Article pubs.acs.org/est
Copper Speciation in Variably Toxic Sediments at the Ely Copper Mine, Vermont, United States Bryn E. Kimball,*,†,∥ Andrea L. Foster,‡ Robert R. Seal, II,† Nadine M. Piatak,† Samuel M. Webb,§ and Jane M. Hammarstrom† †
U.S. Geological Survey, Reston, Virginia, 20192 United States U.S. Geological Survey, Menlo Park, California, 94025 United States § Stanford Synchrotron Radiation Lightsource, Menlo Park, California, 94025 United States ‡
S Supporting Information *
ABSTRACT: At the Ely Copper Mine Superfund site, Cu concentrations exceed background values in both streamwater (160−1200 times) and sediments (15−79 times). Previously, these sediment samples were incubated with laboratory test organisms, and they exhibited variable toxicity for different stream sites. In this study we combined bulkand microscale techniques to determine Cu speciation and distribution in these contaminated sediments on the basis of evidence from previous work that Cu was the most important stressor in this environment and that variable observed toxicity could have resulted from differences in Cu speciation. Copper speciation results were similar at microscopic and bulk scales. The major Cu species in the more toxic samples were sorbed or coprecipitated with secondary Mn (birnessite) and Fe minerals (jarosite and goethite), which together accounted for nearly 80% of the total Cu. The major Cu species in the less toxic samples were Cu sulfides (chalcopyrite and a covellite-like phase), making up about 80−95% of the total Cu, with minor amounts of Cu associated with jarosite or goethite. These Cu speciation results are consistent with the toxicity results, considering that Cu sorbed or coprecipitated with secondary phases at near-neutral pH is relatively less stable than Cu bound to sulfide at lower pH. The more toxic stream sediment sites were those that contained fewer detrital sulfides and were upstream of the major mine waste pile, suggesting that removal and consolidation of sulfide-bearing waste piles on site may not eliminate all sources of bioaccessible Cu.
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INTRODUCTION Copper (Cu) is a micronutrient for most organisms but can be toxic with increasing concentrations. Plants and aquatic animals are particularly sensitive to low concentrations of dissolved Cu. For example, the median toxic concentration of Cu for plants grown in solution culture is 127 μg/L.1 Juvenile salmon exposed to only 5−20 μg/L of dissolved Cu became increasingly unable to escape predation.2 Given the worldwide prominence of historical, current, and future Cu mining operations, understanding the bioaccessibility and bioavailability of Cu under a wide range of conditions is crucial for predicting its possible environmental impacts. Although poor environmental practices at historical mines are largely absent in current mining operations, we can study the processes that work to increase the dispersion and bioaccessibility of Cubearing phases at active and abandoned mines to prevent future contamination. Mining activity at the abandoned Ely Copper Mine (near Vershire, Vermont) was intermittent from 1830 to 1958.3 The mine produced an estimated 0.45 Mt of ore averaging 3.5 wt % Cu.4 Surface water and sediment quality in Ely Brook (EB), which drains the mine site, are currently impacted by acid rock drainage (ARD) and elevated metal concentrations. As a result, © 2016 American Chemical Society
the site was added to the U.S. Environmental Protection Agency’s National Priorities list as a Superfund Site in 2001. Water- and sediment-quality criteria for several metals, including Cu in particular, were exceeded in Ely Brook as reported previously.5 For example, dissolved Cu concentrations in Ely Brook exceeded local aquatic health criteria by 45−222 times, whereas equivalent calculations for dissolved Cd, Ni, Pb, and Zn were always less than 10 times their respective criteria.5 Likewise, Cu in Ely Brook sediments exceeded criteria values by 7−40 times, and Cd, Ni, Pb, and Zn exceeded their respective sediment criteria by only 0.1−1.4 times.5 Previous work describes toxicity for different aquatic organisms from exposure to Ely Brook surface water and Ely Brook sediments combined with dilute laboratory water.5 This previous report includes details on the toxicity tests. Toxicity from exposure to surface water was high in most of Ely Brook, but the sediment toxicity, as measured in the laboratory using the detritus-feeding amphipod Hyalella azteca,6,7 varied in a Received: Revised: Accepted: Published: 1126
August 24, 2015 January 5, 2016 January 6, 2016 January 6, 2016 DOI: 10.1021/acs.est.5b04081 Environ. Sci. Technol. 2016, 50, 1126−1136
Article
Environmental Science & Technology
survival). In a section of Ely Brook 510 m downstream (EB90; Figure 1), streambed sediments displayed low toxicity (91% survival). Total Cu concentrations in both sediment samples used in this toxicity study were high (2700 and 6000 mg/kg for EB600 and EB90, respectively) relative to the background (75 mg/kg). While unexpected, the toxicity results were consistent with parallel acid volatile sulfide-simultaneously extracted metals analyses (e.g., refs 8 and 9), which showed that the amount of bioaccessible Cu was higher upstream (EB600 = 880 mg/kg) than downstream (EB90 = 70 mg/kg). Because Cu was the overwhelmingly predominant trace metal in these sediments, the toxicity results suggest that Cu speciation differed between upstream and downstream sediments. The purpose of this work was to determine if the measured toxicity differences in Ely Brook sediments can be related to differences in the identity, relative abundance, or distribution of solid-phase Cu species. Combining techniques at the bulk-scale (X-ray diffraction, XRD; X-ray absorption spectroscopy, XAS) and microscale (XAS; X-ray fluorescence microscopy, XRF; scanning electron microscopy, SEM), we identified Cu-bearing species in Ely Brook sediments. This work informs remediation of the Ely Copper Mine and improves upon existing processlevel understanding of Cu speciation in acidic, metal-rich drainages for other sites with similar remedial challenges.
counterintuitive way. In the section of Ely Brook above its confluence with one of the major mine waste piles but still downstream of mine workings and mine waste (EB600; Figure 1), streambed sediments displayed high toxicity (only 6%
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STUDY AREA The Ely Copper Mine, along with the neighboring Pike Hill Mine and the better-known Elizabeth Mine, developed Besshitype massive sulfide deposits in the Vermont Copper Belt, located in east central Vermont. The ore was hosted by the Silurian-Devonian Gile Mountain Formation, which consists of carbonaceous metapelite, metagreywacke, and lesser micaceous quartzite, calcareous pelite, hornblende schist, and amphibolite. 11 Nearby formations include the Standing Pond Volcanics, which consist principally of amphibolite, with localizations of fine-grained quartzite with spessartine.11 The ore was stratabound and included the minerals (in decreasing abundance) pyrrhotite, chalcopyrite, sphalerite, and pyrite.11 Ore host rock and gangue included carbonate minerals, but the
Figure 1. Map of the Ely Copper Mine Superfund Site. Stars represent the location of Ely Brook stream and pore water and sediment samples collected in October 2009. The sample numbers represent meters upstream from the confluence with Schoolhouse Brook. The “X” notations represent the approximate location of waste-rock samples collected in August 1998 or October 2002 and characterized by Piatak et al.10
Table 1. Selected Chemical Analyses for Surface Water, Pore Water, and Stream Sediments Collected from Ely Brook in 2006 and Reported Previouslya5 sample site
pH
S.C. (μS/ μm)
alkalinity (mg/L as CaCO3)
Ca (mg/ L)
Mg (mg/ L)
SO4 (mg/ L)
Al (μg/ L)
87 149 123 447
41 11 16 −
13 17 15 21
1.2 3.2 2.4 5.5
4.3 52 36 143
3.4 25 21 4200
22
