21 Engineered Wetlands for Metal Mining-Impacted Water Treatment Herold J. Gerbrandt Montana Tech of the University of Montana, Butte, Montana 1 INTRODUCTION Mining is a major point source of water pollution in America and in the world. In mining zones containing pyritic ore (sulfur-bearing ore), acid mine drainage (AMD) contributes degraded water quality in general and potentially toxic metals in specific to receiving streams, lakes, and oceans. This chapter explains the production of AMD, summarizes common solutions to the problem, and presents engineered wetlands as a possible new control of this very serious pollutant. Waste minimization is not addressed here, although many possibilities for waste minimization exist within the mining industry. 2 BACKGROUND U.S. mining has supplied national and world demand for gold, iron, copper, silver, molybdenum, and a host of other metals for two centuries. Butte, Montana’s “Richest Hill on Earth” supplied copper to electrify America in the 1920s through the 1940s. U.S. metals production helped turn the tide of World War II with an overwhelming armory of guns, planes, tanks, and ships. In more recent times, Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. demand for jewelry and electronic parts has also been met by the national mining industry. The success of this extractive enterprise has come at a cost to the environ- ment. While surface impact is often limited to isolated areas, groundwater and surface water contamination has been more extensive and far more difficult to address. Sulfuric ore bodies, when brought to the surface, have the potential to produce acidic waters when exposed to water and oxygen (air). Acidic waters (pH values between 2 and 5 Standard Units) can then mobilize metals by dissolving them and transporting them downstream or vertically down to the water table. This chapter outlines the problem of mine-impacted surface and groundwa- ter, then discusses conventional approaches to reduce environmental impact. Lastly, results of a demonstration-scale engineered wetlands to clean metals-laden waters are presented as a natural alternative to conventional methods. Throughout the chapter, illustrations of various topics are drawn for the nation’s largest Superfund site, the Butte Area/Clark Fork River Superfund Project. The site stretches from the hills above Butte, Montana, 120 miles down the Clark Fork River to the Milltown Reservoir just outside of Missoula, Montana. The Butte area has a rich history of mining beginning in the 1860s, as well as a legacy of environmental degradation resulting from the mining and smelting in the vicinity. Major sources of acidic and metals-laden waters include the following. Mine adits. These horizontal tunnels access ore bodies or veins within mountains. Once the underground workings are abandoned, the adits become unwanted drains for the tunnels, shafts, drifts, and stopes that make up the mine works. Waste dumps. Made up of overburden soil and rock as well as ore too low in grade to process, waste dumps can become sources of acid rock drainage (ARD) if not isolated from the environment. Tailings. Metals ore is milled down into fine particles so that the metals can be removed by physical/chemical processes. The remaining fine-grade material is discarded. Modern tailings disposal places this material in lined empoundments. Historic as well as present-day operations such as Freeport-McMoRan’s Grasberg mine (1) often discharged tailings into nearby waterways to be washed downstream. Pit lakes. Large open pit mines have, to date, not been required to backfill after the economical ore has been removed. If the pit intercepts the groundwater table, the groundwater will begin filling the pit when dewatering pumps are turned off. Groundwater flowing into the pit will carry dissolved metals into the lake. When the lake surface rises to the natural groundwater level, acidic water carrying dissolved metals may reverse directions and began flowing out of the lake into the aquifer (2). In the cases of adits and pit lakes, groundwater flowing through the unmined ore body surrounding the mine workings can generate acid and dissolve metals. Waste Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. dumps and tailings become ARD producers when rainfall and snowmelt move through the material. The resulting surface flow or infiltration carries metals and acidity to down-gradient receptors (streams, lakes, aquifers). 