Journal of Environmental Quality 30:1710-1719 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Wetlands and Aquatic Processes
Trace Element Removal from Coal Ash Leachate by a 10-Year-Old Constructed Wetland
Z. H. Ye,
S. N. Whiting,
J. H. Qian,
C. M. Lytle,
Z.-Q. Lin and
N. Terry*
Department of Plant and Microbial Biology, Univ. of California at Berkeley, 111 Koshland Hall, Berkeley, CA 94720
* Corresponding author (nterry{at}nature.berkeley.edu)
Received for publication August 1, 2000.
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ABSTRACT
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This study investigated the ability of a 10-yr-old constructed wetland to treat metal-contaminated leachate emanating from a coal ash pile at the Widows Creek electric utility, Alabama (USA). The two vegetated cells, which were dominated by cattail (Typha latifolia L.) and soft rush (Juncus effusus L.), were very effective at removing Fe and Cd from the wastewater, but less efficient for Zn, S, B, and Mn. The concentrations were decreased by up to 99% for Fe, 91% for Cd, 63% for Zn, 61% for S, 58% for Mn, and 50% for B. Higher pH levels (>6) in standing water substantially improved the removing efficiency of the wetland for Mn only. The belowground tissues of both cattail and soft rush had high concentrations of all elements; only for Mn, however, did the concentration in the shoots exceed those in the belowground tissues. The concentrations of trace elements in fallen litter were higher than in the living shoots, but lower than in the belowground tissues. The trace element accumulation in the plants accounted for less than 2.5% of the annual loading of each trace element into the wetland. The sediments were the primary sinks for the elements removed from the wastewater. Except for Mn, the concentrations of trace elements in the upper layer (0–5 cm) of the sediment profile tended to be higher than the lower layers (5–10 and 10–15 cm). We conclude that constructed wetlands are still able to efficiently remove metals in the long term (i.e., >10 yr after construction).
Abbreviations: AMD, acid mine drainage • NPDES, National Pollution Discharge Elimination System
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INTRODUCTION
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ACID mine drainage (AMD) is produced during the mining, storage, and processing of coal, and in association with the land disposal of the byproducts of coal combustion. Acid mine drainage is characterized by low pH as well as iron (Fe) and manganese (Mn) concentrations of greater than 6 and 2 mg L-1, respectively (Brodie et al., 1989; Tarutis et al., 1999). Acid mine drainage therefore constitutes a major pollution problem (Vile and Wieder, 1993; Tarutis and Unz, 1995), affecting more than 20000 km of streams and rivers in the USA (Perry and Kleinmann, 1991). The turbidity and high concentrations of trace elements of AMD reduce both species diversity and population size of aquatic plants, invertebrates, and fish in the streams receiving these discharges (Koryak and Reilly, 1984). Coal-fired power-generating facilities must achieve regulatory compliance when discharging such metal-contaminated wastewater.
Wastewater discharges in the USA must comply with National Pollution Discharge Elimination System (NPDES) effluent limitations. For example, the NPDES monthly average discharge limitations for Fe and Mn are <3 and <2 mg L-1, respectively, and the pH of the water must be in the range 6 to 9 (USEPA, 2001). Traditionally, AMD was treated by the application of chemical bases to elevate the pH of the wastewater; the pH increase caused Fe and Mn to precipitate from solution due to the pH-dependent solubility of these metals upon exposure to oxygen. Chemical treatment, however, is costly due to the considerable requirements of chemicals and associated labor, and furthermore, the generation of large amounts of sludge that must be disposed safely (Stark et al., 1995).
In the last two decades there has been a surge of interest in the use of constructed wetlands as effective, low-cost, low-maintenance alternatives for AMD treatment (Kleinmann, 1985; Wieder, 1989; Tarutis et al., 1999). Constructed wetlands are able to remove trace elements from the wastewater via a number of complex, interactive physicochemical reactions. The primary mechanisms responsible for the removal and retention of Fe, Mn, and S include the formation and precipitation of metal oxides and sulfides within the sediments (Tarutis and Unz, 1995). Indeed, 40 to 70% of total Fe removed from AMD by some wetlands was found as ferric hydroxides from the hydrolysis of ferric iron or the oxidation of ferrous iron (Henrot and Wieder, 1990).
