Reclamation Cover Research Articles

Fully coupled heat and water dynamics modelling of a reclamation cover for oil sands shale overburden

Numerical models of soil-water dynamics have been widely used in the design of soil cover systems for mine site reclamation; however, in most cases these models only consider water dynamics without consideration of heat dynamics. For cover systems in northern climates, such as those associated with oil sands mines in northern Alberta, Canada, freezing conditions exist for approximately 6 months of the year and snow melt comprises approximately 25% of annual precipitation. This study attempts to assess whether a fully coupled water and heat flow model (CM) provides additional insights into cover performance as compared with a water flow model (FM). The CM and FM are developed for a monitored reclamation cover constructed over oil sands shale overburden. The validated results indicated that the key limitation of the CM was its inability to simulate frozen ground infiltration. This limitation results in an overestimate of snow melt runoff and does not replicate the development of perched conditions on the shale overburden surface as a result of snow melt infiltration. The FM is also unable to simulate the observed infiltration of snow melt deep into the cover when snow melt is represented simply as surface precipitation in early spring following ground thaw. Both models are improved if snow melt infiltration is represented in the model by the preferential filling of macropores across the full depth of the cover and oxidized shale prior to ground thaw. This methodology is incorporated with the CM to produce a coupled water and heat model with enhanced infiltration (CM-EI), and with the FM to produce a flow model with enhanced infiltration (FM-EI). The CM-EI provided an improved simulation of soil temperature dynamics under frozen and unfrozen conditions, as well as soil water dynamics under unfrozen conditions, including the improved representation of the annual water balance components over each water year. Given the small difference in annual water balance components between the CM-EI and FM-EI modelling approaches (∼5 mm/year), it is concluded that a FM-EI provides the best tool with which to assess the performance of these reclamation covers.

Biochar affects aspen seedling growth and reclaimed soil properties in the Athabasca oil sands region

Restoring ecosystem function after oil sands surface mining involves reestablishing the biotic and abiotic ecosystem components that affect biogeochemical cycles and fluxes. In boreal forest ecosystems, pyrogenic carbon is a native soil component that affects a variety of biogeochemical parameters and biochar is its human-made analog. To evaluate the benefits of biochar amendment to reclamation cover soils, we compared characteristics and function of peat–mineral mix (PM) and forest floor–mineral mix (FFM) with and without biochar in an 18 wk greenhouse study. We assessed nutrient bioavailability (NO3, NH4, P, K, S, Mg, and Ca), foliar nutrient concentrations (N, P, K, S, Mg, Ca, Na, and Mo), soil respiration, rhizosphere polysaccharide concentration, soil organic matter stability, and Populus tremuloides Michx. seedling growth. Seedling growth increased significantly on PM cover soil with biochar. Biochar improved K nutritional status and potentially interacted with Na bioavailability in PM, affecting growth. Soil respiration significantly decreased in PM with biochar and increased in FFM. Soil organic matter stability was positively correlated with seedling growth and increased with biochar. Our findings suggest that biochar may have a significant positive effect on upland forest reclamation in the Athabasca oil sands region, especially on sites that are reclaimed with PM.

Ecohydrological functioning of an upland undergoing reclamation on post‐mining landscape of the Athabasca oil sands region, Canada

AbstractEcohydrological functioning of natural Boreal forest in Canada's Boreal Plains is a product of interactions between soil hydrophysical characteristics and hydrogeochemical processes. These interactions create a moisture–nutrient gradient within the surface soils, increasing along low‐relief transitions from upland to riparian zone, and in turn influence the distribution of vegetation communities. It is not yet known if/when analogous ecohydrological functions can be achieved in constructed uplands following industrial disturbance, such as that following oil sands development. Hence, to assess this, we studied interactions between hydrogeochemical processes and vegetation colonization in a constructed upland relative to hydrophysical properties of 2 reclamation cover substrates during a typical continental climate's growing season. Our results indicated that in 3 years of postconstruction, the establishment of a moisture–nutrient gradient that supports vegetation colonization along slope positions was still limited by heterogeneity of cover substrates. Portions of the upland under peat–mineral mix were characterized by lower nutrient availability, high moisture content, and establishment of planted shrubs and trees. In contrast, forest floor materials plots were characterized by poor soil quality, but higher nutrient availability and greater colonization of invasive grasses and native shrubs. We suggest that the colonization of underdeveloped soils by invasive grasses may facilitate pedogenic processes and thus should be accepted by reclamation managers as a successional milestone in the recovery of ecohydrological functioning of constructed uplands. Poor soil structure under forest floor materials could not support edaphic conditions required by plants to efficiently utilize fertilizer, making this practise futile at the early stage of soil development.

