ARD Production and Water Vapor Transport at the Questa Mine

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1 ARD Production and Water Vapor Transport at the Questa Mine R. Lefebvre INRS-Géoressources, Quebec Geoscience Centre, Quebec, Qc, Canada A. Lamontagne Experts Enviroconseil Inc., Quebec, Qc, Canada C. Wels & A.MacG. Robertson Robertson GeoConsultants Inc., Vancouver, B.C., Canada ABSTRACT: Numerical simulations of acid rock drainage production in the mine rock piles at the Questa Mine were performed to (i) identify the main processes responsible for the observed present-day conditions, such as temperature and oxygen, and (ii) provide an estimate of the significance of gas phase humidity transfer on the water balance of the pile. Numerical simulations show that the mine rock pile geometry on mountain slopes favors strong thermal air convection which results in evaporation of pore water leading to a reduction in the moisture content of the mine rock. Conceptual simulations indicated that air drying could be a significant component of the water balance for the mine rock pile. Numerical simulations considering benches also indicated significant redistribution of water vapor with vaporization of pore moisture in the lower portion of the pile and condensation in its upper portion. However, water balance calculations indicated that the net vapor loss from the entire rock pile is relatively small, accounting for only about 2 % of net infiltration. 1 INTRODUCTION Acid rock drainage (ARD) is caused by oxidation of pyrite-bearing rocks. The rate of pyrite oxidation is controlled by a complex combination of biochemical processes influencing reaction kinetics, as well as by coupled physical processes including the movement of air and water within piles (Figure 1). Wastes Oxygen diffusion Underlying material Heat Water Gas conduction infiltration convection Figure 1. Conceptual model of physical processes acting within waste rock piles (after Lefebvre et al., 21a). Most mine rock piles are partially saturated and contain sufficient pore water to sustain pyrite oxidation. In contrast, the movement of air (and oxygen) within rock piles is often sufficiently slow to control pyrite oxidation which consumes oxygen. Heat production also occurs since pyrite oxidation is strongly exothermic (11.7 MJ/kg pyrite). The release of heat drives temperature up, as high as 7 ºC in places (Gélinas et al., 1992). In high permeability material, this rise in temperature leads to thermal air convection within the piles. This process is a much more efficient oxygen transport mechanism than diffusion and it sustains higher global oxidation rates. Water infiltration through the material picks up soluble components to form an acidic leachate containing sulfate and metals. The quantitative representation of these processes requires numerical simulation. A comprehensive physical and geochemical characterization of the mine rock piles has been carried out at the Questa mine over the last few years to evaluate the potential for current and future ARD production (Shaw et al., these proceedings). As part of this study, the numerical simulator TOUGH AMD was used to model ARD in the Questa mine rock piles. Table 1 summarizes the processes represented by TOUGH AMD (Lefebvre, 1994, Lefebvre et al., 21a, 21b). TOUGH AMD is based on the general multiphase simulator TOUGH2 (Pruess 1991). This paper focuses on the effect of water evaporation within mine rock piles due to thermal gas convection. This process results from the entry of relatively cold air within the pile. Heating of this air flowing through the rock pile increases its water content. Part of the pore water then evaporates and is carried away in the gas phase. This process could lead to partial drying of waste rock locally. This process was of interest for the Questa mine rock piles since the site is located in a relatively dry climate and gas convection was believed to be important (Shaw et al., these proceedings).

2 Table 1. Processes simulated by TOUGH AMD. System Phases and Components The system includes two fluid phases (liquid and gas) and four components (water, air, oxygen in air, and heat) in both phases. The state of the system is fixed by 4 primary variables: fluid pressure, water saturation, oxygen mass fraction in air and temperature. Multiphase Fluid Flow A multiphase formulation of Darcy s Law represents the simultaneous flow of gas and liquid phases under flow potentials including the effects of fluid pressure, temperature and density. Fluid pressures include capillary pressure, which is a function of water saturation. Relative permeability for each fluid phase is also related to water saturation. Heat Transfer Conduction (Fourier s Law), fluid flow (gas and liquid), and gaseous diffusion of heat are simulated. Latent heat related to liquid vaporization and condensation is considered. The heat stored in solids is considered as well as the heat in fluids. A semi-analytical method is used to calculate conductive heat loss to impermeable confining units. Gaseous Diffusion Diffusion of all mass components in the gas phase is modeled using Fick s Law and an effective diffusion coefficient for a partially water-saturated porous media. Pyrite Oxidation Kinetics A reaction core model is used to calculate the pyrite oxidation rate with first order kinetics relative to oxygen concentration. The reaction consumes oxygen and produces heat according to pyrite oxidation thermodynamics. Pyrite remaining and sulphate production are tracked. Dissolved Mass Transport Dissolved mass transport of the sulphate produced by pyrite oxidation is considered by a simple mass balance. 2 SITE DESCRIPTION The Questa molybdenum mine, owned and operated by Molycorp Inc., is located in the Sangre de Cristo mountains in Taos County, northern New Mexico. From 196 to 1983 large-scale open pit mining at the Questa mine produced over 297 million tonnes of mine rock, which was end-dumped into various steep valleys adjacent to the open pit. As a result, the mine rock piles are typically at angle of repose and have long slope lengths (up to 6 m), and comparatively shallow depths (~3-6 m). Molycorp Inc. has initiated a comprehensive physical and geochemical characterization program of the mine rock piles to assist in the development of suitable closure measures (Robertson GeoConsultants Inc., 1999a, 1999b, 2). The climate on the mine site is semi-arid with mild summers and cold winters. Mean annual precipitation at the mill site is about 4 mm. Average net infiltration into the mine rock piles is estimated to range from 3 to 9 mm/yr (Robertson GeoConsultants Inc., 21). This paper focuses on the Sugar Shack South (SSS) rock pile which contains about 18 million m 3 of mine rock covering a surface area of approximately 12 m by 4 m with a maximum thickness (in the valley center) of little over 1 m. Figure 2 shows a section through an edge of the Sugar Shack South mine pile in which three instrumented boreholes were installed in The rocks in the pile are mixed volcanics with grain sizes ranging from a d 1 of.-.1 mm to a d 9 of 2-42 mm. Elevation (m) Mine Rock Surface WRD-3 WRD-4 WRD- Approximate Bedrock Topography Relative Horizontal Distance (m) Figure 2. Section through the Sugar Shack South mine pile showing the location of instrumented boreholes (after Robertson GeoConsultants Inc., 1999a). Figure 3 shows temperatures and oxygen concentrations in three monitoring boreholes in the SSS pile (Robertson GeoConsultants Inc., 2). WRD-3 is located on a bench in the lower half of the pile whereas WRD-4 and WRD were drilled through upper benches (Figure 2). WRD-3 shows a very small increase in temperature and almost atmospheric oxygen concentration. These conditions indicate that the material is less reactive in the upper portion of the pile in this area as indicated also by the paste ph and descriptions of the material drilled there. WRD- 4 and WRD- instead exhibit higher temperatures that are steadily increasing with depth and that are diagnostic of high pyrite oxidation rates over the entire pile thickness.

3 Depth (m) Borehole WRD- Temperature ( o C) Oxygen (vol. %) Borehole WRD-4 Temperature ( o C) Oxygen (vol. %) Borehole WRD-3 Temperature ( o C) Oxygen (vol. %) Depth (m) Depth (m) Figure 3. Monthly conditions observed in the SSS pile in three monitoring boreholes in year 2 (Lefebvre et al., 21). Despite the inferred high reaction rate, oxygen concentrations remain high within the SSS pile. We conclude from these conditions that oxygen supply is important relative to consumption in the SSS pile and that oxygen is provided by strong lateral thermal air convection upslope of the pile. The next section on numerical simulation provides more details on the processes leading to the observed conditions in the pile. 3 NUMERICAL SIMULATIONS 3.1 Properties and conceptual model Two different numerical models were used in this study: 1) a model with a simplified geometry was developed to investigate whether or not the process of moisture transfer in the gas phase could be significant; and 2) a more detailed model with a geometry more representative of the actual SSS pile was used to numerically reproduce the observed conditions in this pile. This second model allowed the identification of the processes acting in the SSS pile and provided the basis to estimate the importance of moisture transfer in the gas phase in the water balance of this pile. Table 2 summarizes the material properties used in the two numerical models. As indicated in the table caption, the "bulk of material" and "boulder layer" properties represent the values used in the "best case" simulation of the SSS pile whereas the "conceptual model" properties were used in the model using a simplified geometry of the mine rock piles. Compared to other sites, the Doyon Mine in Canada and the Nordhalde pile in Germany, the material in the Questa mine rock piles was found to have an intermediate reactivity, a high permeability, and a peculiar geometry due to the mountain slope setting of the site (Lefebvre et al., 2). Figure 4 shows the numerical grid for the first model using a simplified geometry of the mine pile which is not taking into account the benches along the slope of the pile. This initial model was based on partial data provided by the site characterization program and it was based only on the early monitoring data. On this basis and with this simplified geometry, it was found necessary to use four layers with increasing permeability from 1-9 m 2 at the surface of the pile to x1-8 m 2 at its base in order to numerically reproduce the general conditions observed in the SSS pile. At the time, this steady increase in permeability with depth was believed to generally represent the conditions of the site. Further observations showed the actual field conditions to be different from this idealized model. Figure shows the representation of the SSS pile with the two-dimensional vertical irregular grid used for the second set of simulations. The grid takes into account the presence of benches as well as the main changes in waste rock thickness. As before, the model grid was oriented at a 3 angle with the horizontal. Table 2 presents the properties assigned to the two SSS material types considered for the second model: 1) the bulk of the waste rock consists of layers of relatively fine and intermediate size mine rock which can have a lower reactivity locally; and 2) a coarse boulder layer is found at the base of the pile due to material segregation caused by free dumping. Initial parameter values were determined from site characterization data and later modified in

4 the calibration phase of the simulations. The parameters varied in the simulations to take into account the uncertainties in field values were 1) the magnitude and anisotropy of permeability; 2) the oxygen volumetric oxidation constant; 3) water infiltration; 4) the presence and permeability of the "boulder layer" at the base of the pile; and ) the presence and distribution of low reactivity rocks. Figure 6 illustrates the distribution of materials used in the second model representing the SSS pile. The bulk material is present over most of the model but it has a lower reactivity in the lower part of the pile near WRD-3. A more permeable boulder layer is represented at the base of the pile. Table 2. Physicochemical properties of the mine rock pile materials. The "bulk of material" and "boulder layer" properties represent the values used in the "best case" simulation of the SSS pile whereas the "conceptual model" properties were used in the model using a simplified geometry of the mine rock piles. Properties Symbols and units Bulk of Material Boulder Layer Conceptual Model Properties of the mine rock material Pyrite mass fraction in solids w py dim Solids density ρ s kg/m Porosity n dim Average water saturation S w dim Humid global density ρ b kg/m Properties related to the pyrite oxidation rate Volumetric oxidation constant K Ox s -1 1.x1-7 * 1.x1-7 2.x1-7 Diffusive/Chemical total times τ d /τ c dim Properties related to fluid flow Residual water saturation S wr dim van Genuchten "m" factor m dim van Genuchten "α" factor α Pa Horizontal permeability k h m 2 3.x1-9 1.x1-8 1x1-9 to x1-8 Vertical permeability k v m 2 3.x1-1 1.x1-8 1x1-9 to x1-8 Effect. vertical air permeability k a m 2 3.x1-1.9x1-8 variable Water infiltration rate q i m/y Properties related to heat transfer Average thermal conductivity Κ th W/m ºC Heat capacity of solids c ps J/kg ºC Properties related to gas diffusion Effective oxygen diffusivity D e m 2 /s 2.87x x x1-6 *: K Ox = 1. x 1-1 s -1 for low reactivity material Grid Inclined 3 o from Horizontal: 14 Elements (19 Active) and 26 Connections Regular elements with uniform lengths (3. m) and heights (7.6 m) Grid Inclined 3 o from Horizontal: 177 Elements (126 Active) and 299 Connections Regular elements with uniform lengths (3. m) and heights (7.6 m) 6 Elevation (m) Relative Elevation (m) Figure 4. Rectangular numerical grid used for the first model with a simplified mine rock pile geometry Figure. Numerical grid with benches used in the 2 nd model representative of the Sugar Shack South mine rock pile.

5 Low Reactivity Elements Drain Elements Normal Reactivity Elements Boulder Layer Element 4x Vertical Surface Elements Length (m) Figure 6. Material distribution and boundary conditions used for the "best case" of the Sugar Shack South simulations. The boundary conditions used in the second model are: 1) yearly cyclic surface temperature variations (1 C sinusoidal changes around a mean value of 14 C); 2) hydrostatic gas pressure profile with depth; 3) atmospheric oxygen concentration; and 4) fixed water saturation at the surface imposing a constant water infiltration of 1 mm/y. For the purpose of this modeling exercise free drainage is assumed along the base of the rock pile (fractured bedrock). Also, conductive heat loss is represented from the base of the pile to the underlying bedrock. Similar conditions were used for the simplified model except that surface temperature was fixed at 1 C rather than varied. 3.2 Simplified Model Results Figures 7 and 8 illustrate the results of the simplified model. A vertical exaggeration is used in figures showing simulation results for better viewing. Figure 7 shows temperature and gas velocity on the top graph, whereas oxygen mass fraction and fluxes are presented on the bottom graph. This figure shows that a strong penetrative gas convection pattern is reaching the base of the pile and that a higher gas velocity is found in the more permeable basal layer. A steadily increasing temperature profile is obtained with the highest temperatures found near the base of the pile as observed in the monitoring boreholes at the site. Also, enough oxygen is brought in by gas advection to maintain high oxygen concentrations in the pile. The temperatures and oxygen concentrations simulated are thus in the right range compared to field data (Figure 3). The temperature distribution is found to be affected by water vaporization which is cooling the lower third of the pile whereas water condensation heats the upper third of the pile. Figure 8 presents temperature and water vapor transfer fluxes on the top graph, with liquid water saturation and fluxes on the lower graph. This figure supports the concept that major water vapor transport is physically possible under the simulated conditions: 1) the water vapor fluxes are in the same range as liquid water fluxes, and 2) water saturation is reduced very significantly in the core of the pile by vaporization from.3 to.24. These results thus show considerable drying of the core of the pile due to water vapor transport in the gas phase. Other simulations not presented here even reached total material drying locally when using either lower initial water saturation or lower water infiltration. Although conceptually interesting, these initial simulations may not be representative of actual field conditions. First, the physical conditions presented in the previous figures are achieved for a modeling period limited to 1 years. This time frame was selected to obtain relatively fast simulation answers, and finally with the expectation that pseudo-steady state conditions could be reached in the system within that time frame. Our latest results show that such stable conditions may be reached only after a 2 to 2 year period (Figure 11). The second important limitation of these initial simulations is the simplified geometry representing the system. Benches can have an important effect on gas flow patterns as demonstrated by our latest simulation results. Finally, the data available on which to base the values of the material physical properties were not extensive at that time. These initial conceptual simulations still demonstrate that 1) water vapor transfer can be an important component of a mine rock pile water balance, 2) temperature in a mine rock pile can be significantly affected by the vaporization and condensation of water, and 3) an inclined geometry favors the development of strong lateral thermal gas convection. 3.3 Sugar Shack South Model Results The "best case" results for the SSS model is illustrated by Figures 9 and 1. To obtain these results, low reactivity material was assumed to be present in the lower portion of the pile as observed in the top portion of the drill log for WRD-3. This low reactivity material reduces heat production and leads to temperatures more representative of observations. Also, since less oxygen is consumed in the lower part of the pile, more oxygen reaches the upper portion which shows temperature and oxygen values close to observed levels (Compare Figures 9 and 3). As seen in Figure 1, the water vapor fluxes are much lower in magnitude than liquid water infiltration even though there is significant drying of the material as shown by the local water saturation reductions. For the SSS pile, the main properties found to influence ARD were material permeability and reactivity. Permeability controls thermal gas convection bringing the atmospheric oxygen required to sustain pyrite oxidation. Reactivity determines if the oxygen brought in the pile is consumed fast and thus close to the edges of the pile, or rather slowly so that it can penetrate deeper within the pile.

6 4 Temperature Contours ( o C) and Gas Velocity (m/d) T: m/d 4 Grid in meters 4 Oxygen Mass Fraction Contours and Oxygen Fluxes (kg/m 2 d) XOAIR: kg/m 2 d.2 Grid in meters Figure 7. Initial simulation results with a simplified rectangular grid: temperature and gas velocity (top) and oxygen mass fraction and fluxes (bottom). The grid makes a 3 angle with the horizontal. 4 Temperature Contours ( o C) and Water Vapor Fluxes (kg/m 2 d) T: kg/m 2 d 46 Grid in meters 4 Water Saturation Contours and Liquid Water Fluxes (kg/m 2 d) SL: kg/m 2 d Grid in meters Figure 8. Initial simulation results with a simplified rectangular grid: temperature and water vapor fluxes (top) and liquid water saturation and fluxes (bottom). The grid makes a 3 angle with the horizontal.

7 Waste Rock Thickness (m) Low Reactivity Material Temperature ( o C) and Gas Velocity (m/d) WRD WRD WRD- 1 m/day 2 1 T Waste Rock Thickness (m) Oxygen Mass Fraction and Oxygen Mass Fluxes (kg/m 2 d) kg/m 2 day Figure 9. Best case results for Sugar Shack South with low reactivity material : temperature and gas velocity (top) and oxygen mass fraction and flux (bottom). The grid makes a 3 angle with the horizontal. The approximate locations of boreholes WRD-3, WRD-4 and WRD- are shown by rectangles (see also Figure 2). XOAIR Waste Rock Thickness (m) Temperature ( o C) and Water Vapor Fluxes (kg/m 2 d) Low Reactivity Material kg/m 2 day 2 1 T Waste Rock Thickness (m) Water Saturation and Liquid Water Fluxes (m/d) kg/m 2 d Figure 1. Best case results for Sugar Shack South with low reactivity material : temperature and water vapor fluxes (top) and liquid water saturation and fluxes (bottom). The grid makes a 3 angle with the horizontal. SL

8 Temperature ( o C) Pyrite mass fraction Oxygen loss (kg/m 3 y) Oxygen mass fraction Time (year) Homogeneous Anisotropic Boulder Layer Best Case.8 Homogeneous.8 Anisotropic.7 Boulder Layer Best Case Time (year) Homogeneous Anisotropic Boulder Layer Best Case Time (year).1 Homogeneous Anisotropic. Boulder Layer Best Case Time (year) Figure 11. Average conditions through time for the different simulated conditions for the SSS mine pile. Figure 11 shows the evolution through time of the simulated average conditions for the entire pile for the Sugar Shack South under its present-day configuration. Results are shown for parametric runs including 1) a homogeneous case, 2) an anisotropic case where horizontal and vertical permeabilities are different, 3) a boulder layer case where a very permeable basal layer is added, and 4) the best case where low reactivity material is taken into account. Figure 11 shows temperature, remaining pyrite mass fraction, volumetric oxidation rate, and oxygen mass fraction. A summary of the main conditions obtained for these cases is also compiled in Table 3. Figure 11 provides an overview of the general simulated conditions resulting from a given set of properties. Also, the figure gives an indication of the time required to reach pseudo steady-state conditions under which temperature, oxygen mass fraction and oxidation rate remain about constant. Such conditions can be achieved after a relatively short period in some cases (less than 1 years for the Homogeneous Case) but they may just be reached after 2 to 2 year simulation for all the other cases. On all these figures, small cyclic variations can be seen, especially for temperature. These are related to the cyclic surface temperature imposed as boundary condition. The initial homogeneous case is the one reaching the highest temperature, oxidation rate and oxygen mass fraction. This is due to the higher permeability and reactivity used for this case compared to the other ones. All other cases (anisotropic, boulder layer, and best case) exhibit similar behaviors because the permeability and reactivity of the materials used in these cases are nearly the same. The lower average temperature and reactivity of the base case is caused by the inclusion of very low reactivity material for a large part of the pile. The homogeneous case also shows the most important reduction in water saturation compared to other cases because gas convection is more important with the conditions used in that model. Table 3. Approximate average conditions for the Sugar Shack South parametric simulation cases. Simulation Case Maximum Temperature ( C) Maximum Oxidation Rate (kg/m 3 y) Maximum Oxygen Mass Fraction Pyrite Mass Fraction After 2 years Local Minimum Water Saturation Homogeneous Anisotropic Boulder Layer Best Case

9 4 DISCUSSION 4.1 Controls on Gas Flow and Oxidation Rate A comparison of the conditions obtained from initial simulations using the simplified rectangular grid with the results of the more detailed grid with benches provides an indication of the effects of benches on gas flow. The presence of benches is seen to strongly influence gas flow and temperature patterns. In the initial rectangular grid, a single large gas convection cell formed from the base to the top of the system. For the irregular grid, there are instead two main gas convection cells in the lower and upper halves of the pile. In the initial grid, layers of increasing permeability had to be introduced to obtain a convection pattern that could cover the entire system and provide oxygen everywhere. In the final irregular grid, gas convection can be triggered either with homogeneous or anisotropic material and even without the presence of a basal boulder-type material. In other words, the onset of strong gas convection patterns may be controlled more by the geometry of the system, especially the sloping of the pile and the presence of benches, rather than by a specific distribution of materials or specific values of permeability for these materials. Even though the benches do not represent large irregularities at the scale of the Questa waste rock piles, they are preferential gas entry and exit pathways. The horizontal portions of benches reduce the gas flow section and the naturally upward hot gas flow tendency is enhanced by these surfaces. Also, the toes of the slopes starting upslope of these horizontal surfaces provide preferential air entries. This behavior shows the importance of keeping a representative geometry of the system in the numerical grids. 4.2 Gaseous and Liquid Moisture Transfer The initial simulations with a rectangular grid showed that significant drying could occur due to water vapor transfer of humidity under these simulated conditions. Local drying of some cells could even occur when small initial water saturations were used in these simulations. However, for the "best case" simulations shown here, we find that humidity transfer in the gas phase reduces water saturation within the pile but only intercepts a small proportion of the water infiltration. This reduction in water saturation agrees with the observed trends in mass water content reduction with depth in the piles (Shaw et al., these proceedings). The reduction in saturation is quite variable depending on the simulation cases considered but no case with the irregular grid has led to the complete drying of the pile. A water balance was performed for the best case on the liquid and vapor phase water transfer in the entire pile. This calculation shows that only 2% of the infiltrating liquid water is removed from the pile by water vapor transfer (assuming 1 mm/yr net infiltration). The relative contribution of water vapor loss to the overall water balance of the mine rock pile would be proportionally higher for a lower net infiltration. The process of moisture transfer in the gas phase and its effect on the removal of water from a waste rock pile has some practical implications on ARD production from waste rock pile. In the Questa mine rock piles, very few places show leachate production at the toe of piles. There was thus a possibility that partial drying of water infiltration could reduce or at least delay the production of acidity from the pile. If this process had been very important, it would not have been advisable to cover the site and thus reduce this helpful process. The hypothetical effect of a cover on the SSS pile is discussed further by Lefebvre et al. (21). Moisture transfer in the gas phase also has implications on the evaluation of ARD production using temperature profiles. Figure 7 showing the results of the simulations with the simplified grid indicates that the temperature in a pile can be significantly modified by latent heat effects related to evaporation and condensation of water vapor within a pile. When these processes are strong, the temperature near the entry of a pile can be reduced by the vaporization of water: the pile then acts as a cooling system. On the contrary, in the upper portion of the pile, if water condenses following a reduction in temperature, heat is released by this process and contributes to an increase in temperature in this portion of the pile. If oxidation rates were determined in a mine rock pile based on temperature profiles without taking into account latent heat effects, this would lead to an underestimation of heat production and pyrite oxidation in the lower part of a pile whereas an overestimate would be made instead for the upper portion of the pile. Questions remain regarding the prevalence of moisture transfer and latent heat effects in waste rock piles. Previous simulation studies had identified potential water saturation reductions due to this process in other sites in quite different contexts

10 at the Doyon Mine in Canada and for the Nordhalde in Germany (Lefebvre et al., 21b). Given the variability in material properties, it may be very difficult to identify the effect of this process based on measurements of water content in mine rock. Still, in favorable contexts, such as dry climates and sites with strong thermal convection, it may be important to determine the effect of "air drying" on the water balance of a mine rock pile. Field observations using isotopic tracers support the concept of internal vaporization as a significant process within mine rock piles. Sracek (1997) obtained samples of pore waters in the Doyon pile using suction lysimeters and analysed their Deuterium and 18 Oxygen content. The isotopic data suggested that this water had been subjected to internal evaporation and condensation. The magnitude of internal drying due to water vapor transport in mine rock pile may be related to the heterogeneity of the material. The conceptual model of mine rock piles with dipping layers of coarse and fine grained material presented by Wilson et al. (2) would favor vapor transport. In such a model, water infiltrates down fine layers whereas gas is transported upward in coarse layers. Such a system would promote vapor exit from piles because the temperature in the coarse layers could remain relatively high all the way up the surface of the pile. ACKNOWLEDGMENTS Molycorp Inc. is gratefully acknowledged for supporting the work presented in this paper and for granting permission to publish it. 6 REFERENCES Gélinas, P., Lefebvre, R., and Choquette, M., Characterization of acid mine drainage production from waste rock dumps at La Mine Doyon, Quebec. Second Int. Conf. on Environm. Issues and Manag. of Waste in Energy and Mineral Prod., Calgary, Sept. 92. Lefebvre, R Caractérisation et modélisation numérique du drainage minier acide dans les haldes de stérile [In French: Characterization and modeling of acid mine drainage in waste rock piles]. Ph.D. thesis, Faculty of Science and Engineering, Laval University, Quebec, 37 p. Lefebvre, R., Hockley, D., Smolensky, J., and Gélinas, P., 21a. Multiphase transfer processes in waste rock piles producing acid mine drainage, 1: Conceptual model and system characterization. Journal of Contaminant Hydrology, special edition on Practical Applications of Coupled Process Models in Subsurface Environments, in print. Lefebvre, R., Hockley, D., Smolensky, J., and Lamontagne, A., 21b. Multiphase transfer processes in waste rock piles producing acid mine drainage, 2: Applications of numerical simulations. Journal of Contaminant Hydrology, special edition on Practical Applications of Coupled Process Models in Subsurface Environments, in print. Lefebvre, R., D. Hockley, and C. Wels, 2. Le comportement des haldes de stériles [In French: The behavior of waste rock piles]. NEDEM 2, Colloque sur la recherche de méthodes innovatrices pour le contrôle du drainage minier acide, Sherbrooke, Québec, October 3-, 2, 2-23 to Lefebvre, R., Lamontagne, A., and Wels, C., 21. Numerical simulations of acid rock drainage in the Sugar Shack South rock pile, Questa Mine, New Mexico, U.S.A. Proceedings, 2 nd Joint IAH-CNC and CGS Groundwater Specialty Conference, 4 th Canadian Geotechnical Conference, Sept , 21, Calgary, Canada, 8 p. Pruess, K., TOUGH2 - A general-purpose numerical simulator for multiphase fluid and heat transfer. Lawrence Berkeley Laboratory LBL-294, 12 p. Robertson GeoConsultants Inc., 1999a. Interim Report: Questa Waste Rock Pile Drilling, Instrumentation and Characterization Study. Interim Report 27/1 Prepared for Molycorp Inc., September 6, 1999, 13 p. plus attachments. Robertson GeoConsultants Inc., 1999b. Progress Report on Questa Waste Rock Investigation: Workplans for Geochemical and Physical Characterization. Report 27/2 Prepared for Molycorp Inc., November 1999, 2 p. and attachments. Robertson GeoConsultants Inc., 2. Interim Questa Mine Site Characterization Study. Report 28/1 Prepared for Molycorp Inc., November 2, 8 p. and attachments. Robertson GeoConsultants Inc., 21. Progress Report on Water Balance Study for Mine Rock Piles, Questa Mine, New Mexico. Report 28/17 Prepared for Molycorp Inc., February 21, 28 p. and attachments. Sracek, O., Hydrogeochemical and isotopic investigation of acid mine drainage at Mine Doyon, Quebec, Canada. Ph.D. Thesis, Faculty of Science and Engineering, Laval University, Quebec. Wilson, G.W., Newman, L.L., and Ferguson, K.D., 2. The co-disposal of waste rock and tailings. ICARD 2, Fifth International Conference on Acid Rock Drainage, Society for Mining, Metallurgy, and Exploration (SME) of AIME, Denver, Colorado, May 21-24, 2, Shaw, S., C. Wels, A. MacG. Robertson and G. Lorinczi 22. Physical and Geochemical Characterization of Mine Rock Piles at the Questa Mine, New Mexico: An Overview, 9 th International Conference on Tailings and Mine Waste 2: Rotterdam: Balkema.