British Columbia Mine Reclamation Symposium

Beyond ML/ARD : the many faces of neutral mine drainage in the context of mine closure Bright, D. A.; Sandys, N. 2015

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Mine Closure 2015 โ€“ A.B. Fourie, M. Tibbett, L. Sawatsky and D. van Zyl (eds) ยฉ 2015 InfoMine Inc., Canada, 978-0-9917905-9-3 Mine Closure 2015, Vancouver, Canada 1 D.A. Bright  Royal Roads University/Hemmera, Canada N. Sandys  Hemmera, Canada   The ability to predict and manage the interactions between mine wastes and water, with the associated implications for aquatic ecosystem impairment, has evolved immensely over the last two decades and continues to rapidly change. The major focus so far has been on drainage chemistry from sulphidic ores and wastes, since oxidation of pyrite, pyrrhotite and similar sulphide minerals has been the root cause of serious water-quality issues at many mine sites worldwide. However, there are also many examples of compromised surface water and groundwater quality as a result of geochemical processes involving neutral to alkaline dissolution and aqueous transport. Neutral mine drainage (NMD) is often taken to mean down-gradient runoff from sulphidic source materials that are undergoing oxidation following the subsequent reaction with neutralising minerals such as carbonates; this results in water with circumneutral pH, high hardness and high sulphate levels. There is no standardised definition of NMD, and this term is increasingly used by researchers and managers to describe a variety of geochemical processes and issues. The predictive models and tools for managing the environmental effects of NMD are more poorly developed than for acidic rock drainage (ARD), especially since predictions depend more on complex interpretations of mineralogy and geochemistry. The flux of various trace major elements and materials from mine spoils to the hydrosphere is generally far greater for ARD than NMD; however, the potential for ecosystem-scale impacts from NMD is expected to increase in proportion with an increase in the spatial scale of mining projects in general. This paper provides a simplified classification of the various types of NMD that have been encountered as a starting point for developing new predictive and management approaches for NMD. Several of the tools for preventing NMD-related environmental impacts, applied during mine development and closure planning, are very similar to the tools that have been developed for ARD; however, the tools and approaches for NMD place greater emphasis on management actions at the watershed scale and on interactions between groundwater and surface water. The effects of mine wastes on water quality are of paramount concern in approving new mines and managing ongoing operations and mine closures, since the influence of mine drainage on vegetation, fish, wildlife and humans can be very large relative to the actual mine footprint. While the spatial scale of influence of mine drainage can be large, past experience has shown that the temporal scale of influence can also be large relative to the period over which direct economic benefits of mining are accrued. From the 1970s through the first decade of the new millennium, it has become apparent that metal leaching and acidic rock drainage (ML/ARD) associated with oxidation of sulphide minerals is among the most environmentally damaging issues associated with contemporary mining operations (Price, 2005). In recognition of the environmental, societal and financial costs of ML/ARD, scientistsโ€™ work over the last three decades has led to significant progress in the knowledge about factors that enhance or inhibit acidic drainage, diagnostic tests and interpretations for prediction and management approaches to reduce the environmental impacts. ML/ARD is caused by oxidation of sulphide minerals such as pyrite.  ๐น๐‘’๐‘†2  +  12 ๐ป2๐‘‚ +  154  ๐‘‚2  โ†”  ๐น๐‘’3+  +  2๐‘†๐‘‚42โˆ’ +  ๐ป+     (1) The acidity associated with pyritic mineral oxidation, per the simplified equation (1), is the basis of the term โ€œacidic rock drainageโ€ (ARD). Additional acidification results from the further oxidation of pyrites by ferric Beyond ML/ARD: the many faces of neutral mine drainage in the context of mine closure D.A. Bright and N. Sandys 2 Mine Closure 2015, Vancouver, Canada iron (equation 2) and the secondary precipitation of iron oxides along water flow paths (equation 3) (Price, 2009).  ๐น๐‘’๐‘†2  +  12 ๐ป2๐‘‚ +  14๐น๐‘’3+  +  154  ๐‘‚2  โ†”  15๐น๐‘’3+  + 2๐‘†๐‘‚42โˆ’ + ๐ป+ (2)  ๐น๐‘’2+  + 14 ๐‘‚2  +  52 ๐ป2๐‘‚ โ†”  ๐น๐‘’(๐‘‚๐ป)3(๐‘ )  +  2๐ป+ (3) The tendency of various metals to leach from minerals at higher rates under low pH conditions (metal leaching, or ML) is an obvious concern for ecological and human health risks associated with mine-affected surface waters and groundwater. While ML/ARD issues are typically at the forefront of management concerns for proposed or operating mines and mine closure, scientists have long recognised that other chemical weathering and dissolution reactions can be an important influence on water quality, based on both a theoretical understanding of geochemical processes and real world examples. Nicholsonโ€™s (2004) review of water-quality issues associated with neutral mine drainage (NMD) included a working definition of NMD that involves two factors: (i) runoff from sulphidic source materials that are undergoing oxidation following the subsequent reaction with neutralising minerals such as carbonates, which results in water with circumneutral pH, high hardness and high sulphate levels; and (ii) rapid and accelerated chemical weathering of mineral types other than sulphides. Nicholsonโ€™s review focusses on the second of these.  An extensive search of peer-reviewed and other published literature for the terms โ€œneutral mine drainageโ€ or โ€œneutral drainageโ€ reveals that research efforts to the present time have overwhelmingly focussed on mine drainage chemistry from oxidation of sulphidic deposits and the associated secondary and tertiary geochemical reactions following neutralisation, either in situ or along the water flow path. Nicholsonโ€™s (2004) review on NMD was intended above all as โ€œa starting point for discussion and awareness of an environmental issue that has had less attention to date than acidic drainage, and one that may become important at operations without acid drainage issues or where acid drainage has been mitigated.โ€ In the last decade, however, this issue has not received much attention. The theoretical, conceptual and practical approaches for detecting, predicting and managing NMD are arguably already available; these approaches are discussed in recent guidance documents such as Prediction Manual for Drainage Chemistry from Sulphidic Geological Materials (Price, 2009) and Global Acid Rock Drainage Guide (INAP, 2009). The names of these documents suggest, incorrectly, that they focus exclusively on sulphidic mineral dissolution and drainage as opposed to NMD associated with chemical weathering of other mineral types. Furthermore, the very success of recent international efforts to refine pragmatic approaches and tools for predicting and managing ML/ARD may be blinding some practitioners at times to potential environmental issues associated with NMD, as discussed below. The objective of this paper is to catalyse discussions among scientists and managers about NMD based on broader definitions as proposed by Nicholson (2004). We briefly discuss types of NMD, adequacy of the tools currently available to assess mine drainage chemistry and water quality and especially the need to combine scientific thinking about mine drainage with research on chemical weathering in general. This field offers a larger focus on pedogenesis and evolution of the geosphere at many scales: from alterations at the mineral surface to local- and regional-scale weathering, catchment-scale erosion and solute transport, and continental denudation. NMD includes two major categories of mineral types and in situ weathering reactions. The first category is associated with sulphide mineral oxidation accompanied by subsequent neutralisation reactions such that the drainage is circumneutral or โ€œnear neutralโ€ as defined by Price (2009) (e.g., pH in a range of 6 to 8). The second category is based on accelerated dissolution at the mineral face of any nonsulphide mineral. The influence of weather based on either of these categories become increasingly important at large geographic scales, where there is significant mass of geological materials of interest relative to the size of both Case Studies Mine Closure 2015, Vancouver, Canada 3 catchments that support ecologically productive surface waters and of the recharge areas that support local and regional aquifers or interact with surface flows.  A generalised discussion of mine drainage chemistry and trace element dissolution potential for all economically viable mineral deposits is a daunting task. The economic geology โ€œcheckerboardโ€ classification by Dill (2010) or similar classifications can assist us by providing information on the lithological regimes that may host economically relevant amounts of specific elements. The lithological regimes used by Dill include the following: ๏‚ท igneous rocks (ultrabasic, basic, intermediate, felsic, alkaline and carbonatites); ๏‚ท structures (pipes, faults); ๏‚ท sedimentary rocks (duricrusts-regolith-veinlike deposits, coarse-grained clastics, fine-grained clastics and massive rocks, limestones, evaporates, special sedimentary rocks); and ๏‚ท special facies, including carbon bearing hosts (coal-oil shales-hydrocarbons).  