Rate Controls on the Chemical Weathering of Natural Polymineralic Material

Applied Geochemistry Applied Geochemistry 21 (2006) 377–403 www.elsevier.com/locate/apgeochem Rate controls on the chemical weathering of natural polymineralic material. II. Rate-controlling mechanisms and mineral sources and sinks for element release from four UK mine sites, and implications for comparison of laboratory and field scale weathering studies K.A. Evans a,*, D.C. Watkins a, S.A. Banwart b a Groundwater Protection and Restoration Group, Department of Civil Engineering, University of Sheffield, Mappin St, Sheffield S1 3JD, UK b Camborne School of Mines, Tremough Campus, Treliever Road, Penryn, Cornwall TR10 9EZ, UK Received 9 May 2005; accepted 24 October 2005 Editorial handling by R. Fuge Abstract Predictions of mine-related water pollution are often based on laboratory assays of mine-site material. However, many of the factors that control the rate of element release from a site, such as pH, water–rock ratio, the presence of secondary minerals, particle size, and the relative roles of surface-kinetic and mineral equilibria processes can exhibit considerable variation between small-scale laboratory experiments and large-scale field sites. Monthly monitoring of mine effluent and analysis of natural geological material from four very different mine sites have been used to determine the factors that control the rate of element release and mineral sources and sinks for major elements and for the contaminant metals Zn, Pb, and Cu. The sites are: a coal spoil tip; a limestone-hosted Pb mine, abandoned for the last 200 a; a coal mine; and a slate-hosted Cu mine that was abandoned 150 a ago. Hydrogeological analysis of these sites has been performed to allow field fluxes of elements suitable for comparison with laboratory results to be calculated. Hydrogeological and mineral equilibrium control of element fluxes are common at the field sites, far more so than in laboratory studies. This is attributed to long residence times and low water–rock ratios at the field sites. The high water stor- ativity at many mine sites, and the formation of soluble secondary minerals that can efficiently adsorb metals onto their surfaces provides a large potential source of pollution. This can be released rapidly if conditions change significantly, as in, for example, the case of flooding or disturbance. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction * Corresponding author. Present address: Australian National University, RSES, Building 61, ANU, Mills Road, Canberra Mining activities expose large volumes of fresh ACT 0200, Australia. Fax: +61 2 61250738. rock to atmospheric conditions. Subsequent weath- E-mail address: [email protected] (K.A. Evans). ering and release of rock constituents, especially via 0883-2927/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2005.10.002 378 K.A. Evans et al. / Applied Geochemistry 21 (2006) 377–403 sulphide oxidation, can lead to acidic solutions and oratory results to field situations. Laboratory- elevated concentrations of environmentally undesir- derived mineral dissolution rates are often 2–4 able elements in surrounding waterways (Younger, orders of magnitude faster than those measured 2000; Banks et al., 1997). This global problem is for minerals in the field (e.g. Schnoor, 1990; White exacerbated by the lack of an effective framework et al., 1996). for determination of legal liability, which means A number of factors have been proposed to that resources for remediation of contaminated account for this discrepancy. These include differ- abandoned sites are limited (Younger, 1997). Fortu- ing pH, grain size, temperature, hydrology, cation nately, pre-mining environmental assessments are exchange characteristics, availability of reactants now required in the majority of countries, and for such as O2, degree of physical and chemical heter- this, as well as for cost-effective remediation, it is ogeneity, secondary mineral behaviour, mineral necessary to develop the ability to predict the sever- surface characteristics and proximity to chemical ity and longevity of mining-related contamination saturation with respect to the dissolving minerals, on a site-specific basis. An understanding of the pro- between weathering environments (e.g. Sverdrup cesses affecting weathering is also relevant to cli- and Warfvinge, 1995; Malmstrom et al., 2000; matic modelling, where a better knowledge of White and Brantley, 2003). Algorithms that quan- field-scale mineral dissolution rates is necessary to titatively account for some of these factors have facilitate assessment of relationships between tem- been devised (e.g. Sverdrup and Warfvinge, 1995; perature, precipitation, elevation and weathering Malmstrom et al., 2000). These are based on the rate (e.g. White and Blum, 1995). premise that each of the different parameters that Laboratory assessments are the most conve- affect dissolution rates can be considered sepa- nient method of measuring the contamination rately, and a scaling factor calculated for each. potential of spoil. Detailed studies of the dissolu- The combination of the scaling factors then allows