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Instrumentation and monitoring of an engineered soil cover system for acid generating mine waste /
Abstract
Thesis (M.Sc.)--University of Saskatchewan, 1995. Includes bibliographical references (leaves 222-240) Photocopy.
... Erosion of the cover surface materials should also be assessed as part of the runoff monitoring system. Ayres (1998) andO'Kane (1996). • Oxygen consumption test ...
... The principles behind measurement of soil moisture content with a neutron probe have been described in detail elsewhere (Silvestri et al., 1991;Kramer et al., 1992;O'Kane, 1996) and are reviewed briefly here. Neutron moisture gauges contain a source of fast neutrons and a detector of slow neutrons. ...
... Access tubes must be installed into the soil in order to use the neutron moisture probe for measuring soil water content in situ (O'Kane, 1996). A hole with the proper diameter must be created in the soil profile prior to installing the access tube. ...
... A comprehensive assessment of the spatial and temporal performance of single-layer cover systems applied in seasonally humid regions would help to better understand the interactions between climatic conditions, cover material water dynamics and environmental receptors, and highlight its effectiveness related to typical performance criteria. Such an assessment requires a range of field instrumentation to measure key parameters such as PPT, actual evapotranspiration (AET), cover material moisture storage, pore-gas concentrations, soil temperature, surface runoff (R), and water quality (e.g., O'Kane et al. 1998). This level of assessment on singlelayer covers in humid climates has been limited to a handful of studies at only small test WRPs (e.g., Ayres et al. 2003;Adu-Wusu et al. 2007). ...
... As shown in Table 3, the mean annual NP rate between 2012 and 2016 was 374 mm (mean NP/PPT = 28%), demonstrating that the cover did not significantly limit water influx to the waste rock. As expected, it is higher than multilayer soil covers that are able to maintain a high degree of saturation throughout the year (e.g., O'Kane et al. 1998;Adu-Wusu et al. 2007). Nevertheless, it is similar to previous NP rates through singlelayer soil covers (e.g., Barber et al. 2015) and does provide a reduction from pre-cover NP estimates: using the NP/PPT ratio of 34% demonstrated on similar waste rock/fill material (King et al. 2003), the pre-cover NP was estimated at 434 mm. ...
Cover systems are commonly applied to mine waste rock piles (WRPs) to control acid mine drainage (AMD). Single-layer covers utilize the moisture “store-and-release” concept to first store and then release moisture back to the atmosphere via evapotranspiration. Although more commonly used in semi-arid and arid climates, store-and-release covers remain an attractive option in humid climates due to the low cost and relative simplicity of installation. However, knowledge of their performance in these climates is limited. The objective of this study was to assess the performance of moisture store-and-release covers at full-scale WRPs located in humid climates. This cover type was installed at a WRP in Nova Scotia, Canada, alongside state-of-the-art monitoring instrumentation. Field monitoring was conducted over 5 years to assess key components such as meteorological conditions, cover material water dynamics, net percolation, surface runoff, pore-gas, environmental receptor water quality, landform stability and vegetation. Water balances indicate small reductions in water influx to the waste rock (i.e., 34 to 28% of precipitation) with the diminished AMD release also apparent by small improvements in groundwater quality (increase in pH, decrease in sulfate/metals). Surface water quality analysis and field observations of vegetative/aquatic life demonstrate significant improvements in the surface water receptor. The WRP landform is stable and the vegetative cover is thriving. This study has shown that while a simple store-and-release cover may not be a highly effective barrier to water infiltration in humid climates, it can be used to (i) eliminate contaminated surface water runoff, (ii) minimize AMD impacts to surface water receptor(s), (iii) maintain a stable landform, and (iv) provide a sustainable vegetative canopy.