What such schemes point out is that many elements of interest for mining occur in ore bodies and minerals that are not predominantly sulphidic. Chromite (FeCr2O4), for example, is the predominant mineral source for chromium, occurring especially in coarse-grained clastic rocks. Recoverable rare earth element (REE) deposits in Canada and elsewhere occur in bastnasite [(Ce,La)(CO3)F], monazite [(Ce,La,Nd,Th)PO4] and ionic clays (Parliament of Canadian, 2014). Perhaps a more simple mineral taxonomy for framing discussions about NMD is the older classification of the more than 4,700 known minerals into the major groupings shown in Figures 1 and 2 (Gaines et al., 1997). The Dana classification was developed in 1848 by Professor James Dana of Yale University and includes nine basic classes: native or pure elements, sulphides, oxides/hydroxides, halides, carbonates/nitrates/borates, sulphates/chromates/selenates, phosphates/arsenates/vanadates, organic minerals and silicates. Pure elements are not included in Figures 1 and 2, since water soluble forms at concentrations suitable for extraction are rare. It is important to understand the potential for dissolution of various trace elements from minerals in the ore body of interest both prior to and following excavation and extraction. The mineralised zone almost invariably contains complex mixtures of other minerals (including gangue minerals) that can interact in highly complex ways to influence chemical weathering rates and secondary interactions. It is challenging, therefore, to predict the drainage chemistry of large bulk waste deposits and physically disturbed areas from first principles. In consideration of NMD, accessory sulphide minerals often occur in association with nonsulphide minerals of economic interest and can contribute disproportionately to aqueous concentrations of various metals and metalloids such as arsenic. Mineralised zones and mine wastes are also typically highly heterogeneous at both microscopic and macroscopic scales, which leads to variations in reactivity and dissolution potential that are challenging to fully appreciate. The level of effort applied to detailed mineralogical analysis of ore bodies, host rock, waste rock and tailings, therefore, is generally commensurate with the accuracy of mine drainage predictions, as discussed in Section 3 of this paper. As equations (1) and (2) illustrate, sulphate is a major product of sulphide mineral oxidation; it substantially remains in solution after a change to circumneutral pH in affected waters (e.g., following reaction with carbonate minerals). Sulphate is often the most effective tracer in water of sulphide mineral oxidation, even in systems and mine wastes that are non-acid generating.    Beyond ML/ARD: the many faces of neutral mine drainage in the context of mine closure D.A. Bright and N. Sandys 4 Mine Closure 2015, Vancouver, Canada  Beyond immediate toxicological concerns for human health (based on drinking water ingestion) and aquatic life in affected areas, elevated sulphate in freshwater systems can profoundly alter wetland and sediment diagenetic reactions. In particular, organic matter mineralisation in freshwater sediments occurs predominantly via methanogenesis, as opposed to dissimilatory sulphate reduction, since natural sulphate levels are limiting (Herlihy and Mills, 1985; Brandl et al., 1990; Bright et al., 1994). Oxides/hydroxidesOld Dana Classification New Dana Classification(Gaines et al., 1997)Common FormsSulfides Sulfides, inc. selenides and tellurides pyrite (FeS ),pyrrhotite (Fe S )27 8SulfosaltsType I NMD:  U neutralization byproduct dissolutionnprecipated sulfide oxidation products  and Type II NMD:  Accelerated weathering of all other non-sulfide mineralsSimple oxidesOxides containing uranium and thoriumForms containing hydroxylMultiple oxidesMultiple oxides with Nb, Ta, and Timagnetite ( ),hematite ( )gibbsite ( )aFe OAl(OH)3 43 Fe O2 3xi es/ y r xi esHalides Anhydrous and hydrated halidesOxyhalides/hydroxyhalidesHalide complexes, alumino-flouridesCompound halideshalite (NaCl)flourite( )CaF2Carbonates/nitrates/borates Acid carbonatesAnhydrous carbonatesHydrated carbonatesCarbonates - hydroxyl or halogencalcite ( )CaCO3Compound carbonatesSimple nitratesNitrates - hydroxyl or halogenCompound