occurred in all tailings with increased biosolid rate. Overall, there was little evidence of potential for adverse environmental impacts resulting from biosolid application to these Cu mine tailings.

Abbreviations: MCL, maximum contaminant level