... The majority of geoengineering applications occur in the unsaturated, or vadose zone, which can be up to hundreds of metres in depth depending on the ground profile and climate. Unsaturated soil applications include foundations, excavations, and buried infrastructure ( Fig. 1a; Costa et al. 2003;Machmer 2012;Jung et al. 2016), infiltration and landslide triggering (Fig. 1b;Iverson 2000;Blatz et al. 2004;Tohari et al. 2007;Cascini et al. 2010;Rahimi et al. 2010;Zhang et al. 2011;Robinson et al. 2017), compacted materials (Fig. 1c;Siekmeier 2007), cover systems ( Fig. 1d; Wilson et al. 1994;O'Kane et al. 1998;Aubertin et al. 2009;Dobchuk et al. 2013;Huang et al. 2015;Knidiri et al. 2017), evaporation-aided mine waste consolidation (Bussière 2007;Qi et al. 2017;Simms 2017), sampling and load tests (Fig. 1e;Konrad 1990;Vanapalli and Mohamed 2007;Costa et al. 2003), as well as cryogenic suction processes and contaminant migration ( Fig. 1f; Konrad and Morgenstern 1980;Lenhard and Parker 1987;Azmatch et al. 2012). The unsaturated component of these examples can be broadly grouped into flow, strength, and deformation phenomena. ...
... Transient unsaturated flow phenomena are inherently more complicated compared with steady-state and require a higher level of material property measurements. For example, in cover systems (Fig. 1d, Wilson et al. 1994;O'Kane et al. 1998;Aubertin et al. 2009;Dobchuk et al. 2013;Huang et al. 2015;Knidiri et al. 2017) transient analysis allows for consideration of soil-weather interactions, which are coupled problems that include heat transfer, vapour flow, and liquid flow, to calculate evaporation from the soil surface (Bittelli et al. 2008;Lehmann et al. 2008;Or et al. 2013). In this section, transient flow phenomena are illustrated using unsaturated transparent soil. ...
The majority of geoengineering applications occur in the unsaturated (vadose) zone, which is the near-surface region forming the connection between meteorological phenomena above and saturated ground below. The key characteristic of the unsaturated zone is that water is in tension or, put another way, pore-water pressure is negative. Moisture content, as well as most material properties, vary spatially and temporally in the unsaturated zone and coupled processes are common. In geoengineering applications in the vadose zone, unsaturated soils may be present during part or all of their design lives. The question is how or when to consider the unsaturated soils’ principles in an analysis or design. Although most geoengineering applications have an unsaturated component, use of unsaturated soil mechanics in practice lingers behind the prolific number of publications due the uncertain benefit of accounting for unsaturated effects, complexity, and conservativeness among other reasons. The focus of this colloquium is to continue bridging the gap by illustrating unsaturated soils’ principles using application-driven examples in the areas of capillarity as well as flow, strength, and deformation phenomena. As principles of unsaturated soils become more understood and demand increases for incorporating climate change effects in design, use of unsaturated soils’ principles in practice will continue to increase.
... Weeks and Wilson 2005), others have monitored a range of parameters, including water percolation, soil temperature, matric suction, gas concentrations and moisture content (e.g. O'Kane et al. 1998;Adu-Wusu and Yanful 2006). To a lesser extent, water quality evolution in environmental receptors has also been monitored to obtain an indirect indicator of remedial performance (e.g. ...
... 90% or higher) (e.g. Yanful 1993;O'Kane et al. 1998). Furthermore, this 'post-cover installation' NP exhibits a substantial reduction from the 'pre-cover installation' NP estimate equal to 34% PPT (King et al. 2003). ...
Cover systems are commonly placed over waste rock piles (WRPs) to limit atmospheric water and oxygen ingress and control the generation and release of acid mine drainage (AMD) to the receiving environment. Although covers containing geomembranes such as high-density polyethylene (HDPE) exhibit the attributes to be highly effective, there are few, if any, published studies monitoring their performance at full-scale WRPs. In 2011, a HDPE cover was installed over the Scotchtown Summit WRP in Nova Scotia, Canada, and extensive field performance monitoring was conducted over the next five years. A range of parameters within the atmosphere, cover, waste rock, groundwater and surface water, were monitored and integrated into a comprehensive hydrogeochemical conceptual model to assess (i) atmospheric ingress to the waste rock, (ii) waste rock acidity and depletion and (iii) evolution of groundwater and surface water quality. Results demonstrate that the cover is effective and meeting site closure objectives. Depletion in oxygen influx resulted in slower sulphide oxidation and AMD generation, while a significant reduction in water influx (i.e. 512 to 50 mm/year) resulted in diminished AMD release. Consistent improvements in groundwater quality (decrease in sulphate and metals; increase in pH) beneath and downgradient of the WRP were observed. Protection and/or significant improvement in surface water quality was evident in all surrounding watercourses due to the improved groundwater plume and elimination of contaminated runoff over previously exposed waste rock. A variably saturated flow and contaminant transport model is currently being developed to predict long-term cover system performance.