tion of individual mineral phases have determined extrapolation of laboratory-based weathering rates dissolution mechanisms and relationships between to the field. Calculations are based on fundamental dissolution rates and pH, temperature, surface relationships between reaction rate and the physi- and crystallographic characteristics, and rate- cal parameter of interest, and so site-specific cali- determining concentrations of reactants that influ- bration is not required for most parameters. ence kinetic mass action (e.g. .Wieland et al., Thus, the algorithms should prove to be robust 1988; Xie and Walther, 1992; Martello et al., and generally applicable. The algorithm of Malm- 1994; Peiffer and Stubert, 1999; Holmes and Crun- strom et al. (2000) has been demonstrated to be dwell, 2000), but it is difficult to use such work to successful in a study of the weathering of granitoid account for interactions between the components waste rock from the Aitik Mine, Sweden (Malm- of the phases in a polymineralic assemblage. Rudi- strom et al., 2000). mentary acid–base measurements on polymineralic However, application of this type of algorithm materials are popular (e.g. Adam et al., 1997), involves the implicit assumption that the same because of the relative ease and rapidity with mechanism determines the rate of element release which they can be undertaken and interpreted. in the laboratory and in the field, and that mineral Such methods measure the total potential of a sources and sinks play the same role in the two sample for contamination, but do not provide environments. This is not necessarily the case. The information on the rate at which contamination present study investigates rate-determining mecha- is produced, and are thus of limited use (e.g. Jam- nisms and mineral sources and sinks at four UK bor, 2000). Batch and column experiments have mine sites via a year-long mine-water monitoring also been used (van Grinsven and van Riemsdijk, program and bulk and mineralogical analysis of 1992; Stromberg and Banwart, 1999; Banwart samples from the sites. Results are related to those et al., 2002), to measure rates and to distinguish produced by laboratory experiments using results of rate-controlling mechanisms of mineral dissolu- hydrogeological analyses of the sites. Results for tion. Column experiments can be more useful than one site are then compared to column and batch batch experiments because they involve water:rock experiments on material from the site (Evans and ratios and hydrological solute transport processes Banwart, 2006). The implications of results for similar to those found in the field. Regardless of prediction and treatment of mine-water related method, however, it is difficult to extrapolate lab- pollution are discussed. K.A. Evans et al. / Applied Geochemistry 21 (2006) 377–403 379 2. Field sites (Gandy, 2002) with associated publications (Gandy and Evans, 2002; Gandy and Younger, 2003), which Locations of all sites are shown in Fig. 1. The have developed groundwater flow and chemical criteria for field sites were that they should: be models for the site. hydrologically well defined with point dis- charges;have a well-documented mining history; 2.2. Grattendale exhibit commonly observed contaminant characteristics; be representative of the selected type of Limestone hosted Pb/Zn deposits in the valley of mining environment; have available data for rain- Grattendale in the Peak District, Derbyshire, Eng- fall, discharge, topology, and geology, and have a land (Fig. 1b), were mined up to the end of the reasonably simple geological structure. 18th Century. Grattendale is in the White Peak area, which comprises an inlier of Carboniferous 2.1. Quaking Houses limestone surrounded, except to the extreme SW, by younger fluvio-deltaic sandstones (Stevenson The spoil tip at Quaking Houses, near Newcastle- et al., 1982). Mississipi Valley Type (MVT) Pb upon-Tyne, County Durham, England (Fig. 1a), and Zn ores formed when cooling metal-rich fluids comprises spoil excavated during working of the passed sub-horizontally through the limestones Morrison Busty pit between 1922 and 1973. The (Ewbank et al., 1995). Ore minerals include galena, tip overlies sand and clay drift deposits, which them- sphalerite, baryte, and fluorite. Mineralisation was selves overlie Carboniferous interbedded mudstones, controlled by lithology and by pre-existing struc- sandstones and coal seams (Pritchard, 1997). The ture, and resulted in a geometry of near vertical or area of the tip is approximately 35 ha and the height horizontal sheets, known as rakes and flats respec- varies from 4.25 to 11 m. The spoil material is a het- tively, and occasional linear pipes (Ford and Rieuw- erogeneous mix of shale, ash, coal, and coal dust, erts, 1970). Mineralised zones rarely penetrate the with scattered cobbles, sandstone boulders, timber, volcanic horizons, known locally as toadstones, and traces of red burnt shale. The vast majority of and are thus found mainly within higher levels of particles are smaller than 5 cm diameter. Recently the formation (Ford and Rieuwerts, 1970).

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