nitratesIodates - anydrous and hydratedIodates - hydroxyl or halogenCompound iodatesBorates - anydrousAnydrous borates with hydroxyl or halogenHydrated borates containing hydroxyl or halogenCompound boratesCase Studies Mine Closure 2015, Vancouver, Canada 5  Methylation of mercury in lentic and lotic sediments is expected to occur under anoxic conditions and especially in association with microbial sulphate reduction (Gilmour et al., 1992). Thus, mining-related inputs of sulphate into freshwater can significantly increase the rates of mercury methylation and biomagnification potential. Even in the absence of successive reactions, trace element sulphide mineral oxidation can result in enhanced dissolution of various metals/metalloids, the continued presence of which in the dissolved state is insensitive to further neutralising reactions. For example, oxidation of arsenopyrite (FeAsS) results in production of arsenate oxyanions (equation 4), which will tend to remain in solution across a relatively large pH range in the absence of a strong sorptive phase such as precipitated iron oxyhydroxides.  ๐น๐‘’๐ด๐‘ ๐‘† + 72๐‘‚2 + 4๐ป2๐‘‚ โ†’ ๐ป๐ด๐‘ ๐‘‚42โˆ’ + ๐น๐‘’(๐‘‚๐ป)3 +  ๐‘†๐‘‚42โˆ’ +  4๐ป+ (4) Thus, the aqueous environmental concentrations of arsenate in association with mine drainage are predominantly controlled by two factors: (i) the rate of arsenopyrite oxidation independent of the amount of co-occurring neutralisation potential (and rates of neutralisation); and (ii) the availability of iron oxyhydroxide precipitates (e.g., resulting from oxidation and precipitation of ferrous iron, as shown in Type II NMD:  (continued)Accelerated weathering of all other non-sulfide mineralsOld Dana Classification New Dana Classification(Gaines et al., 1997)Common FormsPhospates/arsenates/vanadatesapatite ( )Ca (F,Cl,OH) (PO ) 4 35Anydrous acid phosphatesAnydrous phosphatesHydrated acid phosphatesHydrated phosphatesAnydrous phosphates containing hydroxyl or halogenHydrated phosphates containing hydroxyl or halogenCompound phosphatesAntimonatesAcid and normal antimonites, arsenites, phosphitesBasic or halogen-containing antimonites, arsenites, phosphitesVanadium oxysaltsAnydrous molybdates and tungstatesBasic and hydrated molybdates and tungstatesOrganic mineralsSulfates/chromates/selenates Anydrous acid and sulfatesHydrated acid and sulfatesAnhydrous sulfates containing  hydroxyl or halogengypsum ( )CaSO H 04 2Hydrated sulfates containing hydroxyl or halogenCompound sulfatesSelenates and telluratesSelenites - tellurites - sulfitesAnydrous chromatesCompound chromatesSalts of organic acids and hydrocarbonsSilicates Numerous: see Gaines et al., 1997Beyond ML/ARD: the many faces of neutral mine drainage in the context of mine closure D.A. Bright and N. Sandys 6 Mine Closure 2015, Vancouver, Canada equation 3) as a potential strong sorptive phase. A lack of appreciation of the expected fate of arsenicals under near-neutral conditions is behind many of the interpretative errors we have encountered in our reviews of applied mine site hydrogeochemical interpretations.  As discussed by Price (2009), antimony, arsenic, molybdenum, selenium and sulphur tend to occur as oxyanions in oxidised environments. These oxyanions typically remain in solution at circumneutral pH in the absence of a strong sorptive phase. Price (2009) also points out that many trace elements that may be solubilised during sulphide oxidation are relatively soluble under near-neutral pH; these include cobalt, molybdenum, nickel and zinc. Demers et al. (2013) provide a compelling example of the ongoing aqueous mobilisation of nickel from older waste rock deposits at the Tio mine site in eastern Quebec, Canada. Hemo-ilmenite has been mined here for over 50 years, and waste rock deposits contain mostly plagioclase (labradorite), ilmenite, hematite and some pyrite, micas, spinel and chlorite. Drainage water from waste rock deposits has remained near neutral, while nickel concentrations have increased in recent years, and the discharge limit of 0.5 mg/L in effluent has been exceeded with increasing frequency (albeit still sporadically). Mineralogical analysis of the waste rock demonstrated that the major source of nickel in the waste rock is millerite (NiS), present as inclusions in grains of pyrite. Temporal trends in effluent concentrations of nickel, under near-neutral drainage conditions, were determined to be associated with secondary sorption. In particular, an increasing trend in effluent-phase dissolved nickel concentrations was determined to be the result of a trend towards saturation of sorption sites over time, following 25 years in which nickel concentrations in waste rock drainage were not identified as an issue of concern. Type I NMD is defined here as runoff from sulphidic source materials that are undergoing oxidation after subsequent reaction with neutralising minerals such as carbonates, resulting in water with circumneutral pH, high hardness and high sulphate levels. Equations (5) and (6) provide simplified reactions for neutralisation of acidity arising from sulphide mineral oxidation through reaction with calcium carbonate.  