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Home , Acid mine drainage, Constructed wetland

MINE DRAINAGE TREATMENT WITH A COMBINED /ANOXIC DRAIN: GREENHOUSE AND FIELD SYSTEMS'

by

J. Skousen*, A. Sexstone, J. Cliff, P. Sterner, Joe Calabrese, and P. Ziemkiewicz'

Abstract: The most common methods for treating (AMD) involve applying a strong base to nentralize the acidity and to precipitate . Because of the enormous expense of chemical treatment, passive treatment systems have been installed on many sites to treat AMD. Constructed have been used for over a decade on hundreds of sites. Wetlands remove acidity and metals from AMD under most conditions, but problems arise when acid and loads are too great for wetland size or as metal hydroxides fill the wetland, restricting substrate and water contact. Passive systems often incorporate limestone, which is a very cost effective acid neutralizer, to treat AMD. Complications also arise in limestone systems because ferric (Fe3+) in AMD coats the limestone (armoring) reducing limestone's contact with acid water and because aluminum (Al) and Fe hydroxides plug limestone pores decreasing water flow. Limestone use in AMD treatment has been largely confined to anaerobic wetlands, anoxic limestone drains (ALDs) and open limestone channels. IfFe3+ and Al could be removed from AMD before introduction into limestone systems, then the use of limestone for AMD treatment could be greatly expanded. We developed and monitored a passive AMD system to determine if AMD containing Fe'+ and Al could be pre-treated by flowing through a wetland making it more suitable for introduction into an underlying limestone drain, similar to a vertical flow wetland. Theoretically, pre-treatment by wetlands could reduce Fe'+ to ferrous iron (Fe'+) through Fe reduction and precipitate Fe+ as ferrous (FeS.) through sulfate reduction. Further, Fe and Al may be adsorbed to organic matter in the wetland thereby eliminating the formation of metal hydroxides with subsequent plugging oflimestone pores. A field scale wetland/anoxic limestone drain (WALD) system located at Douglas, WV exported net alkaline

water (mean of 127 mg/L as CaC03) for one year. However, dissolved and Fe data suggest that poor hydraulic conductivity caused this system to act as an Fe-oxidizing system, rather than an Fe-reducing system. As such, the system's long term effectiveness for treating AMD was compromised. After five years of operation, the system still reduces the acidity of the water from

about 500 mg/Las CaC03 to about 150 mg/L. A small scale Greenhouse system performed more like an Fe-reducing system, decreasing acidity for seven months and exporting Fe'+, although the water exiting the wetland did not contain excess alkalinity. While complications arose in our systems due to high flows in the Douglas system and high acidity in the Greenhouse system, pre-treating AMD with organic material can improve the condition of the water for proper treatment by an ALD or underlying limestone. For low to moderate flows ( <400 L/min) and low Fe concentrations ( <50 mg/L), a passive system that pre-treats AMD with organic substrates and then directs the water into limestone may be effective for many years.

Additional Key Words: aluminum, iron, iron coating of limestone, limestone treatment, passive treatment, metal precipitation

11999 National Meeting of the American Society for Corvalis, OR. Pat Sterner, Northwest Recycling, Surface and Reclamation, Phoenix, AZ, August Fairmont, WV. Joe Calabrese, Lock Haven Univesity, 13-19, 1999. Lock Haven, PA. Paul Ziemkiewicz, National Mine Land Reclamation Center, West University, Morgantown, WV. This research was supported by the 'Jeff Skousen and Alan Sexstone, Division of Plant and U.S. Bureau of Mines, Contract No. 1432-H0339009, and Sciences, University, Morgantown, by funds appropriated under the Hatch Act. WV 26506-6108. John Cliff, Oregon State University, *Corresponding author [email protected]).

