Acid drainage

Acid drainage is a persistent environmental problem in many mineralized areas. The problem is especially pronounced in areas that host or have hosted mining activity (e.g., Lind and Hem, 1993), but it also occurs naturally in unmined areas. The acid drainage results from weathering of sulfide minerals that oxidize to produce hydrogen ions and contribute dissolved metals to solution (e.g., Blowes et al., 2005). 

These acidic waters are toxic to plant and animal life, including fish and aquatic insects. Streams affected by acid drainage may be rendered nearly lifeless, their stream beds coated with unsightly yellow and red precipitates of oxy-hydroxide minerals. In some cases, the heavy metals in acid drainage threaten water supplies and irrigation projects. 

Where acid drainage is well developed and extensive, the costs of remediation can be high. In the Summitville, Colorado district (USA), for example, efforts to limit the contamination of fertile irrigated farmlands in the nearby San Luis Valley and protect aquatic life in the Alamosa River will cost an estimated 100 million or more (Plumlee, 1994a). 

In this chapter we construct geochemical models to consider how the availability of oxygen and the buffering of host rocks affect the pH and composition of acid drainage. We then look at processes that can attenuate the dissolved metal content of drainage waters. 

In this chapter we construct geochemical models to consider how the availability of oxygen and the buffering of host rocks affect the pH and 

---

The existence of acid drainage has been recognized for some time ... 

However, the seriousness of the problem, and its underlying causes, have only relatively recently been addressed by the mining industry (Ripley et al., 1996). As a consequence, many mining operations conducted before the 1980s were not managed to control the deleterious effects of acid drainage. 

The above reaction represents, in a simplified way, the origin of acid mine drainage. Streambeds in areas of acid drainage characteristically become coated with an orange layer of ferric precipitate. We can write a reaction representing the overall process by swapping ferric hydroxide in place of the ferrous ion ... 

Finally, whereas most laboratory experiments have been conducted in largely abiotic environments, the action of bacteria may control reaction rates in nature (e.g., Chapelle, 2001). In the production of acid drainage (see Chapter 31), for example, bacteria such as Thiobacillus ferrooxidans control the rate at which pyrite (FeS2) oxidizes (Taylor et al., 1984 Okereke and Stevens 1991). Laboratory ob-... 

Acid drainage results from the reaction of sulfide minerals with oxygen in the presence of water. As we show in this section, water in the absence of a supply of oxygen gas becomes saturated with respect to a sulfide mineral after only a small amount of the mineral has dissolved. The dissolution reaction in this case (when oxygen gas is not available) causes little change in the water s pH or composition. In a separate effect, it is likely that atmospheric oxygen further promotes acid drainage because of its role in the metabolism of bacteria that catalyze both the dissolution of sulfide minerals and the oxidation of dissolved iron (Nordstrom, 1982). 

The concentrations of other metals attenuate when the metals sorb onto the surfaces of precipitating minerals (see Chapter 10). Hydrous ferric oxide, the behavior of which is well studied (Dzombak and Morel, 1990), has a large specific surface area and is capable of sorbing metals from solution in considerable amounts, especially at moderate to high pH HAO may behave similarly. The process by which hfo or HAO form and then adsorb metals from solution, known as coprecipitation, represents an important control on the mobility of heavy metals in acid drainages (e.g., Chapman etal., 1983 Johnson, 1986 Davis etal., 1991 Smith et ai, 1992). 

We do not concern ourselves with the precipitate that lines sediments in the stream bed, since it formed earlier while in contact with the drainage, and hence would not be expected to continue to sorb from solution. Smith et al. (1992), for example, found that in an acid drainage from Colorado (USA), sorption on the suspended solids, rather than the sediments along the stream bed, controls the dissolved metal concentrations. 

Fig. 31.5. Minerals formed during reaction at 25 °C of a hypothetical acid drainage water with calcite (top), and fractions of the amounts of arsenite, arsenate, copper, lead, and zinc present initially in solution that sorb onto ferric hydroxide over the course of the reaction path (bottom). Bottom figure is plotted against pH, which increases as the water reacts with calcite.

Lind, C. J. and J. D. Hem, 1993, Manganese minerals and associated fine particulates in the streambed of Pinal Creek, Arizona, U.S.A., a mining-related acid drainage problem. Applied Geochemistry 8, 67-80. 