... The investigation of engineered waste rock storage techniques (such as covers, capillary barriers, or the placement of more acid generating rock within non-acid generating cover material) to mitigate ARD generation (e.g., Yanful and Woyshner, 1993;O'Kane et al., 1998;Haug and Pauls, 2002;MEND, 2004;Fala et al., 2005;Amos et al., 2009;Raymond et al., 2020) was beyond the scope of this study. ...
Placement methods and material availability during waste rock pile (WRP) construction may create significant heterogeneities in physical and geochemical parameters (such as grain size, permeability, mineralogy, and reactivity) and influence the internal pile structure. Due to the enormous scale of WRPs, it is difficult to capture the influence of heterogeneities on mine drainage composition and evolution. Although laboratory- or field-scale experimental studies have provided much insight, it is often challenging to translate these results to full scale WRPs. This study uses a numerical modeling approach to investigate the influence of physical and chemical heterogeneities, structure, and scale on the release of acid rock drainage (ARD) through 2D reactive transport simulations. Specifically, the sensitivity of drainage quality to parameters including grain size distribution, sulfide mineral weathering rates, abundance and distribution of primary minerals, and pile structure as a function of construction methods are investigated. The geochemical model includes sulfide oxidation, pH buffering by calcite dissolution, and ferrihydrite and gypsum as secondary phases. Simulation results indicate that the implications of heterogeneity and construction method are scale-dependent; when grain size distribution trends observed in a pile's core are applied to the entirety of a pile, results between push- and end-dumping methods vary substantially—however, predicted drainage for different construction methods become more similar when features such as traffic surfaces, structural variation, and multiple benches are also considered. For all scales and construction methods investigated, simulated results demonstrate that pile heterogeneity and structure decrease peak mass loading rates 2 to 3-fold, but cause prolonged ARD release compared to the homogeneous case. These findings have implications for the economics of planning water treatment facilities for life of mine and closure operations.
... Previous studies (O'Kane et al. 1995;O'Kane et al. 1998) at the Equity Silver mine show that the generated waste rock is relatively coarse in nature and primarily consists of volcanic and minor sedimentary material, with relatively few silt-and clay-sized particles. It is reported that the D 10 ranges from 3.5 to 0.45 mm, and the D 80 ranges from 28 to 127 mm. ...
The leaching process and drainage chemistry of acid rock drainage is mainly controlled by geochemical reactions and how water carries soluble reaction products in waste rock piles. This paper studied how water flows through a full-scale waste rock pile at the Equity Silver mine site in British Columbia, Canada and elaborated on their effects on the leaching process and drainage chemistry. A revised dual-permeability model coupled with mass transport was adopted to investigate preferential flow and matrix flow in the pile simultaneously while considering water and geochemical products leaching/exchange between them. Furthermore, a particle-level water film model and also the effects of air flow, oxygen transport, and heat transfer in pile scale were integrated to account for geochemical reactions. Simulation results of full-scale iron discharge showed that aqueous concentrations in drainage water did not vary significantly relative to variations in drainage flow rate, which was confirmed by 12 years of field monitoring data. In addition, the comparison of pre- and post-cover simulations with measured lime consumption during ARD treatment, and mass-balance/dilution calculations for the entire pile, indicates that roughly 15–20% of total precipitation passes through the cover.
... The underlying concepts behind CCBEs were developed in the 1980s (Nicholson et al. 1989;Aubertin et al. 1995Aubertin et al. , 1999 and the effectiveness of CCBEs was later documented through both laboratory and in situ studies (Aubertin et al. 1995;O'Kane et al. 1998;Yanful 1993;Yanful et al. 1999;Bussière et al. 2004Bussière et al. , 2006Bussière et al. , 2007Bussière et al. , 2009. A CCBE is typically made of three layers or more. ...
Annual precipitation, temperature, and the frequency and duration of drought events are expected to increase in Québec (Canada) south of the 50th parallel as a result of climate change. Oxygen barriers such as covers with capillary barrier effects (CCBE), which are used to control acid mine drainage (AMD), are sensitive to climate change. Increases in precipitation may have positive effects on maintaining the saturation of a moisture-retaining layer (MRL), which is necessary for reducing oxygen fluxes to reactive tailings; however, drought events could cause temporary desaturation of a MRL and, consequently, increase the potential for acid generation. The Lorraine mine site, in western Québec, was reclaimed with a CCBE in 1999 and was used as a case study to assess the effects of climate change on reclamation performance. A two-dimensional numerical model was developed and validated using 3 years of meteorological and hydrogeological data. Numerical simulations were performed to assess the CCBE’s performance, considering the influence of: (1) climatic conditions (i.e., precipitation, temperature, relative humidity, wind speed, albedo and solar radiation) by 2100 for three different climate-change scenarios, and (2) extreme drought events. Performance targets were reached for all tested climate-change scenarios, thus indicating that the Lorraine CCBE design is robust with respect to climate change and extreme droughts. More broadly, this study illustrates how numerical simulations can be used to quantify the influence of climate change on the long-term ability of CCBEs to control the generation of AMD.
... Geotechnique for unsaturated media is a rapidly expanding field, which is related to a wide variety of applications in soil mechanics and soil physics including: estimation of field capacity (Meyer and Gee 1999); efficiency analysis of covers with capillary barrier effects (Nicholson et al. 1989;Aubertin et al. 1993Aubertin et al. , 1995aAubertin et al. , 1996Aubertin et al. , 1997Khire et al. 1995;O'Kane et al. 1998;Morris and Stormont 1999;Bussière 1999); bearing capacity of unsaturated foundation materials (Rassam and Williams 1999); seepage through dams Fredlund 2000a); compressibility and swelling soil response (Fredlund, 1987;Rampino et al. 2000;Hung and Fredlund 2000); shear strength and constitutive behavior (e.g. Alonso et al. 1990;, Vanapalli et al. 1996Cui and Delage 1996); contaminant transport (Badv and Rowe 1996;Lim et al. 1998;Barbour 1998, Esposito et al. 1999; land subsidence (Thu and Fredlund 2000); freeze-thaw response of road structures (Konrad and Roy 2000). ...
... Matric suction outside the sensor is determined indirectly by using a calibrated relationship between output of the temperature sensing device, and the matric suction applied to the exterior of the sensor (Fredlund D.G. 1992). The use of TC sensors has become more common because they provide long term measurements without the need of regular maintenance (O'Kane et al. 1998, Marjerison et al. 2001, Nichol et al. 2003). ...
Golden Sunlight mine is located northeast of Whitehall, MT. Two cover systems were installed on the west waste rock complex and tailings area in order to minimize water infiltration and acid drainage. The covers were designed to function as a store-release system and are composed of coarse and fine materials being 0.80 m thick on the waste rock and 1.8 m thick at the tailings area. In order to study the influence of thickness, material properties, vegetation and climate on the performance of the cover systems, a laboratory testing program was carried out to identify the properties of the cover at different depths. In addition, thermal conductivity sensors were installed at three field monitoring stations located on the waste rock dump and tailings area in order to evaluate the variation of suction in the cover profiles. These data were used to calibrate a numerical modeling program that assessed infiltration rates through the covers under different vegetation and climate conditions. The simulation results were used to correlate internal variations in grain size and hydraulic conductivity with cover performance in terms of measured suction profiles and infiltration rates. The results show that vegetation plays a critical role in cover performance and becomes most important during wet and very wet years. The simulations and field measurement also revealed that the internal properties of the cover control infiltration patterns, showing upward and downward fluxes at different zones within the cover. It is shown that while increasing layer thickness reduces the dependence of cover performance on vegetation conditions, the establishment and sustainability of vegetation remains most important with respect to satisfactory long-term performance of store-release cover systems. Additional Keywords: Store-release cover, numerical modeling
... A more common methodological approach in the geotechnical context has been to use laboratory and numerical methods to estimate evaporation and other water and gas fluxes within soil covers (Fayer et al. 1992;Joshi et al. 1993;Yanful and Aube 1993;Wilson et al. 1994;Khire et al. 1997;Yanful and Choo 1997;Wilson et al. 1997;O'Kane et al. 1998;Choo and Yanful 2000;Lefebvre et al. 2001aLefebvre et al. , 2001bYang and Yanful 2002;Swanson et al. 2003;Yanful et al. 2003). In this work, the importance of modelling coupled heat and mass transfer processes is highlighted to pre-dict liquid and vapour flow within and out of soil covers. ...
