A parametric study of the efficiency of cover systems as oxygen barriers

T. Pabst

Dans les comptes rendus d’articles de la conférence: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: ENERGY RESOURCES - I Mining and Tailings

ABSTRACT: The generation of acid mine drainage (AMD) remains a major environmental challenge for the mining industry. Thepreferable approach for effectively limiting the environmental impact of AMD involves controlling the generation of acidthrough the use of preventative measures that inhibit sulphide oxidation reactions at the source. The goal of this projectwas to assess the behaviour of tailings-cover systems under various conditions and evaluate their efficiencyfor two acid-generating tailings sites, located in Quebec (Canada). Large column tests have been set up to evaluate thehydrogeological response of the specific covers placed on the reactive tailings, following wetting and drying cycles. Thewater content, suction,and oxygen concentrations were monitored over time. Experimental data were used to calibrateand validate numerical models constructed with Vadose/W (GeoSlope Int.). Additional simulations were then conductedusing field conditions to evaluate the effect of various influence factors, such as the depth of the water table, climaticconditions or the thickness of the cover. Alternative cover configurations were also tested. Special attentionwas paid toincluding waste rock from a nearby mine in the cover design. The results presented in this paper focus on unsaturatedwater flow in the tailings-covers system and on the diffusive oxygen flux to the reactive tailings.R†SUM†La g‡n‡ration de drainage minier acide (DMA) demeure un d‡fi majeur pour l…industrie mini•re. L…approche la plusefficace pour limiter la production de DMA consiste g‡n‡ralement — employer des m‡thodes pr‡ventives afin de limiterl…oxydation des sulfures —la source.Cette ‡tude visait— ‡valuer le comportement et l…efficacit‡ de plusieurs types derecouvrement sous diverses conditions, pour deux sites miniers g‡n‡rateurs de DMA situ‡s au Qu‡bec (Canada). Desessais en (grandes) colonnes ont ‡t‡men‡spour‡tudier le comportement hydrog‡ologique de couverturesmonocouches plac‡s sur les r‡sidus r‡actifs,soumis — des cycles r‡p‡t‡s de mouillage-s‡chage. Les r‡sultatsexp‡rimentaux ont permis de calibrer et de valider des mod•les num‡riques construits avec Vadose/W (GeoSlope Int.).Des mod•les num‡riques suppl‡mentaires ont alors ‡t‡ r‡alis‡s afin d…‡valuer l…effet de diff‡rents facteurs d…influencecomme la profondeur de la nappe phr‡atique, les conditions climatiques ou l…‡paisseur des couvertures. Desconfigurations alternatives ont ‡galement ‡t‡ test‡es. On s…est attach‡ dans cette ‡tude — maximiser l…utilisationpotentiellede roches st‡riles provenant d…une mine situ‡e — proximit‡ d…un des sites. Les r‡sultats pr‡sent‡s dans cetarticles se concentrent plus particuli•rement sur les ‡coulements non satur‡s et les flux d…oxyg•ne.1INTRODUCTIONTailings from hard rock mines often contain iron sulphidesthat can react with oxygen and waterto generate acidmine drainage (AMD). The resulting effluent is usuallycharacterized by a low pH andhigh concentrations ofheavymetals and sulphates (e.g. Jambor, 1994; Aubertinet al., 2002). Several control and reclamation approachesexist to prevent the long-term production of AMD underrelativelyhumid climatic conditions.These methodsusually rely on the low oxygen flux through water ornearly-saturated porous media (Collin and Rasmusson,1988; Aachib et al. 2004).For instance, water coverstypicallyconsistofdisposing reactive mine tailingsunder a 1 m (or more)thick water layer(e.g. Yanful and Catalan, 2002; Awoh etal., 2013). The oxygen flux in water being approximately10000times slower than in the air, the oxygen supply inthis case is sufficiently low so the generation of AMDremains negligible. Although efficient to prevent theoxidation of the reactive tailings, building dikesthat willsustain the indefinite lifetime of tailings impoundments(Vick, 2001) is amajorchallenge (Aubertin et al.,2002,2016).An alternative to watercoversaremultilayeredcovers,whichcan serve the same purpose(Nicholson et al. 1989;Yanful, 1993; Aubertin et al., 1999; Bussi•re et al., 2003,2006, 2007; Dagenais et al. 2005).Such type ofcover ismade of several soils(or synthetic)materials placed ontop of each other (Aubertin and Chapuis, 1991; Aubertinet al., 1995, 2002). The number of layers can varydepending on the design, the climate and the position ofthe water table (Aubertin et al., 1995). Typically, a 3-layer-cover madeof geological materials includesa 0.5 to 1mthick retention layer (fine grained material)sandwichedbetween two coarsermaterial layers. The coarse-grainedmaterial layers are expected to drain relatively fast,reaching lowhydraulic conductivities,thus preventing

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