J. Great Lakes Res. 25(4):663–682 Internat. Assoc. Great Lakes Res., 1999 Anthropogenic Copper Inventories and Mercury Profiles from Lake Superior: Evidence for Mining Impacts W. Charles Kerfoot*,1, Sandra Harting1, Ronald Rossmann2, and John A. Robbins3 1Lake Superior Ecosystem Research Center and Department of Biological Sciences Michigan Technological University Houghton, Michigan 49931 2United States Environmental Protection Agency Mid-Continent Ecology Division, Large Lakes Research Station 9311 Groh Road Grosse Ile, Michigan 48138 3NOAA Great Lakes Environmental Research Laboratory 2205 Commonwealth Blvd Ann Arbor, Michigan 48105 ABSTRACT. During the past 150 years, the mining industry discharged more than a billion tons of tailings along Lake Superior shorelines and constructed numerous smelters in the watershed. Given the vast size of Lake Superior, were sediment profiles at locations far offshore impacted by nearshore activi- ties? Did copper and associated precious metal mining modify regional fluxes for copper and mercury? Samples from thirty sediment cores document that background concentrations of copper are high (mean 60.9 ± 7.0 µg/g), due to the proximity of natural ore sources. Anthropogenic inventories uncorrected for focusing also are high, ranging from 20 to 780 µg/cm2 (mean 187 ± 54 µg/cm2). Focusing factor correc- tions decrease the mean estimate and reduce variance (144 ± 24 µg/cm2). Several approaches to estimat- ing inputs suggest that only 6 to 10% of historic copper deposition originated directly from atmospheric sources, emphasizing terrestrial sources. Moreover, coastal sediment cores often show synchronous early increases in copper and mercury with buried maxima. Around the Keweenaw Peninsula, twenty-two cores trace high copper and mercury inventories back to mill and smelting sources. Direct assays of ores from thirteen mine sites confirm a natural amalgam source of mercury in the stamp mill discharges. Core records from inland lakes (Michigamme Project) also reveal patterns of copper and mercury inputs from a variety of mining sources: historic tailing inputs, amalgam assay releases, and atmospheric smelter plumes. INDEX WORDS: Lake Superior, copper, mining, mercury, sediment. INTRODUCTION perior, contaminants from shoreline or atmospheric The Great Lakes ecosystem is susceptible to sources move between solid and dissolved phases, loading of trace metal contaminants from both ter- adsorb onto settling organic particulate matter, and restrial and atmospheric sources, yet the relative become buried in sediments. One beneficial conse- importance of various sources and the details of cy- quence of this process is that sediment cores offer cling in nearshore and offshore environments are excellent opportunities for reconstructing contami- poorly understood (Galloway et al. 1982, Hong et nant loading histories (Edgington and Robbins al. 1996). Patchy industrial discharges react with 1976, Christensen and Osuna 1989). However, the shoreline, wetland, and forest ecosystem compo- sediment inventories of large lakes are often spa- nents. For example, in the dilute waters of Lake Su- tially heterogeneous, reflecting site-specific inputs and the long-term consequences of sediment move- ment. How to utilize sediment cores to clarify in- *Corresponding author. E-mail: [email protected] puts and to provide insight into element cycling is a 663 664 Kerfoot et al. FIG. 1. Lake Superior drainage basin showing the location of 30 1983 NOAA inventory cores plus the location of an additional core (EV11A). challenge (Galloway and Likens 1979, Galloway et weenaw Waterway system, and into Lake Superior al. 1982). between 1850 and 1968 (Kerfoot et al. 1994). Ker- The IJC (1989) asserts that 95% of the contami- foot et al. (1994) presented an extensive geographic nants which enter Lake Superior come from long- discussion of these historical discharges. Beaches distance atmospheric deposition. While many near many Keweenaw Peninsula towns consist of organic compounds are subject to long-distance tailing fans that erode by wave action and that rede- transport, copper and mercury loadings may be posit coarse and fine-grained material along the much more closely associated with regional water- shoreline and into deeper waters. These tailings are shed activities than previously suspected. Early cal- stamp sands, i.e., crushed basalt and conglomerate, culations apparently incorporated some serious that are distinctive in color, mineral content, and errors, and previous coring efforts failed to detect physical characteristics from natural lake sedi- important coastal patterns. Two early investigators ments. Deposits of stamp sand material near mill of Lake Superior sediments, Nussman (1965) and sites and along the