British Columbia Mine Reclamation Symposium

The requirement for a sustainable restoration project Thorpe, Mark B. 1991

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Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  Thorpe, M.B. Environmental Co-ordinator, Placer Dome Inc., Endako Mines Division, Endako, B.C. VOJ ILO, Canada. Land disturbed by mining should be restored to its original state when the mine is decommissioned. Before this can be accomplished, the problems associated with the site, such as toxicity, moisture supply and texture, must be overcome. The biological processes in the disturbed land are drastically disrupted and must be rehabilitated to establish the fundamental building blocks of the ecosystem. The land may need to pass through several "conditioning" stages before the appropriate conditions are present to allow the desired species to become established. Links nee to be formed to maintain a functioning ecosystem. One of the key aspects to rehabilitation is to increase the organic matter content of the substrate. This improves soil structure, increases the moisture holding ability and provides a pool for nutrient cycling. Mine wastes often have little or no organic matter and methods overcome this problem must be applied before nutrients for the desired ecosystem can be supplied, primarily, from within that system. Théorie de la réhabilitation soutenable. La croissance initiale et l'établissement de Pinus contorta sur les murs de barrages à résidus par M.B. Thorpe Endako Mines Division, Placer Dome Inc. i.  Théorie de la réhabilitation soutenable Trois composantes principales définissent un écosystème: l'atmo- sphère, le sol ou la lithosphère et la composante biologique. Ces trois composantes interagissent, produisant un système flexible gui évolue dans le temps. Cette communication porte sur l'approvisionnement en éléments nutritifs de la végétation, telle qu'influencée par le sol et 1'atmosphère, et développe l'idée théorique de système. Dans un système naturel, il n'y a pas d'addition d'engrais inorganique car les éléments nutritifs proviennent de la décomposition des matières organiques. Le système perturbé, qui ne possède aucune matière organique, donc aucune décomposition et substances nutritives, est ensuite expliqué. Les facteurs d'importance, pour l'édification d'un système permettant la fermeture d'un site minier, sont ensuite expliqués. Un système soutenable doit être instauré pour fournir les éléments nutritifs au système entier. Le concept de niveau de matière organique souhaitable dans le système en question est ensuite examiné en détail. 1 Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  The disturbance of land is readily apparent when looking at a mine. However, when considering closing all or part of a mine, the thoughts of Leopold (1934) should be remembered, and considered an idealistic aim: "The time has come for science to busy itself with the earth itself. The first step is to reconstruct a sample of what we had to begin with". This statement is particularly apt in the U.S.A., as the legislation requires companies to replace, wherever possible, the ecosystem that was present before the mine was established. This policy does not yet apply to Canada, although Canadian legislation appears to be shadowing that found in the US. Thus, in the future, disturbed lands in Canada may have to be restored to their original state. At mine sites, the ecosystem on the land to be rehabilitated is usually drastically disturbed and the biological processes are severely degraded or at a standstill, due to disturbance, toxicity or erosion. To establish the original qualities of the ecosystem, the biological processes must be restored, to achieve a functioning ecosystem with the normal interactions between plants and soil, so that nutrient release, uptake by plants and nutrient cycling can proceed at a normal rate. Once this has been achieved, a sustainable system is produced and no external influence is required to maintain the biological processes occurring in that system. Nature can achieve this without any human assistance but it usually takes many decades. For example, work carried out in Alaska on glacial moraines has shown that it takes up to 40 years before proper vegetation cover is established. Changes occur within the developing ecosystem as the vegetation improves the soil, and succession continues for approximately 100 years, until a stable community for the present climatic regime is reached (Crocker and Major, 1955). Such an extended time frame is not acceptable when rehabilitating mined land, so methods to speed up the establishment of vegetation must be found. To achieve this, it is first necessary to understand the interactions in, and the requirements of, the final ecosystem that is required at the site. The problems associated with the land must then be determined and finally the damage to the system must be repaired. It may be necessary to go through several conditioning stages, where conditions in the degraded ecosystem are improved using soil amendments to reduce toxicity and exotic species to improve soil fertility and structure (land rehabilitation). Once the building blocks for the desired ecosystem have been established, the restoration of the land can begin. Exotic species can be removed and the desired species encouraged to establish the necessary links required to provide a stable, functioning system (land restoration). In some areas, legislation may allow this second stage to occur naturally, as the ecosystem evolves following mine closure. For disturbed, mined land, such as tailings ponds, waste rock dumps and open pits, the whole ecosystem has disappeared and must be restored. To achieve this a comprehensive understanding of the ecosystem is needed beyond the simplistic view of restoring the land by stabilizing the surface, controlling pollution and aesthetically improving the site. Broad engineering principles can provide the aesthetic improvements that are often seen as the goal but the land itself is of value, so some productivity should also be included in the aims of the rehabilitation plan. Finally, if the land is not only to be rehabilitated but restored to its former state, a full appreciation of species diversity and interactions between these species is required to establish the complex ecological community that usually forms a sustainable ecosystem. 