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TECHNICAL REPORTS

Heavy Metals in the Environment

Metal Release from Bottom Sediments of Ocoee Lake No. 3, a Primary Catchment Area for the Ducktown Mining District

^a Dep. of Earth System Science, Yonsei Univ., Seodaemun-gu, Shinchon-dong 134, Seoul, 120-749, Korea
^b School of Earth Sciences, 125 S. Oval Mall, Ohio State Univ., Columbus, OH, 43210
^c School of Environment and Natural Resources, 210 Kottman Hall, Ohio State Univ., Columbus, OH, 43210-1085
^d Environmental Sciences Div., EPA NERL, Research Triangle Park, NC, 27711

^* Corresponding author (ghlee{at}yonsei.ac.kr).

Received for publication May 4, 2007.

	ABSTRACT

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Ocoee Lake No. 3 is the first reservoir receiving suspended sediments contaminated with trace metals discharged by acid mine effluents from the Ducktown Mining District, Tennessee. Bottom sediments (0–5 cm) from the lake were sampled to assess the potential for future adverse environmental effects if no remediation controls or activities are implemented. The sediments were found to include a major component (173 ± 19 g kg^–1) that dissolved in 6 mol L^–1 HCl within 24 h. This acid-soluble and relatively labile fraction contained high concentrations of Fe (460 ± 40 g kg^–1), Al (99 ± 11 g kg^–1), Mn (10 ± 8 g kg^–1), Cu (2000 ± 700 mg kg^–1), Zn (1300 ± 200 mg kg^–1), and Pb (300 ± 200 mg kg^–1). When the pH of water in contact with the sediment was decreased experimentally from 6.4 to 2.6, the concentrations of dissolved trace metals increased by factors of 2200 for Pb, 160 for Cu, 21 for Zn, 9 for Cd, 8 for Ni, and 5 for Co. The order in which metals were released with decreasing pH was the reverse of that reported for pH-dependent sorption of these metals in upstream systems. Substantial release of trace metals from the sediment was observed even by a modest decrease of pH from 6.4 to 5.9. Therefore, the metal-rich sediment of the lake should be considered as potentially hazardous to bottom-dwelling aquatic species and other organisms in the local food chain. In addition, if the reservoir is dredged or if the dam is removed, the accumulated sediment may have to be treated for recovery of sorbed metals.

Abbreviations: ERL, effects range-low

	INTRODUCTION

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MINING and processing of sulfide ore deposits often cause the release of large amounts of toxic trace metals into natural water systems (Chapman et al., 1983; Filipek et al., 1987; Nordstrom, 1982). The trace metals that are introduced into streams may be sorbed to suspended metal hydroxides and clay minerals, boulder coatings, and organic matter depending on the pH of the water (Davis et al., 1991; Filipek et al., 1981, 1987; Johnson, 1986; Johnson and Thorton, 1987; Paulson, 1996, 1997; Sholkovitz, 1978). In particular, iron hydroxides have been identified as major scavengers of trace metals in such environments due to their ubiquity and relatively high surface area (Johnson, 1986). The formation of iron hydroxide precipitates and the sorption of positively charged metal ions onto them are promoted by increases in pH that can occur when acidic mine water mixes with the neutral water of a receiving stream. The dynamics of such systems depend on the pH and alkalinity of the tributary streams and on climatic conditions that control stream discharge (Karlsson et al., 1988a, 1988b; Rampe and Runnells, 1989). For example, Lee et al. (2002) showed that neutralization of acidic mine waters caused sequential precipitation of Fe-, Al-, and Mn oxides/oxyhydroxides with increasing pH, resulting in the removal of trace metals by sorption to the precipitates. They found that sorption of 50% of the total dissolved trace metals occurred at the pH where the dominant metal oxide/oxyhydroxide precipitate formed.

The trace elements sorbed to suspended metal hydroxides are ultimately discharged into reservoirs or lakes and thus affect the chemical composition of water and sediment in the catchment. Because the sorption and desorption of trace elements is pH dependent, the contaminated sediment may be affected by subsequent changes in water chemistry (Davis et al., 1991; Milliward and Liu, 2003; Nystroem et al., 2005; Shin et al., 2006; Saha et al., 2003). For this reason, metal-rich sediments have become a major concern worldwide because of the potential bioavailability and toxicity of some metal species to aquatic fauna and flora (Harrington et al., 1998; Hudson-Edwards et al., 1999; Tao et al., 2005; Tessier et al., 1985; Tuncel et al., 2007; von Gunten et al., 1997). Lakes and reservoirs with known sources of contamination should therefore be investigated to prevent negative ecological impacts or adverse human health effects.