3 CHEMISTRY OF METALS-IMPACTED WATERS Iron sulfide ores marcasite and pyrite (both FeS 2 ) are prevalent above and below coal seams as well as in metal deposits. They differ by crystal structure. Pyrite is the predominant of the two, and is the major source of acid mine drainage. Stoichiometric reactions are FeS 2 + 7 ⁄ 2 O 2 + H 2 O = Fe 2+ + 2SO 4 2− + 2H + (1) Once the reaction is started and the ferrous iron is generated, this reaction looses significance compared to the following three. Fe 2+ + 1 ⁄ 4 O 2 + H + = Fe 3+ + 1 ⁄ 2 H 2 O(2) The oxygenation of ferrous iron in the chemistry lab is very slow, approximately 1000 days. Fe 3+ + 3H 2 O = Fe(OH) 3 (s)+ 3H + (3) Ferric iron is hydrolyzed and 3 moles of acid (H + ) are produced. FeS 2 + 14Fe 3+ + 8H 2 O = 15Fe 2+ + 2SO 4 2− + 16H + (4) This reaction rate is rapid (20–1000 min), and 16 moles of acid are produced. It can be seen that the ferrous iron produced in Eq. (4) will feed the reaction represented by Eq. (2). In the presence of excess pyrite, reactions (2), (3), and (4) are now self-sustaining, yielding large quantities of acid in steps 3 and 4. How is the “acid” produced? The hydrogen ions generated in Eqs. (3) and (4) combine with sulfate or phosphate ions to form their respective acids: 2H + + SO 4 2− = H 2 SO 4 sulfuric acid (5) 3H + + PO 4 2− = H 3 PO 4 phosphoric acid (6) Sulfate ions, and often phosphate ions, are present in excess in pyritic ore. In a hypothetical chemistry lab, acid generation should not be a problem, as Eq. (2), the oxygenation of ferrous iron, has an extremely slow rate of reaction. In nature, microorganisms speed up the rate of reaction by mediating the oxidation of ferrous iron. This is performed principally by Thiobacillus and Ferrobacillus ferrooxidans, as well as other sulfide-reducing bacteria (SRB). pH is also important, as the reactions are slow unless pH < 4. Thus, potential ARD material may lie nonreactive for years until the Eq. (1) reaction lowers the pH to Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. around 4. Then, the generation of acid is fast. ARD from adits, waste dumps, tailings, or pit lakes may have pH values less than 2. The final result of the acidification of water in contact with ARD materials is the dissolution of metals in the same materials or in the downstream environ- ment. It has long been known that most metals will dissolve at extremely low or extremely high pH. A typical metals concentration-versus-pH plot shows a V-shaped zone of solids precipitate, with the bottom of the V near neutral (see Figure 1). Outside the V-shaped zone, metals tend to be in solution as dissolved FIGURE 1 Zone of precipitation of Fe(OH) 3(s) in contact with hydroxoiron (III) complexes at 25˚C (3). Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. ions. Figure 1 shows equilibrium concentrations of hydroxo iron(III) complexes in a solution in contact with freshly precipitated Fe(OH) 3(solid) (3). Most metals show a similar trend. It can be seen, then, that at pH ranges between 2 and 4, many metals will dissolve and be carried down-gradient is surface water and infiltration. 4 IMPACTS ON THE ENVIRONMENT The acidic water leaving the mine site may prove toxic to aquatic flora and fauna in the receiving streams and lakes. Most aquatic life flourishes at near-neutral pH, and may die or fail to flourish when pH values drop significantly. A lower pH may also cause reactions in plant nutrients, changing them to forms in which they are unavailable to the aquatic floral community (4). The low pH values additionally can cause large concentrations of metals to enter the aquatic environment: iron, aluminum, manganese, copper, cadmium, zinc, nickel, and lead, to name the typical major constituents. In higher concen- trations, these metals can themselves prove toxic to aquatic life. Additionally, precipitation of metals (principally iron oxides) in downstream channels and lakes will affect the aquatic life found on the beds and banks of streams and lakes. 5 TRADITIONAL SOLUTIONS Traditional solutions can be grouped into two categories: source control and downstream treatment (5). Source control seeks to prevent or stop ARD produc- tion at the source—the waste dumps, adits, and tailings. 