Amy Berger helped me write Chapter 10 (Surface Complexation), and Chapter 31 (Acid Drainage) is derived in part from her work. Edward Warren and Richard Worden of British Petroleum s Sunbury lab contributed data for calculating scaling in North Sea oil fields, Richard Wendlandt first modeled the effects of alkali floods on clastic reservoirs, and Kenneth Sorbie helped write Chapter 30 (Petroleum Reservoirs). I borrowed from Elisabeth Rowan s study of the genesis of fluorite ores at the Albigeois district, Wendy Harrison s study of the Gippsland basin, and a number of other published studies, as referenced in the text. 

MEND 1995. Review of in-pit disposal practices for the prevention of acid drainage - case studies. MEND Report 2.36.1. 

Ferguson, K.D. Morin, K.A. 1991. The prediction of acid rock drainage -Lessons from the data base. Second International Conference on the Abatement of Acidic Drainage, Montreal, Canada, 3, 83-106. 

Site 2 is situated on the south shore of Moore s pit (Figs. 1 2b). This site consists of i) a pyritic waste rock pile, covered with a thin veneer of Fe-oxide tailings and ii) a down stream acid drainage affected area that is characterized by a Fe oxide + sulfur-rich hard pan surface layer that overlies a stratified unit comprised of intensely altered (Fe-oxide stained) and unaltered Quaternary glaciolacustrine deposits. A small creek (product of beaver activities) flows along the eastern margin of the site into Moore s pond. 

At the Fabius Coal Preparation Plant in Jackson County, Alabama, a constructed wetland treatment system was built to treat acid drainage from a coal pile. In 1985, the total cost of the wetlands was 43,000. The annual costs from 1985 to 1990 were approximately 13,000 due to repairs and extensive monitoring. In 1991, operation and maintenance costs were estimated to be 1000 annually (D12459E, p. 164). 

In 1990, an anoxic limestone drain was installed upstream of a wetland treating acid drainage from another coal-contaminated area of the Fabius Coal Preparation Plant. The total installation cost was approximately 19,000 (D124607, p. 135). 

Stanton, M. 1973. The role of weathering in trace metal distributions in subsurface samples from the Mayday Mine Dump near Silverton, Colorado. In Church, S. E. (ed) The USGS Preliminary Release of Scientific Reports on the Acidic Drainage in the Animas River watershed San Juan County, Colorado, US Geological Survey, Denver, CO, USA, 77-85. 

Douglas, J. H. (1992). Constructed wetlands reduce cost of treating acid drainage. Environmental Update, April, 4-5, 15. 

Part of the North Branch of the Potomac River runs crystal clear through the scenic Appalachian Mountains, but it is lifeless—a victim of acid drainage from abandoned coal mines. As the river passes a paper mill and a wastewater treatment plant near Westemport, Maryland, the pH rises from an acidic, lethal value of 4.5 to a neutral value of 7.2, at which fish and plants thrive. This happy "accident comes about because calcium carbonate exiting the paper mill equilibrates with massive quantities of carbon dioxide from bacterial respiration at the sewage treatment plant. The resulting soluble bicarbonate neutralizes the acidic river and restores life downstream of the plant.1... 

The addition of lime to control acid drainage from mining wastes typically produces calcium arsenates (Pichler, Hendry and Hall, 2001). Bothe and Brown (1999) further concluded that lime precipitates As(V) as a number of hydroxyl and hydrated calcium arsenates (Ca4(OH)2(AsC>4)2 4H2O, CaslAsCLLOH (arsenate apatite), and/or Ca3(AsC>4)2 3H2O) rather than anhydrous tricalcium orthoarsenate (Ca3(As04)2). Calcium arsenates also occur in coal combustion byproducts (Chapter 7). In the flue gas treatment systems of coal combustion facilities, volatile arsenic can readily react with calcium to form the arsenates on the surfaces of flyash and injected lime (Seames and Wendt, 2000 Yudovich and Ketris, 2005, 175). 

Nicholson, R. (1999) Prediction of acidic drainage and metal leaching from sulfide mine waste beyond yes or no answers. Abstracts with Programs. The Geological Society of America, 31(7), 334. 

---