2๐ป+  +  ๐ถ๐‘Ž๐ถ๐‘‚3  โ†”  ๐ถ๐‘Ž2+  +  ๐ป2๐ถ๐‘‚3 (5)  2๐ป+  +  ๐ถ๐‘Ž๐ถ๐‘‚3  โ†”  ๐ถ๐‘Ž2+  +  ๐ถ๐‘‚2(๐‘Ž๐‘ž)  + ๐ป2๐‘‚ (6) Type I NMD will generally occur (as opposed to ML/ARD) when alkalinity released from minerals such as carbonates at or down-gradient along an aqueous flow path occurs at a sufficient rate to neutralise the acid generated from sulphide mineral oxidation. Carbonates, hydroxides and silicates may exhibit neutralising potential to varying degrees. In general, neutralisation reactions result in the dissolution of base cations such as Ca2+ and Mg2+ at the site of neutralisation, along with co-occurring elements such as iron and manganese.  Especially for calcium and magnesium in carbonate minerals, rates of dissolution or precipitation are very rapid relative to the surface or groundwater velocities or other types of reactions; therefore, dissolution/precipitation reactions are said to be controlled thermodynamically (or to be equilibrium-controlled) as opposed to kinetically (Price, 2009). Rates of dissolution or precipitation are highly sensitive to solution pH or dissolved CO2 pressure (pCO2). Aquatic animals and plants are generally insensitive to increased water hardness associated with higher dissolved concentrations of base cations, and it is well known that increased hardness is associated with a decrease in the bioavailability and toxicity to aquatic life of metals such as cadmium, copper, lead and zinc. Under some circumstances, however, secondary precipitation of carbonates from NMD within the aquatic receiving environment (i.e., partial-to-complete concretion) can have significant results: reductions in the permeability of and flow rates through aquatic substrates such as spawning gravels, profound alterations in benthic habitat, smothering and loss of benthic invertebrates and occlusion of respiratory epithelia of substrate associated biota. Calcite deposition has been studied largely as a problem for industrial processes and in the context of natural precipitation reactions associated with geothermal springs or cave deposits, especially in karst settings. As reported in Windward Environmental et al. (2014), calcite precipitation has been observed in the Elk River watershed, British Columbia, in reaches influenced by coal mining discharge. Calcite deposition has also been Case Studies Mine Closure 2015, Vancouver, Canada 7 observed in association with coal mining within the McLeod River watershed on the eastern slopes of the Rocky Mountains in Alberta. Calcite deposition is driven by calcium supersaturation in aqueous solutions, especially in high ionic strength groundwater emanating from coal waste deposits, when carbonic acid concentrations decline and pH increases as result of a reduction in pCO2. A reduction in pCO2 may occur especially as a result of increased air-water interactions and off-gassing (e.g., when groundwater emerges at the ground surface) and as result of photosynthesis within the aquatic receiving environment via the localised draw-down through uptake and fixation of CO2 by primary producers during photosynthesis. When calcium becomes oversaturated, it tends to precipitate out onto available solid surfaces as various carbonate minerals, including calcite. As shown in Table 1, a large number of different minerals may be present in mineralised areas and waste deposits at a given site. Our primary interest is in primary and secondary dissolution reactions of nonsulphide minerals in association with natural or accelerated weathering rates.  Price (2009) defines weathering as โ€œthe process by which geological materials are altered on exposure to atmospheric conditions and agents.