An accurate evaluation of the water balance and net percolation through mine waste-rock piles is critical to the development,of decommissioning,plans for these wastes. One of the most difficult components,of the water balance to quantify is evaporation. Evaporation and the surface energy balance were directly measured,from a fine- to medium- grained sand waste-rock pile near Key Lake, Saskatchewan, Canada between 6 June and 25 August 2002 using the eddy covariance method. Actual evaporation for the 81 day study period was 145 mm, which was significantly less than both a potential evaporation of 247 mm,and rainfall of 236 mm,during this period. The waste-rock pile had a dis- tinct radiation and energy balance as high surface albedo reduced available energy for evaporation. The greatest frac- tion of daytime available energy was partitioned into latent heat, followed by soil heat flux, and sensible heat flux, respectively. The Bowen ratio averaged,0.45 ± 0.10 for the entire study period. A simple actual evapotranspiration model, which is an extension of Penman and consistent with the complementary approach, was used to simulate evapo- ration atop the pile. Model results indicate 134 mm,of evaporation for the study period and agree well with observa- tions. Key words: evaporation, waste-rock, energy balance, eddy covariance, acid rock drainage. Résumé : Une évaluation précise de l’équilibre d’eau et de l’infiltration nette à travers des piles de stériles de roc est
... The performance of a dry cover depends both on the characteristics of the materials used and on climatic conditions. Several researchers have studied dry covers with different configurations and materials (including Barbour 1990;Nicholson et al. 1989;O'Kane 1996;Skousen 2000;Swanson 1995;Yanful et al. 1993). ...
Pyritic coal wastes produced by a coal beneficiation plant in the State of Santa Catarina in southern Brazil are acid generating.
In this paper, we report the results from the first phase of a study evaluating the performance of dry covers for minimizing
generation of acid mine drainage and its release to the environment. The first phase includes our investigation of locally
available materials for dry covers. Numerical modeling led to four configurations being selected for testing: a dry cover
using a double capillary barrier with bottom ash, a dry cover with a single layer of clay, a dry cover with mixed waste, and
waste with no cover. Modeling also showed that the best position for the lysimeter was at the bottom of the excavation, where
it would not interfere with the water flux in the waste. A subsequent paper will describe the construction of the experimental
unit and the results of the geotechnical and geochemical studies.
Saline contaminants from potash mining endanger aquatic ecosystems. Uncovered piles of potash tailings release high amounts of chloride and sodium. Conventional coverage systems with a transpiration-intensive vegetation on a soil layer reduce percolation water only to some extent and do not protect the surrounding environment sufficiently. Powerful sealings are used to cover other mining deposits but are still uncommon for potash tailings piles. Because clay for constructing a sealing is limited, an alternative coverage system is required. The aim of this study is to identify a coverage system that conserves resources but also effectively minimises the release of contaminants. Water balance components (evapotranspiration, runoff, storage) of 44 different coverage systems were modelled with BOWAHALD. 4 model settings describe common conventional coverage and 40 settings include an additional sealing. The chosen coverage systems differ in soil layer thickness (1.5 to 3 m), vegetation productivity as well as in the thickness (10 and 50 cm) and impermeability of the sealing (kf values range between 8 × 10⁻⁰⁸ and 5 × 10⁻⁰⁹ m/s). The most impermeable sealing is also common whereas the less impermeable sealings are new designed. The results show that an additional sealing always outperforms (max. 26% percolation rate) conventional coverage without an additional sealing (28.7% percolation rate). Furthermore, a coverage system with less impermeable sealing and lush vegetation can substitute a coverage system with a more impermeable sealing and sparse vegetation without changing the percolation rate. Site-specific examinations, considering geographical orientation and inclination, show that a more impermeable sealing works best on north-facing slopes, whereas lush vegetation performs better on south-facing slopes. A mixed coverage according to site-specific performance reduces the percolation rate more than uniform coverage and requires less sealing material. I conclude that a mixed and site-specific coverage system is the most effective one to improve water quality in post-mining landscapes.
Blending of potentially acid generating (PAG) waste rock with non-PAG waste rock to create a rock mixture which performs as non-PAG is a possible approach to permanent prevention of acid rock drainage (ARD) for PAG waste rock. In 2012, a field weathering study using 300 kg samples was implemented at Teck Coal's Quintette Project located in northeastern British Columbia, Canada to test the prevention of acid generation in the PAG waste rock by dissolved carbonate leached from overlying non-PAG waste rock and direct neutralization of acidic water from PAG waste rock by contact with non-PAG waste rock.
After eight years of monitoring the experiments, the layered non-PAG on PAG barrels provided proof-of-concept that as the thickness of the PAG layer increases relative to the thickness of the non-PAG layers, acidic waters are more likely to be produced. The PAG on non-PAG layering has resulted in non-acidic water and no indications of metal leaching despite accelerated oxidation in the PAG layer shown by sulphate loadings. The study has demonstrated that the scale of heterogeneity of PAG and non-PAG materials is a critical consideration for providing certainty that rock blends designed to be non-PAG will perform as non-PAG in perpetuity. This is contrary to the standard paradigm in which an excess of acid-consuming minerals is often considered sufficient alone to ensure ARD is not produced.
The development of a cover for inclined acid-generating areas, such as the external face of dykes and the slope of waste rock piles, is undoubtedly one of the biggest technical reclamation challenges at several mine sites. The LaRonde mine site, owned and operated by Agnico Eagle Mines (Quebec, Canada) is currently engaged to identify an optimal reclamation scenario for the Dyke 1 of its acid-generating tailings storage facilities. One of the promising reclamation options for controlling water infiltration in the acid-generating waste rock on the Dyke 1 is the use of an inclined cover built with available mine waste materials. An instrumented inclined cell with an inclination angle of 18.3 degrees was built on a slope of this dyke to validate if low sulfide tailings and non potentially acid-generating waste rock can be used as cover material to reclaim the Dyke 1. The instrumented inclined cell was monitored for 3 years (2017 to 2019) using volumetric lysimeters, suction sensors, and volumetric water content sensors. The monitoring was done under natural climatic conditions and artificial wetting events. Under natural conditions, less than 1% (5 mm) of incident rainfall percolated in the volumetric lysimeters installed along the slope of the inclined cell. Under controlled conditions associated with artificial wetting events of 6.4 mm/h over a period of 12 h, net percolation values between 1 and 9% (4 to 60 mm) of the sum of incident precipitation were measured. The distance between the top of the cell and the Down Dip Limit (DDL) point was greater than the slope length of the cover under natural conditions and the DDL point moved from the bottom toward the top to reach values between 12 and 20 m from the top of the slope when the wetting events were applied on the cover. These results confirmed the suitability of mining materials as an inclined cover material to control water infiltration in reactive mine waste rocks.