Wetland ecosystems can be characterized by their emergent macrophytes, and these plants are crucially important in many ways for removing trace elements from the wastewater. Although the direct uptake of trace elements into the plant tissues appears to account for only a small proportion of the total removal by some wetlands (Mitsch and Wise, 1998), plants potentiate metal retention by filtration, adsorption, and cation exchange, and through plant-induced chemical changes in the rhizosphere (Dunbabin and Bowmer, 1992). Plants also provide habitat and energy sources (organic carbon) to maintain and stimulate a diverse microbial population in the sediments (Skousen et al., 1994); these microbes drive the immobilization of contaminants in the sediments through both oxidative and reductive processes (Johnson, 1998). Precipitation of metal oxides, following microbe-mediated oxidation, is thought to be one of the most important removal mechanisms in wetlands (Skousen et al., 1994; Stark et al., 1996).
Wetlands are capable of removing large quantities of trace elements from wastewater. There is, however, considerable variation both among metals and also between wetlands in the degree to which each metal is removed (Ye et al., 2001). A further significant problem with AMD is that it is produced for many tens or even hundreds of years as the coal combustion by-products continue to weather, releasing sulfates and trace elements. For wetlands to be a reliable and sustainable treatment technology, their pollutant removal rates must be consistently high in the long term (such as >10 yr). Some studies indicate that constructed wetlands have a finite lifespan with respect to metal retention and that they could eventually fail to remove some elements (Wieder, 1993; Horne, 2000). For example, the capacity of wetlands to retain Fe (primarily as oxides) might eventually be exhausted, and the acidity of the mine water will overcome the capability of the wetland to neutralize the water (Stark et al., 1995).
This study examined trace element removal by an overmatured (10-yr-old) wetland constructed at the Widows Creek Fossil Plant in Alabama. The Tennessee Valley Authority (TVA) created this wetland as an ecological reclamation system treating acidic leachate from an abandoned coal ash pile. We investigated whether this 10-yr-old wetland could still efficiently remove trace elements from the wastewater, and determined the fate of the six primary contaminants (Fe, Mn, S, B, Cd, and Zn) loaded into the wetland. Because high concentrations of trace elements have been sequestered in the wetland sediments over a 10-yr operation, particular attention was paid to the wetland development (vegetation) and trace element uptake by the dominant plant species.
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MATERIALS AND METHODS
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Constructed Wetland
The wetland was constructed in June 1986 at the Widows Creek Fossil Plant (Jackson County, AL) along the toe of the coal runoff pile. Three shallow cells were excavated to create a marsh-type surface-flow wetland with an original cattail-vegetated area of 4800 m2 (Fig. 1). Cell 2 had six finger dikes that extended into the cell to prevent short-circuiting of the water flow through the wetland. In August 1987, the cattails were almost entirely eradicated by an infestation of cattail armyworms (Simyra henrici). In the next year, the wetland plants reemerged and were dominated primarily by cattail, but with a significant number of soft rush, along with some spike rush [Eleocharis palustris (L.) Roem. & Schult.] and woolgrass bulrush [Scirpus cyperinus (L.) Kunth]. During this study period (May 1996 through May 1997), Cell 1 and part of Cell 2 (mostly within last four finger dike sections) were vegetated predominantly with cattail and soft rush, which this study primarily focused on. Cell 3 was not examined because sodium hydroxide solution was continuously added to the water flowing into this cell to ensure sufficient metal removal by chemical precipitation.
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Fig. 1. Plan diagram of the Widows Creek constructed wetland in Jackson County, Alabama. Cells 1, 2, and 3 enclosed 630, 3120, and 1050 m2, respectively.
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Collection of Samples
The plants, water, and sediments were sampled in Cells 1 and 2 only. To ensure randomized sampling, three 1-m2 areas per cell were chosen using the standard grid sampling technique (USEPA, 1989). Three whole-plant samples (root and shoot) of the dominant species, cattail and soft rush, were collected from each cell. The plants were collected in triplicates on a monthly basis except for November, December, and January, when the plant shoots were senescent; samples of plant fallen litter were collected during the winter months. Samples were collected in triplicate on a quarterly basis from each cell. Water samples were collected from the inlet to Cell 1 and the outlet from Cell 2. Sediment profiles (5-cm cores of the 0- to 15-cm layers as described by Boulding, 1994) and sediment pore water (Rhizon soil moisture filter, 10 cm, 0.1-µm pore size [Ben Meadows, Canton, GA]) were collected in the randomly assigned sampling areas. Plant density was determined from monthly counts of cattail and soft rush shoots in five randomly selected 0.25-m2 areas within each cell. The biomass of cattail and soft rush per unit sampling area was determined as the product of the dry weight of each whole shoot (an average of five shoots) and the plant density in each sampling area. The pH and temperature were measured monthly using a Corning Checkmate modular system (Fisher Scientific, Pittsburgh, PA) at the center of each cell, where possible.