Keweenaw Waterway contain Kemp et al. (1978), suspected that copper enrich- copper in concentrations that locally average 850 to ments in Lake Superior sediments came from sev- 3,750 µg/g (Kraft and Sypniewski 1981, Kerfoot eral intensively mined regions: The Keweenaw and Robbins 1999). In addition, 50 million metric Peninsula of Michigan; Thunder Bay, Marathon, tons of mine tailings were discharged directly into and Sault Ste. Marie regions of Ontario, Canada Lake Superior along the western coastline (Freda/ (Fig. 1). Values near the Keweenaw Peninsula coin- Redridge region) of the Keeweenaw Peninsula by cided with the path of the Keweenaw Current, sug- five stamp mills that operated between l895 and gesting that copper-rich sediments were transported l968 (Champion, Trimountain, Adventure, Baltic, considerable distances from their original sources and Atlantic Mills; Babcock and Spiroff 1970, Ker- by currents or ice rafting (Kerfoot et al. 1994). foot et al. 1994). Smaller amounts were released Investigations initially concentrated on the Ke- from Eagle River and Copper Harbor. Along the weenaw Peninsula region (Fig. 2), where a half bil- eastern coastline of the Keweenaw Peninsula, 23 lion metric tons of copper mine tailings were million metric tons were sluiced into Keweenaw discharged into rivers, the interconnected Ke- Bay at Gay between 1902 and 1932, with other Copper and Mercury Profiles in Lake Superior Sediments 665 FIG. 2. The Keweenaw Peninsula region illustrating a) the position of stamp mills, smelters, and Michigame Project lakes (round symbols). Prevailing wind direction (west) is indicated in lower left. smaller inputs occurring at Bete Grise Bay and (Michigamme Project) allowed inspection of local- L’Anse Bay (Mass, Michigan Mills). ized sediment profiles that result from different To what extent did mining activities in the Lake types of mining-related activities (stamp mill tail- Superior watershed disturb the entire lake ecosys- ings, smelter plumes, assay laboratory discharges). tem and continue to this day to influence metal cy- cling? How important and widespread are mining METHODS AND MATERIALS signatures in sediments, and what is the relationship with long-distance and localized atmospheric in- Offshore Inventory Cores and puts? Thirty inventory cores taken from Lake Supe- Focusing Factor Corrections rior by NOAA in 1983 (Fig. 1) were analyzed for Offshore sediment core samples were provided copper and mercury. These cores provided a much by the NOAA Great Lakes Environmental Research more extensive coverage of Lake Superior than pre- Laboratory from cores originally collected at 30 vious efforts (six sampling sites of Kemp et al. stations in 1983. The cores were taken from the (1978) or of Kolak et al. (1998). The cores allowed R/V Limnos as part of a National Water Research calculation of copper inventories and examination Institute program (study leader Rick Bourbonniere) of associations between copper and mercury pro- and archived. The core sampling sites were distrib- files from a wide variety of depositional environ- uted over several sedimentation basins (Fig. 1): Du- ments. Sites were distributed throughout the lake luth Basin (I7, EV4, EV8, SV180), Thunder Bay basin and included nearshore bays and eastern Basin (S24, EV13), Chefswet Basin (EV12, G18, trough sediments, suspected sites of high sedimen- SV157, SV169, B247, JAR), Isle Royale Basin tation. Cesium-137 inventories were used to correct (EV21, EV24, EV26A, SJE, PS8, 25A, PS16), for sediment focusing effects, a technique not avail- Marathon Basin (T46A), Keweenaw Basin Trough able to Kemp et al. (1978). A second set of twenty- (KB, SV62), Caribou Basin (L42C, L42A), Lake two additional cores from nearshore environments Superior Troughs (SV42, SV43, SV45), Wawa (Keweenaw Waterway, L’Anse Bay) permitted trac- Trough (EV23), Batchwana Bay (PS20), Goulais ing high copper and mercury inventories back to Bay (PS21), and Whitefish Bay (EV11A). Cores shoreline sources. An independent, third set of were obtained with a 7.6 cm diameter gravity corer cores from small, isolated inland lakes which did not free-fall, but was carefully lowered 666 Kerfoot et al. into the sediment, minimizing loss of surface layers Cores were sliced into 2 cm sections and analyzed and reducing compaction artifacts. All cores were for Cu, Ag, and Hg. Element analysis followed sectioned immediately upon retrieval. The majority methods outlined above. In addition, copper ore of the cores were sectioned at 2 cm intervals to a samples from fourteen mine sites (poor rock pile depth of 20 cm. Selected cores were sectioned at samples and archived museum ore samples) were 0.5 cm intervals to a depth of 10 cm and at 1 cm in- assayed for mercury concentrations. tervals