ORIGINAL ARTICLE Sinks of iron and manganese in underground coal mine workings Natalie A. S. Kruse Æ Paul L. Younger Received: 18 February 2008 / Accepted: 7 July 2008 / Published online: 22 July 2008 Ó Springer-Verlag 2008 Abstract The sources and sinks of manganese in under- ground coal mine workings are poorly understood compared to those of iron. The geochemical system in the secondary egress drift of Caphouse Colliery near Horbury, UK, is an ideal system for studying these processes. Five locations along the drift and four secondary inflows to the drift were sampled 24 times through the year commencing May 2005. During the sampling period, the pH in the main channel varied from 6.73 to 7.93 and increased along the flow path. The secondary inflows to the drift from the strata were of higher alkalinity (mean = 385 mg/L as CaCO 3 ) than the main flow (mean = 330 mg/L as CaCO 3 ); the affects of mixing between the less alkaline main channel and the more alkaline secondary inflows and of carbon dioxide exsolution are evident in the form of carbonate and hydroxide precipitates. SEM and XRD analysis of precip- itates collected from the drift confirm the presence of calcium and manganese carbonates and ferric hydroxide. PHREEQC speciation and solubility modelling confirms supersaturation of the water in the main channel with respect to ferric oxy-hydroxides; iron, manganese, mag- nesium and calcium carbonates; and manganese oxides. Keywords Coal Á Mining Á Manganese Á Iron Á Water Á UK Introduction Mine closures continue to create new pollutant discharges; characterization of the source of mine water is key in the approximation of longevity of pollution (Younger 1997). Since many mines are driven either below natural groundwater level (Adams and Younger 2001) or in incompetent strata (Crowell 2001) and may have mine gas hazards in the absence of artificial ventilation (Hall 2007), opportunities for underground characterization are limited. Iron is commonly found in mine waters and is usually taken to be a product of weathering of iron sulphides. Pyrite (FeS 2 ) weathering is seen as the most common source of iron in many mine systems, especially from abandoned coal mines, and the series of weathering reac- tions have been studied extensively (e.g. Wiersma and Rimstidt 1984; McKibben and Barnes 1986; Rimstidt and Newcomb 1993; Williamson and Rimstidt 1994; Holmes and Crundwell 2000). Sinks of manganese in natural aquatic systems are less well defined than sources of iron. Iron oxy-hydroxide precipitates are a common sink of iron, although the pre- cipitation reaction releases acidity. Efflorescent ferrous/ ferric hydroxysulphate salts are less-well studied, but can be significant sinks for metals including iron, especially in underground mines where there is insufficient water to transport the products of mineral weathering away from the mineral surface (Younger et al. 2002), or where mine water can evaporate to dryness. Black substrates associated with mine waters are gen- erally attributed to manganese-rich precipitates, although some more crystalline ferric hydroxide salts are also black. Oxidation and hydrolysis of manganese gives rise to manganese oxides and hydroxides (e.g. pyrolusite and pyrochroite). The appropriate geochemical conditions can Electronic supplementary material The online version of this article (doi:10.1007/s00254-008-1478-7) contains supplementary material, which is available to authorized users. N. A. S. Kruse (&) Á P. L. Younger Hydrogeochemical Engineering Research and Outreach, Sir Joseph Swan Institute for Energy Research, Newcastle University, Newcastle upon Tyne NE1 7RU, UK e-mail: natalie.kruse@ncl.ac.uk 123 Environ Geol (2009) 57:1893–1899 DOI 10.1007/s00254-008-1478-7