โ€ Type II NMD focusses predominantly on chemical weathering rates and consequences โ€” those based on the dissolution and precipitation potential of substances and sorption/desorption kinetics in their existing state (e.g., based on anion and cation exchange); hydration; hydrolysis; oxidation/reduction; and dissolution and precipitation potential of minerals based on chemical reactions (hydration, hydrolysis and oxidation/reduction). Physical weathering and biologically mediated weathering may play a role in mine drainage chemistry, but there is insufficient knowledge about these to develop pragmatic predictive models, and an improved understanding of chemical weathering is likely to be the priority over the foreseeable future. Nicholson (2004) notes that several trace elements can exist in a substantially dissolved form under near-neutral pH conditions; these elements include antimony, arsenic, cadmium, chromium, cobalt, copper, iron, manganese, mercury, molybdenum, nickel, selenium, sulphate, uranium and zinc. Studies on dissolution rates and water-borne concentrations of REE (Medas et al., 2013a, 2013b) under near-neutral conditions are still in their infancy; however, this is likely to be a topic of increasing interest over the next decade. According to Medas et al. (2013a), the solubility of REE generally decreases at higher pH; however, solubility can be enhanced through REE complexation with carbonate ligands or humic substances. Complexation with phosphate ligands, however, decreases solubility.  Selenium flux to the Elk River watershed in British Columbia, Canada, provides a good example of Type II NMD (Lussier et al., 2003). Since the mid-1990s, it has become apparent that selenium concentrations in coal mine-affected areas of the Elk River watershed have been increasing. Coal has been extensively mined in the Elk Valley since 1897, and it is currently mined from five large open pits. Though sulphides are recognised as important Se-bearing minerals, Lussier et al. (2003) did not observe a significant correlation between selenium and sulphide concentrations in representative samples collected from the pit walls of the five coal mines. Detailed studies on 16 samples that were generally representative of the Mist Mountain Formation overall revealed that from 5% to 21.3% of the total Se was water soluble, 60% to 84% was associated with sulphides and organic matter and 5.9% to 24.7% is in the silicate matrix. The extent to which watershed-level effects was associated with direct dissolution (i.e., Type II NMD) rather than sulphide oxidation (i.e., Type I NMD) in various source areas remains uncertain. Selenium water-quality issues associated with coal deposits occur worldwide. Ziemkiewicz et al. (2011) provide results for Se dissolution in coal mine drainage in West Virginia, U.S.A. Water-quality issues associated with selenium are more commonly associated with discharge from low-sulphur waste deposits originating from southern West Virginia coal seams than from pyrite-rich seams found in the northern portion of the state. While the study results were not definitive, they suggested that selenium in the West Virginia coal deposits exists separately from pyrite, its dissolution rate is independent of pyrite reactions and it exists primarily as Se0 as opposed to selenide impurities in sulphide minerals. Beyond ML/ARD: the many faces of neutral mine drainage in the context of mine closure D.A. Bright and N. Sandys 8 Mine Closure 2015, Vancouver, Canada Advances in our understanding of ML/ARD have resulted in the development and wide-scale use of simplified diagnostic tests, including acid-base accounting (static tests) and kinetic tests such as humidity cell tests. Excellent descriptions of mine drainage prediction methods are provided by Price (2009), INAP (2009), Morin and Hutt (2001) and others. Figure 3 provides a simplified schematic of mine drainage prediction methods, which is similar to the โ€œwheel approachโ€ devised by Morin and Hutt (2001).  The nine major approaches illustrated in Figure 3, which each involve multiple tests and data needs, are not mutually exclusive. In fact, all nine approaches are needed to varying degrees to develop an adequate understanding of the influence of mine drainage on the surrounding environment.  Some of these tools have been developed to a high level of detail, such that their application and interpretations have become relatively prescriptive. For example, standardised methods exist for acid-base accounting (ABA) methods as do specific variations to account especially for the characteristics of sulphur in tested geological samples. Conversely, preferred approaches for other important tools such as mineralogical analysis or geochemical modelling remain relatively abstract and subject to substantial variations between researchers. The tools currently used in predicting mine drainage chemistry are too reliant on static laboratory tests on limited size geological samples for practical application. Often, the effort applied to the other lines of evidence is limited and subject to outcomes from ABA analysis. For example, the time and cost invested