Release of Trace Metals from Mine Residues
Rationale:
The Keweenaw Peninsula was the scene of intensive mining activities between 1864 and 1930. Shallow deposits of "native" (elemental) copper allowed this area to become one of the most productive mining regions of the country through this time period. This mining activity left many legacies; one of the more unfortunate ones is the widespread dissemination of stamp sands (crushed ore) in area waterways. Because of the need for steam, stamp mills were located near water bodies (lakes, rivers). The ore was crushed in the mills, and elemental copper was separated from the rock by flotation. The metal-rich residue was deposited in tremendous quantities in the immediate vicinity of the stamp mill. Although mining activities ceased at many sites over 70 years ago, many of the stamp sand piles remain devoid of vegetation, lakes into which they were dumped are nearly devoid of benthic animals, and neighboring wetlands are species depauperate. Continued leaching of trace metals from the ores causes high concentrations in surface waters, concentrations that are above toxicity thresholds for many species. To determine appropriate remediation strategies for these areas, it is critical to understand the factors controlling metal leaching from the ore.
Study Sites
Three sites were chosen to represent the range of conditions encountered at stamp sand refuse piles. The Gay site is located on the shores of Lake Superior. Reworking of the stamp sands by waves and ice has created numerous small ponds within the stamp sand deposit. The entire site is large devoid of vegetation. The ponds receiving shallow groundwater inputs that contain high concentrations of DOC have noticeably more aquatic macrophytes than do the other ponds.
Boston Wetland is an extensive wetland that was partially filled in with stamp sands. The stream passing through the stamp sands continues to wash stamp sands as well as dissolved metals into the wetland. At this point, it is not known whether the death of the woody vegetation in the wetland is due to the metals or to flooding caused by construction of a dam on the stream.
Torch Lake, the third study site, is a 970-ha lake; approximately 20% of the lake volume was filled with stamp sands. Many wetlands around the lake margin were completely destroyed by the stamp sand deposits, and the benthic fauna of the lake remains severely restricted. 
Approach
The copper in the stamp sands is being characterized in terms of its mineral phase (sequential extractions, XRD) and lability (leaching experiments, acid-base titrations). Copper and major ions are being measured in surface waters to enable calculation of saturation indices for mineral phases. To determine the aqueous speciation, one student is measuring binding constants of Cu with dissolved organic matter. We will look subsequently at other trace metals.
Preliminary Results
Sequential extractions indicate that carbonate and oxide minerals are likely to be the dominant Cu-bearing phases present at all sites except Torch Lake. Analysis of pure mineral phases indicates that the sequential extraction procedure followed in this study cannot distinguish between oxides and carbonates. However, calculation of saturation indices in surface waters with MINEQL also indicates that tenorite (CuO) is present at all sites and is controlling aqueous concentrations. Preliminary titration experiments suggest that the combined "carbonate plus oxide" fractions are labile (readily soluble over short time periods).
There is a relationship between particle size and the phase of Cu present. Small particles have a larger percentage of total Cu in the authigenic carbonate and oxide phases as shown in the figure below. The large surface areas of these minerals further enhances their lability.
Further work
Additional work is planned to determine the time required for dissolution and precipitation of the copper carbonates (malachite, azurite) and oxides (tenorite). We also plan to examine the role of light and organic matter in promoting dissolution. Ultimately, the work will be extended to other trace metals present in the stamp sands and the mechanisms of the biota for coping with the high trace metal concentrations.
This project is a collaborative effort among Sarah Green (Chemistry Dept) and Noel Urban (Environmental Engineering). The work is being performed by M.S. students Jaebong Jeong and Xingfang Wang.
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Maintained by N.R. Urban
Last updated February 1997