Trace Element Analysis
Samples of unfiltered inlet and outlet waters were digested with a mixture of HNO3, H2O2, and HCl (USEPA, 1983). Plant samples were dried at 70°C, weighed, ground in a Wiley mill, and acid-digested with a mixture of HNO3, H2O2, and HCl (Zarcinas et al., 1987). The sediment profiles were cut into 5-cm sections (0–5, 5–10, and 10–15 cm), air-dried, ground, and acid-digested with HNO3 and H2O2 (Method 3050B; USEPA, 1996a). The total concentrations of Fe, Mn, S, B, Cd, and Zn were measured in the water, plant, and sediment digests by inductively coupled plasma–atomic emission spectroscopy (ICP–AES) (Method 6010B; USEPA, 1996b). The water-soluble trace elements in pore waters were determined directly by ICP–AES.
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RESULTS
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Temperature and pH of the Surface Water
The pH and temperature of the surface water in Cells 1 and 2 were not significantly different (p > 0.05, two-tailed t test), so the data presented in Fig. 2 are averaged for measurements taken in both cells. The initial surface water pH in Cells 1 and 2 (May 1996) was low (acidic), but the pH increased to about 7 in July (Fig. 2) due to a large amount of rainfall. In an attempt to raise the pH of the wastewater and hence improve the efficiency of contaminant removal, the NaOH solution was added to Cell 1 in October, and the pH remained high in the following months, through March 1997. The temperature of the surface water showed typical seasonal variation, with a summer high of about 30°C and a winter low of 5°C (Fig. 2).
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Fig. 2. Temporal changes in the pH and temperature of the surface water in Cells 1 and 2. Error bars indicate standard error of the mean (SE), n = 6.
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Reduction in Trace Element Concentrations
There was considerable variation in the concentration of each trace element between the sampling times (Fig. 3). The lowest concentration of each of the six trace elements was in February 1997, which coincides with the period of high rainfall (i.e., dilution of the leachate). The concentrations of trace elements in the outlet water were lower than those in the inlet in most cases, confirming that the wetland was removing trace elements from the wastewater. The degree to which the concentrations of the trace elements were reduced varied considerably between sampling times; the maximum concentration reduction rates were 99% for Fe, 91% for Cd, 63% for Zn, 61% for S, 58% for Mn, and 50% for B.
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Fig. 3. Temporal changes in trace element concentrations in the inlet water to Cell 1 and the outlet water from Cell 2. Figures above the bars indicate the percentage reduction in trace element concentration for that month.
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Trace Element Concentrations in the Sediments
The concentrations of Fe and S in the sediment, 
70000 and 8000 mg kg-1, respectively, were the highest among the six elements studied (Fig. 4). The concentrations of the other elements were much lower and were in the order of Mn > B > Zn > Cd, which is the same as their relative concentrations in the inlet water (Fig. 4 compared with Fig. 3). In both Cells 1 and 2, the concentrations of Fe, S, B, Cd, and Zn in the top (0–5 cm) layer tended to be higher than in the lower layers; however, these differences were not statistically significant (analysis of variance [ANOVA], all P > 0.05) (Fig. 4). For all elements except Mn and S, concentrations in the top 15 cm of sediments of Cell 1 tended to be higher than those in Cell 2, but again, the differences were not statistically significant (Fig. 4; P > 0.05, two-tailed t test). Furthermore, the concentration of trace elements in the sediments did not increase significantly during the 1-yr period of this study (data not shown). The concentrations of S, Fe, Mn, B, Zn, and Cd in the pore water were 539, 100, 6.4, 1.24, 0.41, and 0.03 mg L-1, respectively, which are identical to their mean concentrations in the inlet water.
