Year: 1990
Abstract:
Between 1979 and 1988, the Plains Hydrology and Reclamation Project (PHRP) investigated interactions of groundwater, soils, and geology as they affect successful reclamation of surface coal mines in the plains of Alberta. The overall goal of PHRP was: (1) to predict the long-term success and the hydrologic impacts of current reclamation practices; and (2) to develop reclamation technology that will allow modification of current practice to assure long-term success and mitigate deleterious environmental consequences. The first phase of the study, which was completed in 1984, included characterization and instrumentation of two study areas: the Battle River study area, which included Diplomat, Vesta and Paintearth Mines, and the Lake Wabamun study area, which included the Highvale and Whitewood Mines. In the Battle River mining area, the study sites at both Diplomat and Vesta Mines were situated in areas that were mined during the transition from small-scale surface mining to modern, larger scale mining practices. At both mines initial instrumentation, which was installed in 1979 and 1980, was situated in areas of older mining that were reclaimed to pre-modern standards. Later instrumentation, which was installed between 1985 and 1987, was situated in newly reclaimed areas that had been mined using current practice. Paintearth Mine was opened in the early 1980's and all instrumentation was installed in newly reclaimed sites. In the Lake Wabamun mining area, the instrumented areas at both Highvale and Whitewood Mines were located in pits that had been mined during the early to mid-1970's using modern mining and reclamation practices. Active mining continued in other pits of these mines throughout the project. Research from the first phase of study led to the focusing on three problem areas in the second phase of the project: (1) the potential salinization of reconstructed soils from shallow groundwater; (2) the potential deterioration of capability for agriculture as a result of differential subsidence; and (3) the potential changes in the chemical and physical characteristics of reconstructed soils. This report summarizes the results of both phases of PHRP. It brings the project's findings together in one coherent document, and as such exhibits the rationale behind a holistic approach to reclamation research. For a more in-depth treatment of any particular topic, the reader is directed to the project's extensive technical reports and publications (Appendix 1). Mining and reclamation of coal in the plains of Alberta affect two important resources: groundwater resources and agricultural resources (soil and landscape). The most important hydrological impact of surface mining of coal in the plains of Alberta is the reduction in groundwater supply capability within mined areas. Groundwater supplies in areas of potential surface mining of coal are derived almost entirely from either fractured coal beds or sandstone overlying the coal. Surface mining removes these aquifers and replaces them with mine spoil, whose properties, in general, preclude its development as a water supply. The agricultural resources disrupted by mining are replaced by a reconstructed landscape that is not initially in a state of either physical or chemical equilibrium. Depending on reclamation practices, evolution of the reconstructed landscape may result in an agricultural resource that may be better, as good as, or potentially degraded with respect to the pre-mining resource. Groundwater Resources: The hydraulic properties of mine spoil in the plains of Alberta preclude development of water supplies above the base of disturbance within reclaimed mine sites. Cast overburden spoil has values of hydraulic conductivity that are considerably lower than those of the pre-mining coal aquifers, in the range of 10-7 to 10-9 m/s. At these values of hydraulic conductivity, the spoil is not capable of supplying water to wells. In addition, the major ion chemistry of groundwater in mine spoil was found to be considerably degraded relative to pre-mining aquifers. Mean Total Dissolved Solids values are generally 5000 to 7000 mg/L, and the water is generally saturated with respect to calcite, dolomite, and gypsum. At these concentrations, the water is unfit for consumption by both humans and livestock. The brackish nature of groundwater in mine spoil appears to be an inevitable consequence of mining in the plains region of Alberta. There is no known method of materials handling that would alter either the hydraulic conductivity of mine spoil or the chemical make up of the groundwater in mine spoil in this region. We conclude that disruption of shallow groundwater supplies within and above the coal is an unavoidable result of mining in the plains region. The only exception to this generalization would be where extensive, thick sand or gravel deposits lie on the bedrock surface or within the unconsolidated drift overburden. As indicated by Trudell and Moran (1986), it might be possible in such an instance to reconstruct a zone with significantly higher hydraulic conductivity by selectively handling and placing this sand or gravel. There is limited potential to replace the shallow groundwater supplies that are disturbed by mining. Deeper coal or sandstone aquifers that are capable of replacing the shallow coal aquifers removed by mining are present only in some areas. In places where the water quality in these aquifers is acceptable for human consumption, these aquifers offer the best option to replace water supplies lost as a result of mining. AGRICULTURAL RESOURCES The impacts of mining on agricultural resources occur in two time frames: (1) immediate effects, and (2) progressive effects that have long-term implications. Immediate effects focus on the product of the soil reconstruction process. Materials handling associated with mining results in the mixing of the pre-existing soils to produce a reconstructed soil mantle of uniform thickness with properties that are an average of the pre-mining soils. Present requirements for the replacement of up to 1.5 m of subsoil material in addition to topsoil above sodic spoil appear to assure immediate postreclamation capability that is comparable to that prior to mining. There is no evidence to suggest that replacement of greater thicknesses of buffer material would further improve capability. Progressive effects focus on limitations and improvements to agriculture that develop over time; specifically, differential subsidence, which leads to ponding, soil salinization in lowland settings, and leaching in upland settings. Differential subsidence forms depressions that are aligned between the original spoil ridges, and appears to be an unavoidable consequence of dragline mining (Dusseault et al. 1985). These depressions, which typically occupy from five to ten percent of the reclaimed surface, increase infiltration and accelerate differential subsidence by ponding water during spring melt and heavy summer rain storms. As a result, cultivation patterns are disrupted, seeding and/or crop growth is restricted within the ponded depressions, and salinization may occur in the fringe area around the depression. Salinization is a natural phenomenon whose conditions for formation are met in lowland reclaimed settings where ponding occurs, particularly if there is also ponding in the adjacent upland. Ponds in the lowland area cause the water table to persist near the surface. Where there is sufficient ponding in the upland to maintain the water table at levels above that in the adjacent lowland, groundwater will flow toward the lowland. In this setting, the fringe area around ponds in the lowland will become salinized. The flatter the landscape in the lowland, the larger the salinized area will be. The impact of the negative progressive effects of mining and reclamation on agricultural resources can be minimized through modifications of materials placement and grading within existing operations. Grading as much of the upland portion of the reclaimed landscape as feasible into open slopes with integrated drainage can minimize ponding. Pauls et al. (in prep) report that slopes in the range of 1.5 to 3 percent along the long axis of subsidence depressions are sufficient to drain about 90 percent of the water that is ponded on existing reclaimed surfaces. Within the lowland areas, the extent of salinization can be minimized by grading to an undulating to rolling landscape with slopes of 3 percent to 5 percent. This will result in narrower zones around the lowland ponds where the water table is within the critical depth of the surface than when the terrain is more nearly level. There is no known method to prevent the formation of lowland areas where overburden is thinner than the threshold value, other than the expensive process of transporting material from other areas in the mine. These lowland areas can be managed as productive hayland, pasture, or wildlife habitat, which adds much needed variety to the reclaimed landscape. In some cases, it may be desirable to design drainage measures into the materials handling system to facilitate management of the future lowland area.