Using bioavailable nutrients and microbial dynamics to assess soil type and placement depth in reclamation

Land reclamation following surface mining in the Athabasca oils sands region will be extensive, with various challenges specific to local reclamation cover soils. The high economic costs associated with pre-disturbance soil salvage and placement in reclamation necessitates judicious management and application of salvaged cover soils. Soil microbial community activity and bioavailable nutrient supply are largely overlooked in reclamation analyses despite their potential in providing a sensitive measurement of ecosystem function. This study evaluates these parameters by comparing two continuous cover soils, a coarse-textured forest floor mineral mix (FFM) and an organic matter-rich peat soil (PM) at Syncrude Canada's Aurora Soil Capping Study. Shallow (10cm) and Deep (20–30cm) placement depths of FFM and PM were compared to a control receiving no cover soil and a harvested jack pine site as a reference. Soil function was assessed by measuring bioavailable nutrient supply rates, soil respiration, phospholipid fatty acid analysis (PLFA), and community level physiological profiles (CLPP). Non-metric multidimensional scaling (NMS) was used to quantify functional similarity with reference conditions. NMS revealed the greatest similarity between FFM and the reference site for bioavailable nutrient supply, PLFA, and CLPP. Deep FFM application shared greatest PLFA similarity to the reference site, while Shallow FFM was more similar in CLPP. Shallow PM was more similar to reference conditions than Deep for all parameters measured, suggesting that shallow cover soil applications might be sufficient for the reclamation target. Soil respiration rates were greatest in FFM, followed by the reference site and PM treatments, with no difference attributable to placement depth. PM had greater nitrogen and sulfur availability, but was lower in phosphorus and potassium when compared to FFM and the reference site. Ecosystem function was more similar in cover soils that mimicked the reference site conditions as much as possible, which in this case meant shallow placement and material salvaged from upland forests.

Influence of tree species and salvaged soils on the recovery of ectomycorrhizal fungi in upland boreal forest restoration after surface mining

Surface mining in the Canadian boreal forest involves the removal of vegetation and soils, resulting in the local loss of biodiversity and ecosystem functioning. Ectomycorrhizal (EM) fungi are critical to ecosystem processes; however, their recovery following reclamation is not well understood. This study investigated the importance of reclamation cover soils (forest floor material, peat, and subsoil) and tree seedling species (Populus tremuloides Michx., Pinus banksiana Lamb., and Picea glauca (Moench) Voss) in structuring the community composition of EM fungi. We used 1-year-old seedlings to assay cover soils in the field for 3 months, and grew seedlings in each of the cover soils in a growth chamber assay for 5 months. Nonmetric multidimensional scaling indicated host identity structured the community composition of EM fungi in the field, while both host identity and cover soil influenced the composition of EM fungi in the growth chamber. However, pre-colonization of seedlings by nursery fungi complicates interpretation of field results. The rate of EM fungus colonization of seedlings collected across both assays was relatively low, approximately 23%. Our results indicate cover soils used in reclamation of surface-mined landscapes retain propagules of EM fungi, and using a wide variety of tree species in upland boreal forest restoration may increase the diversity of EM fungi recovered.

The impact of reclamation cover depth on the performance of reclaimed shale overburden at an oil sands mine in Northern Alberta, Canada