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Fig. 4. Mean trace element concentrations in the sediments of Cell 1 and Cell 2 measured at three depths in the sediment profile: 0 to 5 cm, 5 to 10 cm, and 10 to 15 cm. Means are calculated from measurements taken over the entire study (standard error of the mean [SE], n = 5).
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Plant Productivity and Trace Element Accumulation
The shoot counts in each sampling quadrat showed that soft rush had a much higher density of shoots than cattail over the entire study period (Fig. 5). However, because the shoot morphology of the two species is very different, the larger stature of the cattail resulted in a greater standing biomass than soft rush (Fig. 5). In the winter, the senescence of the standing plants resulted in the accumulation of a significant amount of fallen litter. During the months of November, December, and January, the amount of fallen litter in the sampling areas averaged 260 g m-2 (Fig. 5).
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Fig. 5. Changes in plant density and shoot biomass for cattail and soft rush. Values are averages of samples collected from both cells.
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Notably, the aboveground tissues of soft rush had higher concentrations of all six elements compared with cattail (Fig. 6). For the belowground tissues of cattail and soft rush, the concentration of all trace elements except B tended to be higher in soft rush than in cattail (Fig. 7). Because the concentrations of the six elements in the aboveground and belowground tissues of both cattail and soft rush were similar in Cells 1 and 2 (data not shown for each cell), the data presented in Fig. 6 and 7 are averages calculated from the samples of each species collected in both cells. The concentrations of Fe and Mn in the shoots of soft rush were approximately fourfold greater than the concentrations in the shoots of cattail. There were no seasonal patterns of trace element accumulation in the aboveground tissues of either species (Fig. 6). As for the shoots, there was marked variation between the sampling times in the concentration of trace elements in the belowground tissues, which did not follow any apparent seasonal patterns. For comparison, the trace element uptake by the plants is summarized as averages calculated from samples collected over the entire study period (Table 1). The concentrations of all elements (except Mn) in the belowground tissues were typically greater than the concentrations in the aboveground tissues and, in some cases, more than 60 times greater. The trace element concentrations in the fallen litter were comparable with the belowground tissues (Table 1). Moreover, the concentrations of the six trace elements in both the aboveground and belowground tissues followed the same order as their concentrations in both the sediments and the inlet water, that is, Fe > S > Mn > B > Zn > Cd (Fig. 4 and Table 1).
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Fig. 6. Trace element concentrations in the aboveground tissues of cattail (black circles) and soft rush (unfilled circles). Monthly means are calculated from plant samples collected from both cells, error bars indicate standard error of the mean (SE).
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Fig. 7. Trace element concentrations in the belowground tissues of cattail (black circles) and soft rush (unfilled circles). Monthly means are calculated from plant samples collected from both cells, error bars indicate standard error of the mean (SE).
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Table 1. Mean concentrations of the six trace elements in the living and senescent tissue of cattail and soft rush (mean SE, n = 12, except fallen litter, where n = 3).
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Trace Element Loading and Retention by the Plants
The total mass of each element accumulated in the standing biomass and fallen litter of soft rush and cattail (Table 2) was calculated from the average biomass of cattail shoots (509 g m-2), soft rush shoots (242 g m-2), and fallen litter (259 g m-2), and the mean concentration of trace elements in these tissues (Table 1). The total mass of trace elements accumulated within each of these plant components was small compared with the trace element loadings. The maximum was for Fe accumulated in the fallen litter (13.4 g m-2), and the minimum was for Cd accumulation by both cattail and soft rush (0.0001 g m-2).
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Table 2. Annual trace element loading into the wetland, the total mass of trace elements accumulated in the plant material, and the contribution of trace element accumulation in the plants to removal by the wetland. Total treatment area of the wetland = 3600 m2, average flow rate in inlet = 70 L min-1. The data for quarterly flow rate in the inlet were provided by the Widows Creek Fossil Plant in Alabama, based on averages of frequent measurements they recorded.
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The estimated loading rates of the six trace elements in the inlet water to the wetland were calculated as the product of the average element concentrations in the inlet water and average flow rate of water into Cell 1 for each quarter on a unit area basis. Because we are interested in the role of plants in trace element retention, the mass of trace elements accumulated by these components is presented as a percentage of the annual loading in the wetland (Table 2). It can be seen that the direct uptake of trace elements by the plants is very small when compared with the loading. Again, Fe accumulation by the fallen litter was the highest. Even so, this only represented 1.38% of the loading. For the other five elements, the accumulation within the aboveground tissues or fallen litter of the plants was generally less than 1% of the mass of each element flowing into the wetland during the year.