2 Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  In most cases, such a noble, and often legislated, goal can be achieved. However, financial constraints play an important part in the rehabilitation/restoration process. Factors such as cost, speed, reliability and the sustainability of the final ecosystem must, therefore, be considered. Leaving the disturbed land untouched will, eventually, fulfil the requirement for a sustainable ecosystem but, as indicated above, nature can be slow in implementing her restoration plan. Consequently, factors that decrease the time to achieve the final product are important. Mine wastes do not usually form an ideal substrate for plants. Typically, tailings and waste rock have low levels of organic matter, little or no macro-nutrients, a poor water supply, which is exacerbated by coarse texture, high surface temperatures, poor structure and may contain chemicals which are toxic to plants. If such a skeleton for an ecosystem is compared to mature ecosystems, which have higher levels of organic matter, an adequate water supply and sufficient nutrient cycling to supply the established vegetation, the problems of establishing a sustainable biological system become apparent.  Many mine wastes contain toxic chemicals, which are detrimental to plant growth. These include; salt, produced as a result of potash mining (Thorpe, 1989); pyrite, which the results in acidity (Ferguson and Erickson, 1987); heavy metals (Bradshaw and M0NeUIy, 1981); and boron, which is found in fly ash (Bradshaw and Chadwick, 1980). Such toxicity is a fundamental barrier to plant growth and, unless mitigated, will preclude the establishment of an ecosystem that is similar to that present before the mine was established. For saline substrates, a cover is often used (Merrill et al, 1983; Thorpe, 1989) and for acidity lime may be added or the material covered with an impervious layer. Heavy metals prove to be a greater problem and tolerant species are needed to stabilize the site (Bradshaw and M0NeUIy, 1981), and it may be millennia before such a site returns to its previous form and global climatic changes may even prevent that. Mine wastes are often coarse textured or are sorted with some areas tending to have larger particles and others fine. In areas where coarse textured material is dumped, the pores in the new soil substrate are large and the ability of the material to hold water is minimal. This then affects the ability of the substrate to provide water for plant growth. Good water storage is not possible until the size of the particles is <0.2 mm, which is uncommon in waste rock and coarser tailings sand. These types of mine wastes tend to be subject to drought, so the vegetation used in the rehabilitation process should be adapted to such conditions. In areas where slimes are present, imperfect drainage may result in anaerobic conditions, again reducing plant growth. In the china clay pits in Britain, two wastes are produced, one coarse and one fine. These are mixed to provide a better rooting medium for vegetation (Bradshaw and Chadwick, 1980). The water supply in a material is also affected by the precipitation at a site. In an area which receives frequent rainfall, the ability of the material to hold water may not be critical. However, in areas with extended dry periods, the ability to hold water is essential to ensure plant growth and the resulting stability of the surface. 3 Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  One of the contributing factors to poor water holding ability is a lack of organic matter. Most mine wastes have little or no organic matter. This results in poor soil structure and the inability of coarser textured substrates to store water. In sustainable ecosystem, organic matter in soil is also the primary source of the nutrients required for plant growth, which are released as a result of the biodégradation of plant material on or in the substrate. Generally, a large pool of organic matter in the soil improves the structure, improves the water holding ability, stabilizes the surface and provides enough nutrients from biodégradation to supply the requirements of the ecosystem. The colonization of mine wastes may be slow, even where there is no physical and lexicological reason. A lack of seed may be the cause, however, a lack of nutrients, particularly nitrogen often limits establishment and growth (Marrs et al., 1983). Initially, plants can be encouraged to grow by applying inorganic fertilizers to the substrate until the organic matter pool in the soil is able to supply the required nutrients. Continuing poor growth indicates that a micro-nutrient may be lacking or a toxin may be present in the substrate. There are other problems which adversely effect vegetation establishment on disturbed, mined land. If the surface of the material is unstable, drifting material may cover the vegetation and blowing sand will abrade the plants. On darker substrates, particularly if they face south, surface temperatures may exceed those within the tolerable range for seedlings. A surface mulch can help to reduce these problems and once the vegetation becomes established, the material is prevented from moving and the increased albedo at the