in humidity cell tests and field kinetic testing is often quite limited, in our experience, if there is no appreciable evidence from static tests of potential acid generating (PAG) materials. Such a simplification of the investigative process may be appropriate in cases where the only truly plausible environmental issues arising from mine drainage chemistry will be in association with ML/ARD; however, over-reliance on static test results will make accurate predictions of environmental issues associated with NMD highly likely to fail. MineDrainageChemistry PredictionBulk Chemistry:OreHost RocksWaste RockTailingsDetailed MineralogicalAnalysisLaboratory-basedstatic tests:shake-flask tests/leachate tests,acid-base accounting,NAG testLaboratorykinetic tests(e.g. column tests; humidity cell tests)Site-basedkinetic testsGeochemicalmodelling at sourceCatchment scalehydrogeological modellingOn-site monitoringand trend analysisSite conditions:meteorology &climate,hydrology,hydrogeologyCase Studies Mine Closure 2015, Vancouver, Canada 9 The relative value of the nine different approaches for developing accurate predictions of mine drainage chemistry and its associated environmental impacts is different for geological materials prone to NMD as opposed to ARD. In particular, NMD predictions require a greater level of effort directed especially to detailed mineralogical analysis, kinetic testing, geochemical modelling (including reactive transport modelling at catchment scales) and validation of predictions through follow-up monitoring. The scale of mining projects worldwide has increased with each passing decade. Contemporary studies of chemical weathering processes and rates are increasingly relevant as a core theme in geomorphological research and soil forming processes (pedogenesis), especially for dissolution and aqueous flux associated with NMD. Such research has largely advanced independently of geochemical evaluations of mining resources and wastes; since the early 1990s, it has focussed on alterations at mineral surfaces, weathering in pedons, catchment-scale erosion and continental denudation (see Schott et al., 2012; Moore, 2008; Phillips, 2005; White and Brantley, 1995). In particular, many of the geochemical models used in mine drainage chemistry prediction assume dynamic equilibrium and, in more complex cases, consider reaction kinetics. A number of recent studies of chemical weathering in the context of pedogenesis, geomorphic change, large-scale biogeochemical cycles and climate change influences have used models based on chemical dissolution and precipitation kinetics for various minerals under either far-from-equilibrium or near-equilibrium conditions. Schott et al. (2012) discuss recent advances in the use of mechanistic numerical models of weathering to predict biogeochemical changes over scales of a kilometre to thousands of kilometres. Such numerical models generally rely on the Surface Complexation approach (SC) combined with Transition State Theory (TST) to describe mineral dissolution kinetics over wide ranges of solution composition. We are not aware that such models have been applied specifically to mining-related problems; however, such approaches may be particularly promising for some of the more intractable issues associated with contemporary mines, such as selenium flux into large watersheds from large-scale coal mining. Neutral mine drainage can arise under a variety of conditions and can result in aquatic environmental impacts as a result of the dissolution, entrainment in mine drainage and persistence in surface runoff into water bodies that support aquatic life. Depending on the minerals present, chemical accessibility, solution chemistry of meteoric water introduced to the geological materials, sorption capacity along the flow path and other factors, NMD can have several results: elevated dissolved-phase concentrations of sulphate, base cations, metal/metalloid oxyanions and metal ions, and precipitation of calcite. Issues associated with NMD arising from nonsulphide minerals have been given little attention in the context of mining. However, relevant information on mineral dissolution rates has emerged based on chemical weathering studies driven by an interest in pedogenesis, geomorphic change, large-scale biogeochemical cycles and climate change implications. The mechanistic numeric weathering models that have been refined through these interests may be of value in a mine-management context. Overall, it is our hope that this paper advances the objectives of Nicholson (2004) in catalysing discussions about NMD. This paper was shaped by early discussions with Bill Price, to whom we are grateful. Beyond ML/ARD: the many faces of neutral mine drainage in the context of mine closure D.A. Bright and N. Sandys 10 Mine Closure 2015, Vancouver, Canada Brandl, H., Hanselmann, K.W., and Bachenhof, R. (1990) In situ stimulation of bacterial sulfate reduction in sulfate-limited freshwater sediments, FEMS Microbiology Ecology, Vol. 74, pp. 21โ€“32. Bright, D.A., Coedy, B., Dushenko W.T. and Reimer K.J. (1994) Arsenic transport in a watershed receiving gold mine effluent near Yellowknife, Northwest Territories, Canada, Science of the Total Environment, Vol. 155, pp. 237โ€“252. Clow, D.W. and Drever, J.I.  (1996). Weathering rates as a function of flow through an alpine soil. Chemical Geology. Vol. 132, pp. 131โ€“141. Demers, I., Molson, J., Bussiรจre. B. and Laflamme, D. (2013) Numerical modeling of contaminated neutral drainage from a waste-rock field test cell, Applied Geochemistry, Vol. 33, pp. 346โ€“356. Dill, H.G. (2010). The โ€œchessboardโ€ classification scheme of mineral deposits: Mineralogy and geology fro aluminium to zirconium. Earth Science Reviews, Vol. 100, pp. 1โ€“420. Gaines, R.V., Skinner, H.C., Foord, E.E., Mason B. and Rosenzwieg, A.  (1997) Danaโ€™s New Mineralogy: The system of mineralogy of James Dwight Dana and Edward Salisbury Dana, John Wiley and Sons Inc., New York. Gilmour, C.C., Henry, E.A. and Mitchell, R. (1992) Sulfate stimulation of mercury methylation in freshwater sediments, Environmental Science and Technology, Vol. 26(11), pp. 2281โ€“2287. Herlihy, A.T. and Mills, A.L. (1985) Sulfate reduction in freshwater sediments receiving acid mine drainage, Applied and Environmental Microbiology, Vol. 49(1), pp. 179โ€“186. International Network for Acid Prevention (INAP) (2009) Global acid rock drainage guide (GARDGuide), International Network for Acid Prevention, viewed 12 May 2015, Lussier, C., Veiga V. and Baldwin, S. (2003) The geochemistry of selenium associated with coal waste in the Elk River Valley, Canada, Environmental Geology, Vol. 44, pp. 905โ€“913. Medas, D., Cidu, R., de Giudici, G. and Podda, F. (2013a) Geochemistry of rare earth elements in water and solid materials at abandoned mines in SW Sardinia (Italy), Journal of Geochemical Exploration, Vol. 133, pp. 149โ€“159. Medas, D., Cidu, R., de Giudici, G. and Podda, F. (2013b) Geochemical behaviour of rare earth elements in mining environments under non-acidic conditions, Procedia Earth and Planetary Sciences, Vol. 7, pp. 578โ€“581. Moore, J. (2008) Biogeochemistry of granitic weathering, PhD dissertation, Pennsylvania State University, State College, PA. Morin, K. and Hutt, N.H. (2001) Environmental geochemistry of minesite drainage: practical theory and case studies, MDAG Publishing, Vancouver, Canada, 19 pp. Nicholson, R.V. (2004) Review of water quality issues in neutral pH drainage: examples and emerging priorities for the industry in Canada, MEND Report 10.1, Mine Environment Neutral Drainage (MEND), viewed 12 May 2015, Parliament of Canada (2014) The rare earth elements industry in Canada โ€“ summary of evidence, Canadian House of Commons Standing Committee on Natural Resources, 6669744& Language=E&Mode=1&Parl=41&Ses=2 ). Phillips, J.D. (2005) Weathering instability and landscape evolution, Geomorphology, Vol. 67, pp. 255โ€“272. Price, W.A. (2009) Prediction manual for drainage chemistry from sulphidic geological materials, MEND Report 1.20.1, Mine Environment Neutral Drainage (MEND), viewed 12 May 2015, MEND Prediction Manual-Jan05.pdf, 579 pp. Price, W.A. (2005) List of potential information requirements in metal leaching and acidic rock drainage assessment and mitigation work, MEND Report 5.1E, Mine Environment Neutral Drainage (MEND), viewed 12 May 2015, http://pebblescience. org/pdfs/MEND_5_10E_Price_%20Final_Report.pdf.  Schott, J. Oelkers, E.H., Benezeth, P., Godderis, Y. and Francois, L. (2012) Can accurate kinetic laws be created to describe chemical weathering? Comptes Rendu Geoscience, Vol. 344, pp. 568โ€“585. White, A.F. and Brantley, S.F. (eds) (1995) Chemical weathering rates of silicate minerals, Reviews in Mineralogy, Vol. 31.  Windward Environmental, Minnow Environmental Inc. and CH2M Hill Ltd. (2014) Elk River Watershed and Lake Koocanusa, British Columbia, Environmental Synthesis Report, 2014, Report to Teck Coal Ltd., Sparwood, BC, gov/DownloadAsset?assetId=52D09453DEB446228DF1838D65F2FFD5&filename=elk_river_aquatic_env_synthesis_report_oct_2014.pdf, 223 pp. Ziemkiewicz, P.F., Oโ€™Neal, M. and Lovett, R.J. (2011) Selenium leaching kinetics and in situ control, Mine Water Environment, Vol. 30, pp.141โ€“150.  


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