621 Introdnction Anaerobic Wetlands

Acid mine drainage (AMD) forms when Anaerobic wetlands encourage interaction of minerals such as and marcasite (FeS2) are exposed water with organic-rich substrates. The wetland substrate to oxygen and water during mining and other large scale may contain a layer of limestone in the bottom of the land disturbances. Acid mine drainage is characterized wetland or the limestone may be mixed within the organic by high sulfate concentrations, high levels of dissolved matter. Wetland plants are transplanted into the organic metals, and pH < 4.5. In 1995, the Enviromnental substrate. These systems are used when the water has net Protection Agency (USEPA 1995) estimated that about acidity, so alkalinity must be generated in the wetland and 10,000 km of were impacted by AMD in the introduced to the net acid water in order to accomplish northern Appalachian area of the US (PA, MD, OH, and significant precipitation of dissolved metals. WV), and a 1996 study showed that 17 of 51 priority streams in WV were affected by AMD (Faulkner and The alkalinity can be generated in an anaerobic Skousen 1998). Historically, AMD treatment is wetland system in two ways (Hedin and Nairn 1990). accomplished by adding a strong base to neutralize Certain , Desulfovibrio and Desulfotomaculum, acidity, raise pH, and precipitate metals. (in various utilize reactions between organic substrate (CH20, a forms), sodium hydroxide, sodium carbonate, and generic symbol for organic ) and sulfate as a anhydrous are the four primary chemicals used source of energy for their metabolism. The bacteria for AMD treatment. Although effective, chemical require relatively simple organic compounds, so only part treatment is expensive when the cost of equipment, of the organic matter is normally usable by them, or they chemicals, and manpower is considered (Skousen 1991 ), require action by fermenting or other bacteria to degrade and responsibility for treatment may be a long term and complex compounds. In the bacterial conversion of possibly never-ending liability. In 1990, the United sulfate to , alkalinity is States industry spent over $1 million per day on produced: 2 active treatment of AMD (Kleinmann 1990). S04 + 2 CH20 - H,S + 2 HCO; Consequently, there has been much interest in the use of The sulfate-reducing bacteria function best in the pH passive systems employing limestone as a more cost range 6 to 9 (Widdell 1988) and can function down to effective means of treating AMD ( du Plessis and Maree about pH 5. However, the bacteria can control their 1994, Hedin et al. 1994b, Maree et al. 1992). microenvironment so that successful sulfate reduction has been reported at influent pH as low as 2.0 to 2.8 (Balis et The primary passive techoologies include al. 1991, Gusek 1998, Tuttle et al. 1969). However, if the aerobic or anaerobic constructed wetlands, anoxic rate of acidity input exceeds the neutralization capacity of limestone drains (ALD), vertical flow wetlands such as the system, pH will decrease and sulfate reduction will successive alkalinity producing systems (SAPS), and stop (Eger 1994). open limestone channels. Natural wetlands are characterized by water-saturated or sediments with Alkalinity can also be generated as the limestone supporting adapted to reducing conditions in under the organic material reacts with acidity in the their . Constructed wetlands are man-made wetland: 2 that mimic their natural counterparts. Often CaC03 + H+ - ca• + HCO,· they consist of shallow excavations filled with flooded The limestone continues to react when kept in an , soil, and organic matter to support wetland plants, anaerobic environment because ferrous iron (Fe2+) is such as , Juncus, and Scirpus. Treatment depends relatively soluble at pH 7 in anoxic water and ferrous on dynamic biogeochemical interactions as contaminated hydroxide does not form and coat the limestone. If water travels through the . Aerobic ferrous iron is oxidized, forming ferric iron (Fe'•), then wetlands promote oxidation and hydrolysis in the surface the ferric iron will precipitate as a coating of ferric water of the wetland. In anaerobic wetlands, the hydroxide on the limestone. Bacterial sulfate reduction metabolic products of sulfate-reducing bacteria, usually and limestone dissolution produce water with higher pH accompanied by limestone dissolution, are major and add bicarbonate alkalinity for metal removal. reactants in raising pH and precipitating metals as sulfides, hydroxides and/or carbonates. The bacteria use The surface water in anaerobic wetlands is organic substrates and sulfate as nutrients; plants are not oxidized, so oxidation and precipitation of Fe and required but are often helpful in mediating the reactions. (Mn) are promoted in this zone to the extent that alkalinity is available. Organic matter and the

