Hydrogeochemistry and microbiology of mine drainage: An update D. Kirk Nordstrom a,⇑ , David W. Blowes b , Carol J. Ptacek b a United States Geological Survey, 3215 Marine Street, Boulder, CO 80303, United States of America b Department of Earth and Environmental Sciences, 200 University Avenue West, University of Waterloo, Waterloo, ON N2L 3G1, Canada article info Article history: Available online 21 February 2015 abstract The extraction of mineral resources requires access through underground workings, or open pit opera- tions, or through drillholes for solution mining. Additionally, mineral processing can generate large quan- tities of waste, including mill tailings, waste rock and refinery wastes, heap leach pads, and slag. Thus, through mining and mineral processing activities, large surface areas of sulfide minerals can be exposed to oxygen, water, and microbes, resulting in accelerated oxidation of sulfide and other minerals and the potential for the generation of low-quality drainage. The oxidation of sulfide minerals in mine wastes is accelerated by microbial catalysis of the oxidation of aqueous ferrous iron and sulfide. These reactions, particularly when combined with evaporation, can lead to extremely acidic drainage and very high con- centrations of dissolved constituents. Although acid mine drainage is the most prevalent and damaging environmental concern associated with mining activities, generation of saline, basic and neutral drainage containing elevated concentrations of dissolved metals, non-metals, and metalloids has recently been recognized as a potential environmental concern. Acid neutralization reactions through the dissolution of carbonate, hydroxide, and silicate minerals and formation of secondary aluminum and ferric hydroxide phases can moderate the effects of acid generation and enhance the formation of secondary hydrated iron and aluminum minerals which may lessen the concentration of dissolved metals. Numerical models provide powerful tools for assessing impacts of these reactions on water quality. Published by Elsevier Ltd. 1. Introduction 1.1. General types of mine drainage Mine drainage, i.e. waters affected by mining and mineral pro- cessing, can be acidic, circumneutral, basic, dilute, mineralized, and saline. This paper provides an overview of the wide varieties of mine water chemistry and the processes that form them. Although the main emphasis pertains to acid drainage from metal mining sites, other drainage will be mentioned including that from coal mines, diamond mines, and iron ore mines. This review is primarily an update since the article by Nordstrom and Alpers (1999a). The same basic geochemical pro- cesses will be briefly summarized and new research will be reviewed along with topics that were not covered in the previous review. 1.1.1. Acid mine drainage and acid rock drainage The term ‘acid rock drainage’ (ARD) is widely used for any acid drainage produced from rock, whether mined or not. There are numerous examples of natural acid rock drainage (NARD, see Nordstrom, 2015), acid drainage from mining activities or acid mine drainage (AMD), and acid drainage from other construction activities that must excavate sulfide-rich rock for construction of buildings, highways, bridges, and dams. Although ARD is not defined exactly in terms of pH, most ARD samples fall in the range of 2–6 and sulfate is the dominant ion. For waters with pH values of 6–9 where buffering is achieved with bicarbonate equilibria, most trace metals are insoluble and strongly sorbed. Anionic met- als and metalloids (e.g. arsenate, arsenite, chromate, and molyb- date) are more soluble at circumneutral to basic pH because of their negative charge. At high pH values, metals become more sol- uble because of their amphoteric nature. The GARD Guide chart in Fig. 1 shows the range of conditions for mine drainage. The lowest measured pH of acid mine drainage is 3.6 (Nordstrom et al., 2000). Such water is very high in dissolved constituents with a density of about 1.43 g/cm 3 . There are also saline waters that are acid but that partly depends on one’s definition for saline water or salinity. Some mine drainage begins circumneutral but, because of the high ferrous iron content and insufficient alkalinity, becomes acid on oxidation (Kirby and Cravotta, 2005). 1.1.2. Saline drainage Saline waters are defined somewhat differently depending on the chosen literature source and the water type. Salinity is defined http://dx.doi.org/10.1016/j.apgeochem.2015.02.008 0883-2927/Published by Elsevier Ltd. ⇑ Corresponding author. Tel.: +1 303 541 3037. E-mail address: dkn@usgs.gov (D.K. Nordstrom). Applied Geochemistry 57 (2015) 3–16 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem