Acid mine drainage (AMD), characterized by low pH and high concentrations of sulfate and heavy metals, is an important and widespread environmental problem related to the mining industry. Sulfate-reducing passive bioreactors have received much attention lately as promising biotechnologies for AMD treatment. They offer advantages such as high metal removal at low pH, stable sludge, very low operation costs, and minimal energy consumption. Sulfide precipitation is the desired mechanism of contaminant removal; however, many mechanisms including adsorption and precipitation of metal carbonates and hydroxides occur in passive bioreactors. The efficiency of sulfate-reducing passive bioreactors is sometimes limited because they rely on the activity of an anaerobic microflora [including sulfate-reducing bacteria (SRB)] which is controlled primarily by the reactive mixture composition. The most important mixture component is the organic carbon source. The performance of field bioreactors can also be limited by AMD load and metal toxicity. Several studies conducted to find the best mixture of natural organic substrates for SRB are reviewed. Moreover, critical parameters for design and long-term operation are discussed. Additional work needs to be done to properly assess the long-term efficiency of reactive mixtures and the metal removal mechanisms. Furthermore, metal speciation and ecotoxicological assessment of treated effluent from on-site passive bioreactors have yet to be performed.

Tailings are ground rock particles from which the valuable minerals or metals have been extracted. An historical overview on hard rock mines shows that since the 1930s, it has become current practice to pump the tailings into storage areas circumscribed by dykes made of the tailings themselves. However, numerous physical and chemical stability problems were observed mainly owing to the particular hydrogeotechnical and mineralogical properties of the tailings. Therefore, modifications to the conventional methods were proposed, but these were relatively costly, not always efficient, and sometimes difficult to implement. New management methods that improve the physical and (or) chemical stability have hence been developed to reduce environmental risks associated with tailings storage, namely, densified tailings, environmental desulphurization, covers built with tailings, and co-disposal of tailings and waste rock. Even if many aspects need to be optimized, these approaches can be considered today as interesting alternatives to conventional tailings management approaches.

The water retention curve (WRC) has become a key material function to define the unsaturated behavior of soils and other particulate media. In many instances, it can be useful to have an estimate of the WRC early in a project, when little or no test results are available. Predictive models, based on easy to obtain geotechnical properties, can also be employed to evaluate how changing parameters (e.g., porosity or grain size) affect the WRC. In this paper, the authors present a general set of equations developed for predicting the relationship between volumetric water content, θ, (or the corresponding degree of saturation, Sr) and suction, ψ. The proposed model assumes that water retention results from the combined effect of capillary and adhesion forces. The complete set of equations is given together with complementary relationships developed for specific applications on granular materials and on fine-grained soils. It is shown that the model provides a simple and practical means to estimate the water retention curve from basic geotechnical properties. A discussion follows on the capabilities and limitations of the model, and on additional tools developed to complement its use. Key words: water retention curve, unsaturated soils, prediction, porosity, grain size, liquid limit.

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Covers installed over waste disposal sites are used to control water and gas exchanges with the surrounding environment. One example involves covers built to limit oxygen flux to sulphidic mining and milling wastes, which can be the source of acidic leachate. In this paper, the authors present an approach to evaluate oxygen flux and its controlling parameters, the effective diffusion coefficient De and reaction (consumption) rate coefficient Kr. A laboratory experimental procedure to determine these two parameters simultaneously is described, and the proposed interpretation method is presented with a few sample results. New analytical solutions are developed to calculate oxygen flux through covers with capillary barrier effects (CCBE). The proposed solutions are compared with results ensuing from a numerical treatment of Fick's laws. Specific applications of these analytical solutions are presented and discussed.Key words: unsaturated soils, covers, capillary barrier, Fick's laws, oxygen diffusion, acid mine drainage, analytical solutions, numerical solutions.

The Kettara site (Morocco) is an abandoned pyrrhotite ore mine in a semi-arid environment. The site contains more than 3 million tons of mine waste that have been deposited on the surface without concern for environmental issues. Tailings were stockpiled in a dyke and pond and in piles, over an area of about 16 ha, and have generated acid mine drainage (AMD) for more than 24 years. The mine waste and secondary precipitates from this mine were characterized using geochemical and mineralogical techniques. The Kettara wastes contain 1.6-14.5 wt% sulfur, mainly sulfide minerals (e.g., pyrrhotite, pyrite, chalcopyrite, galena, and sphalerite). The main gangue minerals were goethite, quartz, chlorite-serpentine, talc, muscovite, and albite. Carbonates occur at very low quantities (less than 1 wt%). The most abundant heavy metals were Cu, Zn, Cr, Pb, Co, As, Cd, and Ni. Acid-base accounting static test results showed that all the samples have low values of acid-neutralizing potential (NP) (0-9 kg CaCO3/t). The mine waste has high acid-producing potential (AP) (51-453 kg CaCO3/t). Abundant secondary mineralogy is present, consisting mainly of halotrichite, goethite, jarosite-hydroanion, hydroniumjarosite, starkeyite, gypsum, alunite, copiapite, butterite, and coquimbite. Hardpans, which can prevent water infiltration to fresh tailings beneath and thereby lessen the rate of sulfide reactivity, were observed during sampling of the fine tailings. Mineralogical analysis indicated that the cementitious phase of the hardpan is mainly goethite. The alteration observed in the tailings pond does not extend more than 5-15 cm.

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Covers with capillary barrier effects (CCBE) have been recently proposed as a viable option for gas migration barriers. However, the effect of geometry on CCBE performance has not been clearly demonstrated. In this paper, the results of a laboratory study performed with an original apparatus called the inclined box are presented. The results obtained show that the hydraulic behavior of layered covers is influenced by the inclination of the slope. Generally, the upper part of the slope contains less water than the lower part. This means that the upper part is less efficient than the lower part for limiting gas migration. The authors have also studied an existing site where a CCBE was built on a sloping surface. After validation of the numerical model with in situ measured data, the model was used to perform a parametric study to quantify the influence of the main CCBE parameters on its performance. The results obtained confirm those measured in the lab and clearly show how geometry influences the performance of the CCBE. Based on these results, a simple relationship is proposed for a preliminary estimation of sloping covers performance to limit oxygen migration by diffusion.Key words: cover with capillary barrier effects, acid mine drainage, slope effects, laboratory investigation, in situ measurements, design criteria.