The weathering of mine waste rock can cause release of metal-laden and acidic drainage that requires long-term and costly environmental management. To identify and quantify the geochemical processes and physical transport mechanisms controlling drainage quality, we monitored the weathering of five large-scale (20,000 t) instrumented waste-rock piles of variable and mixed-composition at the Antamina mine, Peru, in a decade-long monitoring program. Fine-grained, sulfidic waste rock with low-carbonate content exhibited high sulfide oxidation rates (>1 g S kg⁻¹ waste rock yr⁻¹) and within 7 years produced acidic (pH < 3) drainage with high Cu and Zn concentrations in the g L⁻¹ range. In contrast, drainage from coarse, carbonate-rich waste rock remained neutral for >10 years and had significantly lower metal loads. Efficient metal retention (>99%) caused by sorption and secondary mineral formation of e.g., gypsum, Fe-(oxy)hydroxides, and Cu/Zn-hydroxysulfates enforced strong (temporary) controls on drainage quality. Furthermore, reactive waste-rock fractions, as small as 10% of total mass, dominated the overall drainage chemistry from the waste-rock piles through internal mixing. This study demonstrates that a reliable prediction of the timing and quality of waste-rock drainage on practice-relevant spatiotemporal scales requires a quantitative understanding of the prevailing in-situ porewater conditions, secondary mineralogy, and spatial distribution of reactive waste-rock fractions in composite piles.
The performance of cover systems over waste rock piles in humid, temperate regions (i.e., where annual precipitation > annual potential evapotranspiration) is likely to be defined largely by their ability to shed water laterally within the cover. Lateral flow processes in this context are still poorly understood. Here we present a field and modeling study of the mechanisms that produce lateral subsurface flow and vertical percolation in a 7-yr-old cover system at a mining site in Southeast Alaska. The cover consists of a growth medium on top of a coarse drainage layer, underlain by a highly compacted barrier layer. A second coarse drainage layer separates the cover system from the underlying waste rock. We installed a trench to measure lateral subsurface flow in the cover and then successfully modeled this behavior, without calibration, using a two-dimensional physics-based model. Our results show that the cover responds rapidly to precipitation, converting approximately two-thirds of the input to lateral subsurface flow. Lateral subsurface flow is preceded by the development of transient perched water tables at the interface of the coarse drainage layer and compacted barrier layer. Water balance simulations indicate that flow through the barrier layer is driven by a small but permanent vertical pressure head gradient that develops within the barrier layer and results in vertical net percolation of approximately 15% of the precipitation input. These model results correspond well with lysimeter measurements of vertical percolation into the waste rock.
The oxidation of sulfide minerals in waste rock has the potential to generate low-quality drainage that can present a significant challenge to mine owners, regulators, and other stakeholders. Challenges involved in managing waste rock include the large volume of waste rock produced and the difficulty in predicting the quality and quantity of leach water due to the chemical and physical heterogeneities in the waste rock, and the highly non-linear coupling of geochemical and physical processes. Many important studies have been conducted over the past decade, particularly at the field scale, that have investigated the geochemical, hydrological, microbiological, and gas and heat transport aspects of waste-rock. These studies show that although the parameters and processes that influence AMD generation and solute release are fundamentally similar between different waste-rock piles, major differences in the dominant mechanisms of water, gas and heat transport result from differences in the physical and mineralogical properties of the rock piles, and the climatic conditions, including the amount of precipitation and prevailing temperatures. Accurate prediction of the leach water quality from waste-rock requires a detailed characterization of the properties of the rock piles and the coupling of processes specific to the particular conditions. This paper provides a review of the physical and mineralogical characteristics of waste-rock piles, followed by a discussion of the principal processes related to sulfide oxidation and solute loading, and concluding with a discussion on acid mine drainage prediction and prevention techniques.
The calculation of evaporation from soil surfaces is of critical importance in predicting the surface water balance for soil cover systems constructed on waste disposal sites and for geotechnical problems where the prediction of pore-water pressures is important. There are several well-established approaches available for predicting evaporation from horizontal soil surfaces, but the issues that affect evaporation from soil slopes have not been fully investigated for geotechnical applications. The importance of variations in solar radiation with respect to evaporation from soil slopes is examined here, and a method to predict the solar radiation on soil slopes is presented. The method is applied to a fully three-dimensional surface, based on the digital map of a soil cover placed at a mine site, to illustrate variations in potential evaporation over the surface of the site. The implication of radiation variations for actual evaporation on slopes is also illustrated. The results show that north-facing slopes at greater latitudes in the northern hemisphere can receive significantly less net radiation than horizontal or south-facing slopes, resulting in less actual evaporation.Key words: slope, evaporation, solar energy, soil cover, water balance.