5.1 Source Control For dumps and tailings, source control usually consists of isolating the waste materials from water or oxygen or both, covering them with either soil or water. This is often successful when waste materials are contained within impoundments or engineered dumps. When waste rock or tailings are spread over large areas, it is difficult to locate, excavate, transport, and isolate these ARD-producing wastes. Adit discharge has proven even more difficult to address at the source. Conventional approaches include plugging up the adit and attempting to prevent percolation through the mine workings drained by the adit. Plugging an adit often results in the creation of ARD springs in the vicinity of the plugged adit, as the hydraulic pressures inside the mountain create new passages to drain the mine works. Schemes to prevent percolation through the mine workings include attempting to grout fissures which carry rainfall percolation down to the mine works. Large recharge areas and complex geology often foil such attempts (6). Source control for large pit lakes is impossible using current technology, as the lake bank surface area is too large to seal, and the inflowing groundwater too Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. difficult to control. Backfilling of small pits attempts to limit contact between groundwater and oxygen, and may or may not prove successful. Approaches for large pits include institutional controls (fencing, restriction of pumping) if the lake will not affect the groundwater. If the pit lake water will significantly affect the groundwater, then conventional technology calls for perpetual pumping with surface treatment before discharge (7). 5.2 Downstream Treatment Most downstream treatment approaches for surface flow ARD use neutralization followed by precipitation of metal compounds. Neutralization may be achieved by adding lime: limestone (CaCO 3 ), slaked lime [Ca(OH) 2 ], or baked lime (CaO). Limestone is often used to line channels through which the ARD is directed. It is inexpensive and dissolves slowly; however, iron precipitation in the form of FeO 2 or FeOH often coats the limestone particles, decreasing their effectiveness. Slaked lime and baked lime are most often added to ARD streams as a slurry (lime mixed with water). Lime addition to the Clark Fork River near Warm Springs, Montana, is a good example. The Clark Fork is a potential trout stream heavily impacted by metal mining in its watershed. Both waste rock and tailings contribute acidity and dissolved metals to surface and groundwater, which in turn deliver these pollutants to the Clark Fork. Flow rates average 40 cubic feet per second (cfs) in spring and 8 cfs during the rest of the year (8). To raise the pH and drop out metals, a lime slurry is injected into the stream, followed by oxidation using a small waterfall. The stream flow then passes through a series of settling ponds, where metal hydroxides and oxides precipitate out. The water leaves the ponds at a pH 4 range of 7.5–9.0 and with typical metals concentrations several orders of magnitude lower than those in the upstream water. While lime precipitation is successful at reducing metals concentrations down to levels that meet discharge regulations, several factors argue for a better solution. First, the cost of lime is significant, and increases with the purity and fineness of the lime. The Warm Springs lime treatment station spent approxi- mately $89/ton in 1999 for slaked lime, which resulted in an annual cost of approximately $720,000 (9). Since treatment for surface runoff may continue for hundreds of years and pit lake treatment for perpetuity, a less costly alternative with minimal operations is desirable. Second, lime precipitation creates a large volume of CaOH, CaCO 3 , and CaO sludges. These sludges have various waters of hydration attached, and may contain large amounts of water when pumped out of settling chambers. Additionally, these sludges are unstable and may redissolve under changing water conditions. An innovative and still experimental approach for treating mine adit ARD is the U.S. Bureau of Mine’s (USBM) “In-Mine Treatment of Acidic Drainage Using Anaerobic Bio-reactors” (10). The USBM built a water treatment bioreac- Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. tor inside an abandoned mine adit and filled the reactor with limestone and compost. The mine adit discharged around 4 liters per minute with a pH of 3, 500 ppm iron, and 30 ppm aluminum. Mine drainage was directed to the reactor, where, in an anaerobic environment, sulfur-reducing bacteria (SRB) are encour- aged to consume organic materials such as sugar and generate hydrogen sulfide (H 2 S). The H 2 S reacts with metal ions to produce metal sulfide precipitates. The metal sulfide precipitates are retained within the reactor, and have a much smaller volume and are more stable than hydroxide or oxide precipitates. Alkalinity, generated by dissolution of the limestone, raises the pH and precipitates alumi- num as hydroxide [Al(OH) 3 ]. The bioreactor reportedly raised the pH to 6 and reduced the iron and aluminum concentrations by 50% and 99%, respectively. The treatment system was located inside the mine and could thus be operated year-around. The cost of the sugar was expected to be $0.67 per million liters of water. 