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DISCUSSION
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Trace Element Removal by Wetlands and Role of Vegetation
It has been suggested that constructed wetlands have a finite and short life span with respect to metal retention (Wieder, 1993; Horne, 2000). However, this 1-yr study indicates that the 10-yr-old constructed wetland reduced the concentrations of the primary contaminants in the wastewater, and was still able to almost completely remove Fe and Cd from the wastewater (Fig. 3). At the Widows Creek wetland the concentrations of Fe in the outlet were between 0.16 and 22 mg L-1, irrespective of the high variability in Fe concentrations in the inlet water (9.5 to 234 mg L-1). The concentrations of Fe in the outlet water were below the NPDES discharge limits in the months of November 1996 and May 1997. Moreover, Cd was below the NPDES discharge limit in May 1996, February 1997, and May 1997, and Zn met the NPDES limit (0.09 mg L-1) in all months except May 1997. Our results are consistent with the findings of other studies, which demonstrated that wetlands are more effective at removing Fe than other elements, such as Mn and S, from AMD (e.g., Wieder, 1989; Mitsch and Wise, 1998; Ye et al., 2001).
The fate of the six primary contaminants within the wetland was determined with particular reference to the roles of the plants in trace element removal. The shoot biomass of cattail and soft rush indicated that they were tolerant to the high concentrations of trace elements in the water and sediments within the wetland. The standing biomass of cattail shoots was 509 g m-2, which is comparable with the 447 and 502 g m-2 given by Fennessy and Mitsch (1989) and Mitsch and Wise (1998), respectively, for cattails grown in wetlands treating AMD. The aboveground biomass of soft rush was also in the range reported for that species by Vymazal (1995). When averaged over the study period (Table 1), however, the mean concentrations of trace elements in the plants were within the ranges reported by other authors for these species growing in metal-contaminated wetlands (Taylor and Crowder, 1983; Fernandes and Henriques, 1990). Notably, both species accumulated more Mn in their shoots than their belowground tissues, a finding also reported by Taylor and Crowder (1983) and Fernandes and Henriques (1990). Our results indicate that the majority of the Fe, S, B, and Cd taken up or immobilized by the plants were localized within the belowground tissues. This localization of trace elements within the root system, and limited transfer to the shoots, may be a corollary mechanism of tolerance toward toxic trace elements by these wetland plants (Taylor and Crowder, 1983; Ye et al., 1997).
The senescence of the plants in the winter results in a considerable mass of fallen litter in the wetland cells. The concentrations of Fe, S, B, and Cd, but especially Fe and Cd, in or bound onto this fallen litter were higher than those in the aboveground tissues of cattail and soft rush (Table 1). In fact, the concentrations of Fe and Cd in the fallen litter were 161 and 58 times higher than those in the shoots of cattail, respectively. Our field observations documented a significant quantity of brown precipitate on the fallen litter, most probably composed of hydrated iron oxides. This high concentration of Cd in the fallen litter may also be a result of the high binding affinities between Cd and organic matter (Ross, 1994). This indicates that the plant detritus in the wetland might play a significant role in the removal of elements such as Fe and Cd by adsorption–absorption or by providing sites for their precipitation from the water.
Trace element uptake and accumulation in the plant shoots does not contribute significantly to the overall removal of trace elements by the wetland. Means masses of 2.2 g of S, 1 g of Mn and Fe, and <0.05 g of B, Zn, and Cd were accumulated in aboveground tissues of the plants (cattail and soft rush) per square meter (Table 2). With respect to the total loadings of trace elements in the inflow, the aboveground plant tissues therefore retained less than 1.9% of the annual loading (Table 2). This highlights the minor role of metal uptake by the plant shoots in the efficiency of metal removal from wastewater by wetlands. Similarly, Mitsch and Wise (1998) reported the bioaccumulation of Fe in plant shoots as only accounting for 0.07% of the annual loading of Fe into a cattail-dominated wetland. Similarly, the fallen litter retained a maximum of 1.4% of the annual loading of each of the six elements despite containing higher concentrations of Fe, Cd, Mn, and B than the living shoots (Table 2).