Acid mine-drainage from the Ducktown Mining District, Copper Basin, introduces significant amounts of toxic trace metals into tributaries of the Ocoee River, among which Burra Burra Creek, North Potato Creek, and Davis Mill Creek are the most heavily contaminated (Lee, 2001). North Potato Creek discharges acidic water containing high concentrations of dissolved trace metals to the Ocoee River and is the closest upstream tributary to Ocoee Lake No. 3. The discharged, acidic water is rapidly neutralized by the downstream flow of the river, which results in the precipitation of iron hydroxides and the sorption of trace metals to suspended particulates (Lee et al., 2002). The particulates and sorbed trace metals are transported to Ocoee Lake No. 3 as suspended load and presumably accumulate on the lake bottom. The objectives of this study, therefore, were (i) to characterize the bottom sediments of Ocoee Lake No. 3 to evaluate the accumulation of various trace metals derived from the Ducktown Mining District and (ii) to examine the sensitivity of the sediment to changes in pH by experimental acidification of lake water in contact with the sediment. We hypothesize that metals associated with sediments in the oxic zone have remained relatively labile after deposition.

	Materials and Methods

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Study Area
Ocoee Lake No. 3 is located about 5 km downstream from the mouth of North Potato Creek (Fig. 1 ). The lake has a length and breadth of about 7 km and 0.4 km, respectively, giving it a surface area of about 2.4 km². The lake level is regulated by a dam that controls the drainage derived from an area of 1260 km² (Alexander et al., 1984). The water of Ocoee Lake is slightly acidic (pH 5.9 ± 0.9) and is not significantly contaminated by trace metals (Lee, 2001) even though the Ocoee River flows through the Ducktown Mining District, an Environmental Protection Agency superfund site, where nine ore bodies were mined from 1847 to 1987 (Emmons and Laney, 1926; Magee, 1968). Mining and refining of sulfide ore have contaminated several tributaries of the Ocoee River producing elevated acidity, high concentrations of dissolved metals, and an increased load of suspended sediment (Lee, 2001).

View larger version (19K): [in this window] [in a new window]   Fig. 1. Sample collection sites for Ocoee Lake No. 3 bottom sediments. Samples were provided by the USEPA, whose personnel collected the samples and identified their locations using global positioning system coordinates.

Upon arrival at the laboratory, supernatant waters from the bottom sediments were filtered through 0.45-µm membrane filters and acidified to pH <2 with reagent-grade HNO₃. The remaining solids were freeze-dried in a Labconco lyophilizer, gently disaggregated in an agate mortar, and dry sieved for 15 min using U.S. Standard Sieve Series No. 35 (500 µm), No. 60 (250 µm), and No. 120 (125 µm) on an automatic sieve-shaker. The grain-size fractions from 500 to 250 µm, 250 to 125 µm, and <125 µm were arbitrarily designated as being coarse, medium, and fine, respectively. Three of the fine sediment samples (DS3, -12, and -13) were selected to represent a long axis profile of the lake. X-ray diffraction analyses of these samples were conducted to determine their mineralogical composition. The X-ray diffraction analyses were performed using topfill powder mounts and CuK radiation on a vertical, wide-range goniometer (Philips 1316/90 diffractometer; Panalytical Inc., Westborough, MA) equipped with a 1° divergence slit, a 0.2-mm receiving slit, and a diffracted-beam monochromator. Specimens were scanned from 0 to 70° 2 in increments of 0.05° 2 with a 4-s step time. Peak positions were determined by using the Jade 3.0 software of Materials Data, Inc. (Livermore, CA).

Acid Soluble Fraction of Sediment
Winland et al. (1991) reported that most trace elements found in mine drainage sediments were associated with a mineral fraction that was readily soluble in cold 5 mol L^–1 HCl. A similar procedure for leaching of the Ocoee Lake No. 3 bottom sediments was tested as follows: Four subsamples of fine sediment (<125 µm) from sample DS9 were weighed; two were 1 g each, and the other two were 5 g each. Each subsample was transferred to a 50-mL polypropylene beaker. One set of 1- and 5-g subsamples was leached with 50 mL of 2 mol L^–1 and the other set with 50 mL of 6 mol L^–1 reagent-grade HCl. After exposure to the acid for 24 h at room temperature, the solutions were filtered through 0.45-µm filters and diluted to 250 mL using deionized H₂O. The diluted solutions were analyzed by ICP–OES for Fe, Al, Mn, Na, K, Mg, and Ca and by ICP–MS for the trace metals Cu, Zn, Pb, Ni, Cd, and Co. The sediment residues were air dried and weighed to determine the acid-soluble fractions.