6 ENGINEERED WETLANDS TO TREAT MINE-IMPACTED WATER The USBM bioreactor described in the previous section, while using natural mechanisms to precipitate metal sulfides, is still operationally demanding, requir- ing addition of sugar on a semiweekly basis. Additionally, it is limited to warm climates or protected areas where freezing will not inhibit the operation. Also, the discharge treated was approximately a gallon per minute, on the small side of adit discharges and much less than storm discharges of surface water from mining- impacted areas. Thus, a natural technology with larger discharge capacity and reduced operational requirements is desired to address the problem of mining- impacted waters. In the past two decades, natural and created wetlands have been used successfully to clean a variety of contaminants from water, including domestic sewage and industrial waste streams. It is logical to investigate such wetlands for removal of metals from water as well. Dominant reactions and treatments occurring within wetlands are represented by the following five processes (11). Photosynthesis uses light energy and CO 2 to form organic matter and O 2 and reduce the partial pressure of CO 2 . The production of organic matter allows other zones of the system to become anaerobic. Production of O 2 increases the oxidation potential, most probably at microenvironment sites. The change in the partial pressure of CO 2 shifts the water chemis- try, usually toward higher pH. Aerobic respiration involves the formation of energy for cell use by the consumption of organic matter and oxygen. The main benefit of aerobic respiration is the consumption of oxygen. Through aerobic respiration, Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. the deeper portions of the free water surfaces have decreased oxygen concentrations, as do the treatment walls. The decrease of oxygen allows other organisms to exist, which use other electron acceptors for anaerobic respiration. Anaerobic respiration involves the formation of energy for cell use by the consumption of organic matter and electron acceptors such as ferric iron, sulfate, nitrate, and CO 2 . The main benefits are the reduction of sulfate and nitrate and the formation of metal sulfide complexes. Metal sulfide complexes are more stable than metal oxides/hydroxides, which are more likely to form in aerobic environments. Settling and filtering involves the removal of suspended solids from the water. Plantings along the banks and berms of the cells will increase settling and filtering. It is anticipated that solids that are settled and filtered out will reside in and help to create anaerobic zones and transitional zones between aerobic and anaerobic. Oxidation reactions in zones of higher oxidation potential (near the water surface) are expected to form metal oxides/hydroxides, which will settle in the open water system. These complex natural processes are expected to combine to adjust pH values and remove excess metals from the influent water. 7 CASE STUDY: ARCO DEMONSTRATION WETLANDS In 1996, the Atlantic Richfield Corporation (ARCO) began a series of demonstra- tion projects to treat mining-impacted surface and groundwater in Butte, Mon- tana. ARCO had been named in 1983 as one of the Potentially Responsible Parties (PRPs) for the Clark Fork Basin Superfund sites listed in the very first National Priorities List (12). Water components included surface water running through or over widespread waste rock and tailings as well as precipitation percolating through waste rock and tailings and reaching the groundwater. While some adit discharge may add to the storm water, adit flow is considered insignificant. The site includes a pit lake with severely degraded water quality. No pit lake water was treated in the demonstration projects. Three demonstration-scale wetlands were constructed by ARCO to test the hypothesis that natural and chemical processes in engineered wetlands could remove metals from mining-impacted surface and groundwater. The three wet- lands are identified below, with salient features of each presented. Wetlands Demonstration Project 1 (WDP1). Seven cells were constructed, including an initial storage pond, four anaerobic subsurface cells, and two aerobic surface water cells. The volumes of the four anaerobic cells measured 30,000 ft 3 , 20,000 ft 3 , 12,000 ft 3 , and 15,000 ft 3 for Cells 1–4, Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. respectively, with depths varying from 2.5 to 6 ft. Hydraulic retention times varied from 3.7 days to 9.4 days at 5 gallons per minute (gpm). Three different flow paths were possible using