resulting bacterial reduction of 0 2 and Fe create reducing

622 conditions in the subsurface zone. Sulfate reduction and Anoxic Limestone Drains limestone dissolution in this zone consume acidity, produce alkalinty and cause Fe-sulfide precipitation. Anoxic limestone drains (ALDs) are buried cells Increase of pH also causes Al hydrolysis and or trenches of limestone into which anoxic water is prec1p1tation. Some alkalinity is transported to the introduced. The limestone dissolves in the mine water surficial zone by diffusion and minor flow, promoting and adds alkalinity (Watzlaf and Hedin 1993). Under hydrolysis and precipitation. Iron, Al and H+ may also be anoxic conditions, the limestone does not coat or armor 2 transported into the anaerobic subsurface zone for with Fe hydroxides because Fe • does not precipitate as precipitation and neutralization. ferrous hydroxide at pH <8.0. The effluent pH of ALDs is typically between 6 and 7 .5. The sole function of an Several treatment mechanisms are enhanced in ALD is to convert net acidic mine water to net alkaline anaerobic wetlands compared to aerobic wetlands, water by adding bicarbonate alkalinity. The removal of including formation and precipitation of metal snlfides, metals within an ALD is not intended and has the metal exchange and complexation reactions, microbially- potential to significantly reduce the permeabilty of the generated alkalinity due to reduction reactions, and drain resulting in premature failure. The maximum continuous formation of carbonate alkalinity due to alkalinity in effluents from 21 ALDs studied by Hedin et limestone dissolution under anoxic conditions. Microbial al. (1994b) was 469 mg/Las CaC03, with effluent values mechanisms of alkalinity production are likely to be of commonly between 150 and 300 mg/Las CaC03• Based critical importance to long term AMD treatment. on experiments with of differing purity, However, Wieder (1992) documents that the mechanism Watzlaf and Hedin (1993) showed that limestones with and efficiency of AMD treatment varies seasonally and >82% CaC03 differed little from purer limestones in their with wetland age. Like their aerobic counterparts, effectiveness in neutralizing acid. anaerobic wetlands are most successful when used to treat small AMD flows of moderate water quality. Faulkner and Skousen (1994) reported both successes and failures among 11 ALDs treating mine Five anaerobic wetland systems in WV receiving water in WV. In all cases, water pH was raised after 4 to 98 L/min ofnet acid water (110 to 2400 mg/L acidity ALD treatment, but three of the sites had pH values <5.0, as CaC03 and Fe from 10 to 376 mg/L) reduced acidity indicating that the ALDs were not fully functioning or by 3 to 76% and Fe concentrations by 62 to 80% that the acid concentrations and flow velocities were too (Faulkner and Skousen 1994). These wetlands were high for effective treatment. Acidity of water in these generally much smaller in area than that recommended by drains, varying from 170 to 2200 mg/L as CaC03, early formulas published by the U.S. Bureau of Mines decreased 50 to 80%, but Fe and Al concentrations in the based on Fe loads. For example, one of these wetlands, outflow also decreased from influent values. With Fe and Keister, reduced the acidity of a 17-L/min.flow from 252 Al decreases in outflow water, some coating or clogging to 59 mg/Las CaCO, (76% reduction) and increased pH of limestone is occurring with Fe and Al hydroxides from 3.1 to 5.4. Iron was reduced from 23 to 9 mg/L inside the ALD. (62%), Mn from 23 to 20 mg/L (11 %), and Al from 27 to 13 mg/L (52%). The Pierce wetland used an organic At the Howe Bridge and Morrison ALDs, substrate over limestone and treated a 98-L/min flow. alkalinity in effluents increased by 128 and 248 mg/L,