The results of this preliminary leaching experiment (Fig. 2A ) showed that 1-g subsamples yielded slightly larger acid-soluble fractions than the 5-g subsamples in 2 mol L^–1 and 6 mol L^–1 HCl. Because the volume of acid was the same (50 mL), the larger acid-soluble fraction of the 1-g sample presumably resulted from a greater acid to sediment ratio. Subsamples leached in 6 mol L^–1 HCl also yielded larger acid-soluble fractions than when they were leached in 2 mol L^–1 HCl (Fig. 2B). Despite differences in the total material dissolved, the amounts of trace metals released were independent of the acid concentration (Fig. 2C). Based on these preliminary results, the concentrations of the acid-soluble fractions and associated metals in the sediment were determined by leaching 1 g of dry sample (except samples DS16, -17, and -18, which lacked a fine fraction) with 6 mol L^–1 HCl as described previously. After leaching, three of the fine-sediment residues (DS3, -12, and -13) were dried and analyzed by X-ray diffraction using procedures described previously to evaluate changes in mineralogical composition.

View larger version (28K): [in this window] [in a new window]   Fig. 2. Acid-soluble fractions in different acid-leaching schemes using the fine fraction (<125 mm) of sample DS 9. (A) Comparison of acid-soluble fractions (g 100 g–1) from different amounts of sediment (1 g vs. 5 g) in the same volume (50 mL) of acid. (B) Comparison of acid-soluble fractions (g 100 g–1) from the same amount of sediment in different concentrations (2 mol L–1 vs. 6 mol L–1) of acid. (C) Comparison of amounts of metals (mg g–1) released by different concentrations (2 mol L–1 vs. 6 mol L–1) of acid.

Water Analysis
The filtered solutions from the characterization and acidification experiments were analyzed for Fe, Al, Mn, Mg, Ca, Na, and K using ICP–OES and for Cu, Zn, Pb, Ni, Co, and Cd using ICP–MS. The instruments used were an Optima 3000 (ICP–OES) and a Sciex Elan 6000 (ICP–MS) (PerkinElmer, Wayham, MA). Calibration standard solutions were prepared by diluting commercially available single- or multi-element ICP standards (EM Science, Gibbstown, NJ). A blank was prepared as a 1% solution of 6 mol L^–1 HCl in deionized H₂O, and the matrices of all standard solutions were made from this blank. The analytical detection limits were expressed as three times the concentrations of elements detected in the blank. All concentrations were determined based on the average of triplicate readings, and the analytical error of the analyses was better than 5% relative standard deviations.

	Results and Discussion

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Physical Characteristics of Bottom Sediment
With the exception of samples DS16, -17, and -18, the bottom sediment samples in Ocoee Lake were uniform in grain-size distribution, with 77.7 ± 5.6 g 100 g^–1 fine (<125 µm), 20.0 ± 4.7 g 100 g^–1 medium (250–125 µm), and 2.3 ± 1.7 g 100 g^–1 coarse (500–250 µm) sediment plus gravel (>500 µm) (Fig. 3 ). By contrast, samples DS16, -17, and -18 consisted of 93.2 ± 17.6 g 100 g^–1 coarse sand and gravel, 4.4 ± 3.3 g 100 g^–1 medium material, and only 2.4 ± 2.9 g 100 g^–1 fine sediments. The abundance of coarse sediment in these three samples is probably attributable to (i) an anomalously high flow velocity along the sharp bend of the lake at this location and/or to (ii) the influx of coarse sediment from tributaries entering from a northern arm of the lake (Fig. 1). The mineralogical composition of the <125-µm fractions of sediment samples DS13, -12, and -3, collected from the upper, mid-, and downstream portions of the lake, were similar. Major constituents included quartz, mica, kaolinite, K-feldspar, plagioclase, gibbsite, and goethite (data not shown).

View larger version (60K): [in this window] [in a new window]   Fig. 3. Grain size distributions of the bottom sediments (0- to 5-cm depth) from Ocoee Lake No. 3. Samples DS16, -17, and -18 contained anomalously high amounts of sediment grains larger than 500 µm.

The acid-soluble sediment also contained high concentrations of trace metals, among which the concentration of Cu ranged from 3000 to 640 mg kg^–1 (dry weight) (average, 2000 ± 700 mg kg^–1), Zn ranged from 1700 to 900 mg kg^–1 (average, 1300 ± 200 mg kg^–1), and Pb ranged from 770 to 140 mg kg^–1 (average, 300 ± 200 mg kg^–1). The concentrations of Ni, Co, and Cd were lower but still significant (Table 2). When the concentrations of trace metals in the acid-soluble fraction were converted to a whole sediment basis, the concentrations of Cu, Zn, and Ni exceeded the effects range-low (ERL) values for sediments according to the Sediment Quality Guidelines developed for the National Status and Trends Program (NOAA, 1999) (Table 3 ). The ERL indicates the concentrations below which adverse effects such as toxicity rarely occur. When concentrations exceed the ERL but are lower than effects range–median values, the incidence of adverse effects increases to 20 to 30% for most trace metals (NOAA, 1999). In this study, the concentrations of Cu in the bottom sediment reach the effects range–median values (Table 3), so that the possibility of adverse effects increases to 60 to 90% (NOAA, 1999). Consequently, the bottom sediment of Ocoee Lake No. 3 is a potential hazard to bottom-dwelling aquatic species.