The concentrations of the trace elements in the sediments were very high (Fig. 4), and were within the ranges reported by previous investigators in the other cattail-dominated wetlands (Taylor and Crowder, 1983). The wetland sediment was a major pool of the trace elements removed by the wetland, which agrees with the findings of many other studies (e.g., Fernandes and Henriques, 1990). Our data indicate that there was direct deposition of the trace elements to the sediments; there was a trend for higher concentrations of the trace elements except Mn in the top layers (0–5 cm) of Cell 1, and to a lesser extent in Cell 2 (Fig. 4), than in the lower layers (5–10 or 10–15 cm). A similar localized deposition was found at the Springdale constructed wetland in Pennsylvania, also removing trace elements from coal combustion by-product leachate (Ye et al., 2001). Although the sediment contains a large volume of pore water, the low element concentrations in this fraction do not contribute significantly to the overall element budget in the wetland.
Factors Influencing the Effectiveness of Wetland Systems to Treat Acid Mine Drainage
The concentration of Fe in the inlet water was quite variable with time, but the Fe concentrations in the outlet water remained fairly constant (Fig. 3). It appears, therefore, that the efficiency of Fe removal by the first two cells of the wetland was unaffected by the rate of Fe input. This may, in part, be due to the low Fe loading rate in this wetland (2.6 g m-2 d-1). For example, Brodie (1993) indicated that Fe removal by wetlands is generally efficient for loading rates up to 13 g m-2 d-1. Conversely, in February of our study, the dilution effect of the heavy rain resulted in low concentrations of all trace elements in the inlet water. The heavy rainfall was also associated with a low concentration reduction (%) of Fe, Mn, S, and Cd in this month (Fig. 3). This suggests that the addition of rainfall water to the wetland decreased residence time of wastewater in wetlands, along with low-standing biomass, and therefore, decreased the ability of wetlands to remove trace elements from the wastewater (Stark et al., 1994).
Abiotic factors also affect the ability of wetlands in trace element removal from wastewater. The process of abiotic iron oxidation or precipitation is primarily pH dependent, increasing 100 times for every unit increase of pH above 3.5 (Hedin, 1989). Furthermore, Mn requires a higher pH than Fe to be precipitated from solution; Skousen et al. (1994) indicated that oxidized iron precipitates as ferric hydroxide as the pH increases above 3.5, while Mn hydroxides require a pH of at least 7 to precipitate. The results presented here show that pH in the AMD is an important factor in Mn removal by the constructed wetland (Fig. 2). Before the addition of caustic soda into Cell 1, the maximum reduction for Mn was 26%, with a mean pH of 3.9. When NaOH was drip-fed into Cell 1, the pH in the outlet rose to greater than 6 and the average reduction of Mn increased to 58% (Fig. 3). In contrast, the reduction in the concentration of the other elements did not significantly change with caustic soda addition. Similarly, our previous study of the Springdale constructed wetland (Ye et al., 2001) showed that where the average pH in water was 7.1, the average reduction in the concentration of Mn after passing through the wetland was 91%. Therefore, when managing constructed wetlands treating AMD, the pH of the surface water may require chemical manipulation (e.g., addition of chemical bases) to ensure that Mn can be removed efficiently to comply with the NPDES Mn discharge limit.
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CONCLUSIONS
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This study demonstrated that constructed wetlands sustain high efficiency of trace element removal from wastewater in the long term, such as 10 yr after their construction. The concentrations of Fe and Cd in the inlet water were substantially reduced (up to 99%) in some months during the study year. Other trace element concentrations were reduced, but to a lesser extent (less than 63%). Most notably, the direct uptake of the trace elements by cattail and soft rush plants in the wetland accounted for less than 2.5% of the annual element loading rates. It appears that the sediments were the primary sink for the trace elements removed. In this study, the effects of plant–microbial interactions on the localized removal of trace elements were not examined, but were almost certainly a crucial factor in trace element removal by constructed wetlands. We are currently elucidating plant–microbe interactions in the rhizosphere and determining their role in the mechanisms of trace element sequestration.
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ACKNOWLEDGMENTS
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This study was supported by a grant from the Electric Power Research Institute (EPRI). The authors also thank Widows Creek Fossil Plant in Alabama for helping with this project.
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NOTES
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Z.H. Ye, present address: Institute for Natural Resource and Environmental Management, Hong Kong Baptist Univ., Hong Kong.
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ABSTRACT
INTRODUCTION