The Equity Silver mine is located in north-central British Columbia and is the site of a large, well-instrumented soil cover. The soil-cover site was designed to prevent the generation of acid rock drainage from the acid-generating waste rock disposed of at this site. The cover was designed to act as an oxygen barrier by maintaining a layer of saturated soil above the waste rock. The cover consists of a compacted till placed over the waste rock and covered with a non-compacted layer of the same material. Neutron probes and thermal conductivity sensors were used to monitor the moisture content of the cover soil over a 10 year period. Weather data at the site were also collected. A review of the data collected has shown that saturation was maintained in the lower, 0.5 m thick compacted layer of the cover, as per the original cover design. The upper cover layer (0.3 m of noncompacted till) was subject to seasonal variations in moisture content, with drying in the summer months and wetting in the fall and spring. Variations in the moisture of the upper layer correlated well with the weather data collected at the site. Spatial variations were noted in the water contents measured over the cover, but a nearly saturated lower layer appears to have been maintained throughout the area monitored.Key words: soil cover, neutron probes, moisture profile, mine waste covers.
Soil covers are widely used in mine waste and landfill applications to protect the environment. For an effective soil cover, the infiltration and oxygen barrier must have a low hydraulic conductivity and also maintain a high degree of saturation. Soil water evaporation significantly affects water content, and as a result the degree of saturation of the soil. Therefore, knowledge of the rate of evaporation at the soil–atmosphere interface is required to estimate the water content of candidate cover soils. Clayey soil is commonly used, either separately as a single cover or in various combinations with other soils as a layered cover system. In this study, water flow through a single clayey till cover and a layered soil cover were modeled using a coupled liquid flow, vapor diffusion and heat transfer finite-element model (SoilCover). The layered soil cover studied was a typical mine-waste soil cover intended to control oxygen diffusion and infiltration in a temperate climate and consisted of three layers: coarse sand and fine sand as upper and lower capillary barriers, respectively, and clayey till as an infiltration and oxygen barrier. The water level was located at the bottom of the cover. Evaporation and drainage predicted by the model reasonably agreed with experimental results and showed that the clayey till would be an effective oxygen barrier in sulfide-bearing mine waste covers, for which a very low oxygen flux is desired in order to achieve environmental protection. The results emphasize the need to have evaporation and drainage barriers above and below clay barriers designed to retain moisture. Further analysis using SoilCover showed that coarse sand would perform better than either fine sand or silt as a protective top layer over a clayey till barrier.
In the investigation of soil cover design options for final decommissioning of reactive mine waste, it is often necessary to analyze or predict the anticipated cover performance as a function of the cost of implementation, which is governed by the type, number and thickness of the layers in the cover system. An example of such investigation is presented in this study where one-dimensional evaporation from hypothetical moisture-retaining cover systems is simulated to assess the influence of several cover properties and hydrogeologic parameters on performance. The commercially available transient flow model, SoilCover, was used to compute suction and water content profiles for different cover design scenarios. The predicted water content profile and porosity of layers were then used to estimate the oxygen diffusion coefficients of the various layers. The oxygen diffusion coefficients were used to estimate oxygen flux through the cover systems. The oxygen flux was, in turn, related to the maximum acid flux. The studied cover and hydrogeologic parameters included soil type, thickness of barriers, and water table elevation. Two types of infiltration and oxygen barrier and two types of capillary layer with different thicknesses were studied. The water table was either kept constant at the base of the waste (tailings) or dropped by 0.5, 1, 2, and 3m over 120 days. The results showed that the relationship between water table depression and the thickness of capillary layers, on one hand, and desaturation of the infiltration and oxygen barrier, on the other, is not linear. Relationships between oxygen flux and barrier thickness and between cost increase and performance improvement of the studied cover systems are presented. Finally, a method that outlines steps for site-specific and economically feasible design of multi-layer cover systems is introduced.
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