View larger version (13K): [in this window] [in a new window]   Fig. 4. Changes in pH of sediment suspensions in the lake water after the addition of varying amounts of 6 mol L–1 HCl. The amounts of acid added are reported in Table 3.

View larger version (17K): [in this window] [in a new window]   Fig. 5. Amounts of metals released from the lake sediment by acidification.

Proton Balance for Metal Release from the Sediment
An overall decrease in pH by acidification indicates that the amount of protons added exceeded the buffer capacity of proton-consuming reactions in the Ocoee Lake sediment suspensions (Table 4). The y-intercept of Fig. 6 indicates that the total amount of metal ions released in the initial solution (S1) by equilibration of the sediment and lake water was 1.4 cmol kg^–1. The quantity of metal ions released increased in proportion to the protons consumed (Fig. 6) so that, on average, 1.0 cmol kg^–1 of protons were consumed to release 0.56 cmol kg^–1 of metal ions.

View larger version (13K): [in this window] [in a new window]   Fig. 6. The relationship between total metal ions released (by charge) from the sediment and protons consumed during acidification.

Change in Chemical Composition of Lake Water by Acidification
As metals were released from the sediment into the lake water during acidification, the chemical composition of the water changed substantially (Fig. 7 ). At pH >5.9, the lake water was enriched in Mn because it was preferentially released from the sediment; however, as the pH decreased, the lake water became enriched in Fe (Fig. 7A). Likewise, above pH 6, Zn was the dominant trace metal in the lake water. As the water became more acidic, the relative abundance of Cu and Pb increased compared with other trace metals (Fig. 7B). This evolution is the reverse of that observed by Lee et al. (2002) for the neutralization of acid mine waters in the Ocoee watershed. According to these authors, neutralization of acid mine water caused the precipitation of Fe, Al, and Mn oxy/hydroxides to occur sequentially. As a result, the trace metals dissolved in solution were also sorbed sequentially with increasing pH (i.e., Pb, Cu, Zn, and Co, in that order). The results of the current study indicate that a pH decrease in sediment pore waters would cause metals to be released in the reverse order and thereby produce a systematic change in the chemistry of these waters.

View larger version (8K): [in this window] [in a new window]   Fig. 7. Changes in the chemical composition of the water of Ocoee Lake No. 3 through release of metals from the sediment in contact with the water as a result of acidification. Numbers on adjacent data points indicate the final pH of the solution after acidification. Arrows indicate the direction of progressive changes in the chemical composition of the solution. (A) Chemical composition of the solution expressed in the relative abundance of dissolved Fe versus Al versus Mn. (B) The relative abundance of dissolved Pb versus Cu versus Zn.

	Conclusions and Environmental Implications

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The area of Ocoee Lake is 2.4 km², and the acid-soluble fraction of the bottom sediment (0- to 5-cm depth) does not vary significantly over the length of the lake. The dry bulk density of sample DS12, which was collected from the middle of the lake, was 1.72 ± 0.03 Mg m^–3. If this density is representative of the bottom sediment, the upper 5 cm of the lake bed includes 210,000 ± 4000 metric tonnes of sediment that contains 36,000 ± 1000 metric tonnes of acid-soluble metals. Therefore, the lake sediment contains approximately 16,000 ± 500 metric tonnes Fe, 3600 ± 400 metric tonnes Al, and 400 ± 300 metric tonnes Mn in acid-soluble form. In addition, the quantities of trace metals in the acid-soluble sediment are 70 ± 20 metric tonnes Cu, 50 ± 10 metric tonnes Zn, 10 ± 6 metric tonnes Pb, 8 ± 1 metric tonnes Ni, and 8 ± 4 metric tonnes Co.

The results of this study have shown that even a modest decrease in pH of the sediment pore water from 6.4 to 5.9 caused significant release of trace metals to the environment. Consequently, the metal-rich sediment of Ocoee Lake is a potential concern for ingestion by bottom-dwelling aquatic species and would likely require remediation if the lake bottom is subjected to disturbance (e.g., dredging).

	ACKNOWLEDGMENTS

 
The authors thank USEPA personnel for providing sediment samples and gratefully acknowledge the analytical support of Dr. J. Olesik for ICP analyses and of Mr. U. Soto for X-ray diffraction analyses. Reviews by the anonymous reviewers are gratefully acknowledged. This work was supported by a Graduate Student Alumni Research Award from the Graduate School of The Ohio State Univ. and the Korean Research Foundation (KRF-2005-070-C00137).

	NOTES

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