AbstractMultilayer covers are widely accepted reclamation designs in the oil sands region of northern Alberta, Canada, with an ultimate goal of revegetating to species characteristic of predisturbance native plant communities. To determine the optimal depth of reclamation material required to reclaim overburden shale from an oil sands mine, an evaluation was made of the long‐term performance of six reclamation soil cover depths all placed over overburden. The measured soil water contents from different cover thicknesses at South Bison Hills located at the Syncrude Mine site north of Fort McMurray, Alberta, were used to calibrate and validate a dual‐porosity model in HYDRUS‐1D. The calibrated and validated model was then used to evaluate the influence of cover thickness and climatic variability on plant available water for forest growth. The frequency distributions of actual transpiration (Tr) for six cover treatments with a range of leaf area index (LAI) cases were developed. These Tr frequency distributions were then modified by coupling Tr and LAI. The modified frequency distributions for annual Tr for the six simulated cover thickness highlight the strong nonlinearity between the distributions of Tr over a long‐term (60 years) climate cycle in that incremental increases in cover thickness do not produce proportional increases in Tr. The results indicated that, once the cover thickness exceeds 100 cm, there is little incremental increase in the median value of Tr over the 60‐year climate cycle. Copyright © 2014 John Wiley & Sons, Ltd.

Numerical Modeling of Gas Flow in the Suncor Coke Stockpile Covers

The occurrence of thermally driven convective air flow within waste rock or natural soil profiles has been well established; however, the potential impact that convective air flow may have on water storage within reclamation soil covers has not been previously explored. We conducted a numerical modeling study to evaluate the effect that convective air flow may have on stored water within a soil reclamation cover placed over a coke stockpile at an oil sands mine in Alberta, Canada. Coke is a carbon, sand‐like byproduct of heavy oil processing. Two‐dimensional simulations of thermally driven convective air flow were conducted for two different field sites based on available field data. The elevated temperature within the coke stockpile resulted in the development of strong convective air flow cells that drew in drier atmospheric air over the lower slope positions while releasing it across the upper slope and plateau areas of the cover. The magnitude of the gas flux and the intensity of the convection within the cell were a function of the air permeability of the coke and cover material, the depth of the coke, and the slope of the stockpile. It was estimated that convective air movement through the cover could produce as much as 1 to 2 mm/d of enhanced drying of the cover in lower slope positions. Field observations of water content distributions within the cover provided corroboration that the cover has undergone enhanced drying at lower slope positions.

Long-term monitoring and modelling of a reclaimed watershed cover on oil sands tailings

Tailings sand associated with oil sands mining contains saline process-affected pore-water which can have a detrimental impact on the performance of reclamation covers. A reclamation cover site, located at the Syncrude Canada Ltd.'s South West Sand Storage facility (SWSS), consisted of 45 cm of cover soil over tailings sand. The site was instrumented in 2001 to measure real-time local meteorology, soil temperature, water content and suction over an eight-year period. A one-dimensional numerical model (VADOSE/W) was created to calculate moisture fluxes through the cover/tailings interface over a long-term period (51 years). A pore-water volume method was then used to evaluate the potential risk of salt transport into the overlying cover soil. The long-term modelling demonstrated that the cumulative water flux across the cover/tailings interface is downward, towards the water table, when the water table is deeper than 3 m below the cover/tailings interface; although the water flux across this interface in any single year may be upward. The risk of cover salinisation due to the upward movement of saline tailings pore-water from the underlying water table under these conditions was low (less than one occurrence in over 50 years).

Reclamation for aspen revegetation in the Athabasca oil sands: Understanding soil water dynamics through unsaturated flow modelling

Carrera-Hernández, J. J., Mendoza, C. A., Devito, K. J., Petrone, R. M. and Smerdon, B. D. 2012. Reclamation for aspen revegetation in the Athabasca oil sands: Understanding soil water dynamics through unsaturated flow modelling. Can. J. Soil Sci. 92: 103–116. Reclamation of mined areas in the Athabasca oil sands region is required by law, with the ultimate goal of revegetating to species characteristic of predisturbance native plant communities. To develop adequate reclamation strategies, an analysis of soil water dynamics is of utmost importance, as is understanding the impact of the thickness of the reclamation cover. In this work, soil water dynamics and fluxes at the water table were simulated for three reclamation scenarios and compared with the fluxes obtained for natural conditions assuming that aspen is the target reclamation species. According to the simulations, a reclamation thickness between 0.5 and 1.0 m can be used to provide water for revegetation. The numerical simulations show that the reclaimed landscapes have fluxes at the water table that exhibit less fluctuation than natural conditions. To limit the interaction between the water table and atmospheric fluxes, and to limit upward flux, the water table should be deeper than 2.0 m on reclaimed landscapes that use aspen for revegetation, particularly when reclamation takes place during a dry climatological cycle.