Influent pH was 3.3, acidity was 118 mg/L as CaC03, Fe respectively, over influent water, CO2 pressures were near of 10 mg/L, Mn of 8 mg/L, and Al of 9 mg/L. Outflow 0.1 atm and calcite was at about 10% of saturation (Hedin pH was 4.4, acidity was reduced to 57 mg/L as CaCO, et al. 1994a). For the past eight years, the effluent from (52%), Fe decreased to 2 mg/L (80%), Mn was reduced the ALD-wetland system at Morrison has always met by 11 %, and Al by 25%. effluent criteria (pH 6-9, and Fe <3 mg/L). At Howe Bridge, the ALD-wetland system has removed an average A wetland system consisting of six wetland cells of70% of the Fe over the past seven years. (total area of 2500 m') and a sedimentation basin each received a small flow (5 L/min) of AMD with pH of3.0, At the Jennings Env. Center in Slippery Rock, acidity of 217 mg/L as CaC03, Fe of 27 mg/L, Al of 12 PA, an ALD was constructed to document the reduction mg/L, and Mn of 2 mg/L (Hellier 1996). At this site in in permeability due to Al hydroxide precipitation within , the effluent after passing through the the drain. The system, which received 21 mg/L Al at a wetland was raised to pH 5 .1, and the water contained a flow rate of 92 L/min, began to have permeability net acidity of 16 mg/Las CaC03, with about 46% Fe problems within three months and plugged with Al removal, and 56% Al removal. hydroxide after about 6 months (Watzlaf et al. 1994).

623 At the Brandy Camp site in PA, an ALD was Kepler and McCleary (1994) reported on initial employed to treat AMD with a pH of 4.3, acidity of 162 successes for three SAPS in PA. The Howe Bridge SAPS

mg/L as CaC03, Fe of 60 mg/L, Mn of 10 mg/L, and Al reduced acidity from 320 mg/L to 93 mg/L as CaC03, and of 5 mg/L (Hellier 1996). After passage through the removed 2 mg/L ferric iron. The REM SAPS decreased

ALD, the effluent had a pH of 6.0, net alkalinity of 10 acidity from 173 to 88 mg/L as CaC03, and exported

mg/L as CaC03, Fe of 50 mg/L, Mn of 10 mg/L, and Al more ferrous iron than entered. The Schnepp Road SAPS of <1 mg/L. Most of the Fe and Mn passed through this decreased acidity from 84 to 5 mg/L as CaC03, but system and precipitated in subsequent wetlands, while Al removed all 19 mg/L ferric iron, with only 1 mg/L ferrous was precipitated inside the drain. Like wetlands, ALDs iron exiting the SAPS. may be a solution for treating specific types of AMD or for a fmite period after which the system must be Kepler and McCleary (1997) also reported the replenished or replaced. use of SAPs in OH, PA, and WV. In all cases, Al in AMD precipitated in their systems. Their drainage design Longevity of AMD treatment is a concern for incorporates a flushing system called the 'Aluminator.' ALDs, especially in terms of water flow through the This allows for the precipitated Al to be flushed from the 3 limestone. If appreciable dissolved Fe • and Al are pipes thereby maintaining hydraulic conductivity through present in influent water, precipitation of Al and Fe the limestone and pipes. The Buckeye SAPS received 3 hydroxides causes clogging oflimestone pores (Faulkner L/min of pH 4.0 water with acidity of 1989 mg/L as

and Skousen 1994, Watzlaf and Hyman 1995). For CaC03, Fe of 1005 mg/L, and Al of 41 mg/L. Over a waters with high sulfate (>1,500 mg/L), (CaS04) two-year period, the effluent had a pH of 5.9, net acidity may also precipitate (Nairn et al. 1991). In ideal concentration of about 1000 mg/L, Fe of 866 mg/L, and 3 situations, dissolved Fe •, Al, and oxygen should not be

oxygen in the mine water may significantly decrease the from a net acid water (925 mg/L as CaC03) to a net effective life of the ALD. Selection of the appropriate alkaline water (150 mg/L as CaC03), and reduced Fe water and conditions is critical for long term alkalinity from 40 to 35 mg/Land Al from 140 to <1 mg/L. generation in an ALD. At the Brandy Camp site in PA, a SAPS was Vertical Flow Wetlands or SAPS employed to treat AMD with a pH of 4.3, acidity of 162

mg/L as CaC03, Fe of 60 mg/L, Mn of 10 mg/L, and Al Any strategy or system which could of 5 mg/L (Hellier 1996). After passage through the inexpensively raise pH while removing Al, and either SAPS, the effluent had a pH of 7. I, net alkalinity of 115 3 precipitating Fe • or reducing Fe tcr•Fe prior to mg/Las CaC03, Fe of 3 mg/L, Mn of 10 mg/L, and Al of entering an ALD may broaden treatment application.