the seven cells. The anaerobic cells’ substrate was a 20% limestone/80% river cobble mix. Compost was added to three of the four anaerobic cells. Cattails were planted in the anaerobic cells as a renewable source of organic carbon (13). All of the cells except the storage pond were lined with HDPE geomembrane. Inflow water came from the Metro Storm Drain, a small channel collecting storm water at the foot of the “Richest Hill on Earth.” Inflow characteristics are presented in Table 1. Wetlands Demonstration Project 2, Butte Reduction Works (BRW). This wetland consisted of three open ponds separated by two porous treatment walls (13), the total surface area measuring slightly less than an acre with a depth of 4–5 ft. Water flowed sequentially through the five cells. The treatment walls were constructed of river cobble. The second treatment wall contained compost as well. None of the cells was lined. This wetland treated Missoula Gulch surface water, which is also impacted by mine TABLE 1 Dissolved Metals and Major Anions (14) Parameter Units Missoula Gulch LAO groundwater Metro Storm Drain PH mg/liter 7.51 3.58 7.05 Aluminum mg/liter <0.03 2.52 0.04 Arsenic mg/liter <0.04 <0.04 0.05 Cadmium mg/liter 0.019 0.115 0.007 Calcium mg/liter 84.8 134 214 Copper mg/liter 0.096 14.6 0.067 Iron mg/liter <0.021 0.087 <0.021 Lead mg/liter <0.04 0.39 <0.04 Magnesium mg/liter Manganese mg/liter 1.38 17.6 13 Phosphorus mg/liter <0.1 <0.1 <0.1 Sodium mg/liter 37.3 Zinc mg/liter 2.22 18 15.1 Chloride mg/liter 35 0.58 69 Nitrate mg/liter 3.34 0.15 0.87 Phosphate mg/liter 0.09 0.11 Sulfate mg/liter 184 594 650 a Combined POTW: POTW water, LAO groundwater, and LMG water. b Combined w/o POTW: Metro Storm Drain (MSD) water, LAO groundwater, and LMG water. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. wastes distributed throughout the watershed. Missoula Gulch water characteristics can be found in Table 1. Wetlands Demonstration Project 2, Colorado Tailings (CT). The wetland design was similar to BRW, consisting of three sequential open ponds separated by two cobble treatment walls. The five cells are unlined, and are approximately 425 ft long by 185 ft wide, with a depth of around 4.5 ft. The wetlands floor penetrated the groundwater, so the bottom 6–18 in. can be considered groundwater flow, while the top 2–4 ft of water is surface flow. CT included lime addition at the inlet to raise the pH, remove excess metals, and pretreat the water. Influent water was pumped from a collection trench intercepting groundwater, known as Lower Area One (LAO) groundwater. This water is the most degraded of the three. Characteristics are presented in Table 1. 7.1 Summary of Performance on Each Wetland It was hoped that the ARCO wetlands would precipitate metal sulfides via anaerobic respiration. Theoretically, in an anaerobic environment, sulfur-reducing bacteria (SRB) generate hydrogen sulfide (H 2 S). The H 2 S reacts with metal ions to produce metal sulfide precipitates. As stated previously, metal sulfide precipi- tates are more stable and of a much smaller volume than metal oxides or hydroxides. Anaerobic zones are produced in the subsurface cells in WDP1 and in the treatment walls of BRW and CT, as well as at the bottom of surface cells where sediments and decaying organic matter form an anaerobic layer. However, any of the other mechanisms—photosynthesis, oxidation, aerobic respiration, settling, and filtration—may play the major role as well. The metals of interest for the ARCO wetlands are copper, manganese, and zinc. Other metals are present at concentrations low enough to meet anticipated discharge standards. A summary of the performance of each of the wetlands is presented below. 7.2 WDP1 Performance Cell 3, the only upflow cell of the four, failed hydraulically within the first three months. The remaining three horizontal flow cells were monitored continuously from the spring of 1996 until December of 1998. Contaminants of major concern were copper, cadmium, and zinc. Manganese may also be a problematic constit- uent, depending on site-specific cleanup levels yet to be determined. Table 2 presents typical summer and winter performance data from Cell 2. It can be seen that copper and cadmium concentrations were reduced more than 97% on both sampling days. Zinc concentrations were reduced by more than 99% on the summer day, but only 94% reduction was achieved for unfiltered Zn on the winter sampling date. The decrease in Zn removal efficiency is probably due to both a reduction in hydraulic retention time because of freezing of the upper Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. [...]