Assessment of a Reclamation Cover System for Phosphogypsum Stacks in Central Alberta, Canada

Phosphogypsum (PG), a byproduct of the phosphate fertilizer industry, was produced and stockpiled at the Agrium Fort Saskatchewan facility from 1965 to 1991. Upon decommissioning, the outer slopes of the PG stacks were reclaimed by applying 15 cm of topsoil and planting a non-native seed mix. Physical, chemical, and hydrologic evaluations of the cover system confirmed that plants were successfully growing in various soil capping depths and were often rooting more than 200 mm into the PG. Percolation past the substrate into PG during a typical storm event was low (< 10 mm), and runoff from the stacks was negligible. Runoff quality met most guidelines, but some parameters, including fluoride, were up to 18 times higher than provincial or federal guidelines for soil and water quality. However, the cover system, when applied appropriately, does meet basic reclamation objectives. The exceedances are found in areas where the cover system has been compromised by erosion or mixing or in areas where the cover system has not been fully applied, such as roads or the inner basin. In areas where the cover system has been applied successfully, basic reclamation requirements are met.

Comparative probabilistic assessment of the hydrological performance of reconstructed and natural watersheds

AbstractThe oil sands industry has committed to returning the mine sites to a productive condition. The reconstructed soil covers must have sufficient available water holding capacity (AWHC) to supply enough moisture over the growing season, to promote vegetation. In order to assess the sustainability of various soil cover alternatives, a generic, system dynamic watershed model entitled GSDW was used along with the available historical meteorological records to estimate the maximum soil moisture deficit and annual evapotranspiration fluxes. A probabilistic framework was adopted; consequently, frequency curves of the maximum annual moisture deficit values are constructed and used to assess the probability that various reconstructed and natural watersheds can provide the associated moisture demands. In general, the study showed a tendency for the reconstructed watershed to provide less moisture for evapotranspiration than natural systems. Watersheds of various soil types, layering, thicknesses and topography were studied. The gained knowledge was used to predict the possible performance of a hypothetical reclamation cover. The results indicated that the hypothetical cover performed in a similar manner to the thickest existing soil cover which confirmed a high probability of that cover to survive under the same existing climatic conditions. Moreover, this probabilistic framework was found to be useful for integrating information gained from natural watersheds (e.g. the canopy of mature natural systems and transfer the results to the reconstructed system). The results show that the canopy influenced the moisture deficit regime positively which signifies a greater possibility that reconstructed covers will adapt to vegetation type. In brief, the adopted approach enables better understanding of the response of reconstructed systems via multiple simulations of ‘what‐if’ scenarios using different soil/vegetation alternatives. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Fracture‐Dominated Subsurface Flow and Transport in a Sloping Reclamation Cover

The successful performance of reclamation soil covers over saline‐sodic overburden associated with oil sands mining in northern Alberta, Canada, depends on the dynamics of water and salt migration within these covers. Subsurface flow exerts a significant control on the distribution of soil moisture and salts within reclaimed landscapes. A conceptual model for fracture‐dominated lateral subsurface flow and transport in a sloping clay‐rich reclamation soil cover over saline‐sodic shale overburden was developed based on an interpretation of field observations. This model was then verified through the use of numerical simulations. The conceptual model assumes that lateral subsurface flow is dominated by a preferential flow system and that chemical equilibration between fresh snowmelt water stored in the macropores and higher concentration pore water stored in the soil matrix is nearly instantaneous. Numerical modeling of subsurface flow indicated that the discharge rate and cumulative volume are controlled by the bulk saturated hydraulic conductivity and drainable fracture porosity, respectively. A drainable fracture porosity ranging from 3 to 4% yielded a simulated cumulative discharge similar to measured values. A pseudo‐equivalent porous medium transport model was used to simulate the Na+ concentration of collected subsurface flow with time. General agreement between measured and simulated values demonstrates that discharge concentrations increase as the depth of perched water diminishes with time and water drains through macropores associated with a matrix of higher solute concentrations lower in the cover profile.

Controls on the spatial distribution of soil moisture and solute transport in a sloping reclamation cover

A detailed field study was conducted to map the spatial distribution of soil moisture and salt transport within a sloping clay-rich reclamation cover overlying a saline-sodic shale overburden landform. The soil moisture data suggest that: lower-slope positions are wetter in spring due to the down-slope movement of surface run-off; infiltration occurs via preferential flow paths while the ground is frozen; and, interflow occurs along the cover–shale surface when the ground thaws. Soil moisture conditions also remain wetter in lower-slope positions throughout the summer and fall. Salt transport from the shale into the overlying cover is affected predominantly by soil moisture conditions and lateral groundwater flux. Quasi one-dimensional modelling of in situ profiles of pore-water Na+ concentration demonstrate that: (i) increased soil moisture conditions in lower-slope positions accelerate salt transport into the cover through diffusion; (ii) snow melt infiltration water bypasses the soil matrix higher in the cover profile; (iii) drier conditions in the mid- and upper-slope positions limit salt transport through diffusion; (iv) advection accelerates upward salt transport in lower-slope positions; and, (v) interflow and (or) deep percolation are key mechanisms mitigating vertical salt movement in lower- and upper-slope positions.

Preferential Flow in a Reclamation Cover: Hydrological and Geochemical Response

Evapotranspirative covers used for waste containment or land reclamation strategies are intended to function in perpetuity. Pedogenesis of the cover materials caused by biophysical processes may lead to the development of macroporosity (i.e., preferential flow paths), which will alter the hydrological response from the intended design function. Hydrometric and geochemical data were used in this study to examine the contribution of preferential flow to the hydrological response of a reclamation cover on saline-sodic shale mine overburden, in a cold semiarid environment. The hydrometric data suggest that infiltration occurs along preferential flow paths when the ground is frozen or when wet antecedent soil moisture conditions develop prior to precipitation events. Interflow is initiated during the spring snowmelt when the cover thaws and water migrates from the preferential flow paths into the soil matrix, causing a perched water table to form on the cover-shale interface. The cessation of interflow coincides with a recession of the perched water table and an increase in matric suction within the cover in response to elevated evapotranspiration demands. The chemistry and stable isotope signature of the interflow demonstrates that these waters are initially composed of fresher snowmelt water, flowing along preferential flow paths, which then transition to pre-event water dominated by higher concentration water from within the soil matrix. A numerical simulation demonstrates that macroporosity imposes a significant control on the discharge rate and cumulative volume of interflow.

ENHANCED COVER METHODS FOR SURFACE COAL REFUSE RECLAMATION

Controlling acid rock drainage (ARD) can be a major component of surface mining reclamation. An enhanced reclamation cover system is being constructed to control infiltration of rain water and generation of ARD from coal-refuse disposal areas at a closed mine in southern Illinois. Development of the mine reclamation plan required consideration of ARD generation in coal refuse disposal areas located adjacent to an alluvial aquifer used for public water supply. An integrated site characterization was performed at the mine to provide information to develop and support the enhanced reclamation plan. The enhanced cover system is similar to covers required for municipal solid waste landfills by the Resource Conversation and Recovery Act (RCRA), Subtitle D regulations. The system comprises a graded and compacted gob layer, overlain by a compacted clay liner, and a protective soil cover. The results of infiltration modeling and analyses showed that the standard reclamation cover is effective in reducing infiltration by about 18 percent compared to an unreclaimed coal-refuse surface. The modeling results showed that the inhanced cover system should reduce infiltration by about 84 percent. The geochemical modeling results showed that the reduction in infiltration would help minimize ARD generation and contribute to an earlier reclamationmore » of the mine site.« less

Strategies for reclamation of tailings impoundments

ABSTRACT Reclaimed tailings disposal sites must be configured to avoid the possibility of catastrophic dewatering to ensure long term sustainability. This can normally be achieved by dewatering tailings impoundments and breaching tailings dams or by reducing the size of the pond to a shallow ponded area far removed from any dam.. Where tailings are stored in abandoned mine pits, ponding may be acceptable if the scheme is-not vulnerable to breaching, accelerated erosion, or rapid flow releases. Surface erosion of the reclamation cover is a governing issue where the underlying tailings material is highly erodible. Reclaimed tailings storage areas with long uniform side slopes may not be sustainable over the long term. Irregular topography, with well defined swales as found in the natural environment, is preferable. Concepts such as the “safety factor”, “design recurrence interval” and “life of a civil engineering project” need to be replaced with concepts such as robust systems, self-healing capability and ...