... impacts surface and groundwater quality While waste minimization and pollution prevention can prevent generation of AMD in current operations, centuries of historic mining has Copyright 2002 by Marcel Dekker, Inc All Rights Reserved resulted in acid mine drainage from thousands of mine sites across the United States The control of these pollution sources is mostly limited to treatment This chapter has... production of AMD, summarized common solutions to the problem, and presented engineered wetlands as a possible new control of this very serious pollutant A demonstration-scale wetlands was presented to demonstrate treatability of AMD After two years of operation, the three demonstration wetlands in Butte, Montana, are successful to some degree in removing metals from miningimpacted surface and groundwater... Certainly, the ARCO wetlands showed promise for removing metals from water The extent of contaminant reduction and long-term effectiveness must wait to be proven until the wetlands mature Cold water temperatures (in the winter season) obviously reduce the effectiveness of the wetlands At a minimum, engineered wetlands can serve as a passive polishing step With the addition of a small amount of lime to the influent,... Butte-Silver Bow Planning Department, Butte, MT 3 V L Snoeyink and D Jenkins, Water Chemistry New York: Wiley, 1980 4 F F Munshower, The Practical Handbook of Disturbed Land Revegetation Lewis Publishers, 1994 5 Educational Communications, Inc., for U.S Environmental Protection Agency, Surface Mining and the Natural Environment, 1985 6 L McCloskey, D B Kelly, and J Gilbert, Source Control Surface Waste. .. activity of SRBs during cold temperatures (15) It should be kept in mind that Cell 2 has a volume of 20,000 ft3 and a flow rate of approximately 5 gpm It was thought that, upon scaling up, this volumeto-flow rate ratio would require excessive land surface for treating the required flow rate for the Superfund site Early results of the WDP1 performance led ARCO to modify its wetlands design for the second and. .. Department of the Interior, In-Mine Treatment of Acidic Drainage Using Anaerobic Bioreactors Technol News, no 444, December 1994 11 ARCO, Wetlands Demonstration Project 2 Butte Reduction Works Final Design Report, December 1996 12 Montana Department of Health and Environmental Sciences and the U.S Environmental Protection Agency, Progress—Clark Fork Basin Superfund Sites, May 1990 13 A Frandsen, Review of. .. Heavy Metal Attenuation in an Anaerobic Treatment Wetland, Butte, Montana, unpublished, 1999 16 Montana Tech, Wetlands Demonstration Project Progress, September 1997 17 T P Mulholland, A Study of the Mass Deposited, Removal Efficiency, and Types of Minerals Formed in Colorado Tailings Constructed Wetlands, Master’s thesis, Montana Tech of the University of Montana, May 1999 Copyright 2002 by Marcel Dekker,... extreme, with a long and cold winter Piping, valving, and meters were prone to freezing, so recommendations are to use open channels to deliver water and flumes or weirs to measure flow rate Wetlands must be scaled to account for freezing in winter Freezing of the top 12 in of the treatment cells will obviously reduce the flow area and decrease hydraulic retention time Also, bacterial and plant communities... cold season, and storage of contaminated water may be required during the winter until natural degradation processes are once again active in the spring Precipitation of contaminants such as metal sulfides is desirable, and theoretically achievable using natural processes in engineered wetlands At two years of operation, the ARCO wetlands were not mature enough to document precipitation of metal sulfides... the wetlands may be able, with time, to meet treatment goals ACKNOWLEDGMENTS The information concerning passive engineered wetlands presented in this chapter would not have been generated without the support and guidance of the Atlantic Richfield Company and their project manager, Dr John Pantano Copyright 2002 by Marcel Dekker, Inc All Rights Reserved REFERENCES 1 G A Mealey, Grasberg, Freeport-McMoRan . mining of sulfur-bearing ores seriously impacts surface and groundwater quality. While waste minimization and pollution prevention can prevent generation of AMD in current operations, centuries of. source control and downstream treatment (5). Source control seeks to prevent or stop ARD produc- tion at the source—the waste dumps, adits, and tailings. 5.1 Source Control For dumps and tailings,. problem, and presented engineered wetlands as a possible new control of this very serious pollutant. A demonstration-scale wetlands was presented to demonstrate treatability of AMD. After two years of