site, coupled with evapotranspiration will decrease the temperature at the surface of the soil. Important clues for success in restoration projects can be recognized if older, abandoned areas are observed. These indicate which species have potential in either the initial rehabilitation plan or the final restoration plan, particularly if the area adjacent to the mine has not been disturbed and is to provide a template for the desired ecosystem. Adjacent areas provide a valuable source of seed and, following appropriate growth trials, seed or young plants can be propagated for the restoration plan. Wherever possible, local seed should be used as it is specifically adapted to the climate, latitude and altitude of the site. The simplest way to restore an area is to replace the topsoil that was originally at the site before it was disturbed. The topsoil provides a suitable medium for the plants growing in the area and will allow the original vegetation to become established in an "old field succession". If topsoil is replaced immediately, remarkable success can be achieved. In the strip mining areas of the US, topsoil is moved from in front of the advancing pit and replaced behind it and up to 95% of the pre-mining productivity can be achieved (Bradshaw and Chadwick, 1980). Careful handling of the soil is required as its value as a rooting medium decreases with increasing storage time and increasing traffic passing over it. Ideally, the various horizons of the topsoil should be stripped independently and replaced without mixing. 4 Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  Problems also occur if the substrate on which the soil is placed is toxic, as the toxins can move upwards in the profile and render the soil unsuitable for plant growth (Merrill et al, 1983; Thorpe, 1989). The problems of toxicity in the underlying substrate must be addressed before the topsoil is applied. For example, the potential to generate acid can be mitigated if limestone is incorporated into the surface of the aid-generating substrate before the topsoil is applied. Even without a toxic substrate, topsoil does not provide the ultimate solution to the restoration of lands. Handling topsoil is very expensive. For instance, placing 0.3 m of topsoil over 1 ha requires handling 3000 m3 of material. Additionally, this amount of soil will only supply a certain amount of nutrients for plants and will only hold a certain amount of water. If topsoil is placed over a barren substrate, additional organic matter may be required to supplement the nutrient store available in the soil and to improve the moisture holding ability of the soil. A coarse substrate under the soil may result in the potential for drought in a lighter textured soils. Topsoil does not contain any unique properties which cannot be reproduced (Bradshaw, 1987) and, even if the money is available, the topsoil may not be. Buying topsoil to cover a degraded area also produces a devastated area from which the topsoil was removed, so solving one problem and creates another. If there are no phytotoxins in the mine wastes, the need to improve the rooting medium can be achieved by adding organic matter, such as sewage sludge (Thorpe, 1990), cattle manure (Eleizalde, 1981), domestic refuse (Bradshaw and Chadwick, 1980), wood chips or other organic material (Sartori et al., 1985) to the substrate. The nutrient values of various amendments are given in Table 1. This will improve the structure, stability and moisture holding capacity of the substrate, and aid the rehabilitation of the land. However, adding these materials to the derelict land will not solve the problem of low levels of nutrients as biodégradation is required to mineralize the macro-nutrients. Table 1. Typical nutrient contents for organic and other materials that may be of value in rehabilitating disturbed, mined land (Bradshaw and Chadwick, 1980).  5 Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  It is, therefore, necessary to add fertilizers to the new system until the pool of organic matter in the soil builds up to a level where nutrient cycling can supply the nutrients required by the plants in the ecosystem. The breakdown of organic matter occurs optimally if the C:N ratio is <15:1. If the C:N ratio of the organic matter exceeds that, the soil system will use some of the applied fertilizer in the biodegradation of organic matter. Where straw is used as a mulch or is incorporated into the surface layers of a substrate, fertilizer is needed to allow nitrogen to be mineralized because the C:N ratio is so high (Table 1). However, if a sustainable system is to be achieved, the nutrient supply for the vegetation must come from within the system, with minimal atmospheric and aeolian inputs. Of the macro-nutrients (N,P,K), nitrogen can, in most cases, be assumed to be the limiting nutrient. Generally, 1000 kg ha-1 of plant productivity in a system requires 20 kg N ha-1. In a sustainable system, this nitrogen is supplied from the breakdown of the organic matter in the soil. The speed of this biodegradation depends on the type of ecosystem (Table 2) but is primarily governed by temperature, moisture and oxygen. Table 2. The organic soil nitrogen capital needed (kg N ha-1) to satisfy different nitrogen requirements, assuming various decomposition rates (Bradshaw, 1983).  If the production of a site is 2500 kg ha-1 then the plants will need 50 kg ha-1 of nitrogen. In most of Canada, the ecosystems are somewhere between cool temperate and montane, therefore the decomposition rate for the organic.matter pool in the soil will be between 1/64 and 1/16. In a montane ecosystem, the following calculations apply. If the nitrogen in organic matter is multiplied by the breakdown rate for organic matter then the amount of N released per hectare will be found: 3200 kg ha-1 * 1/64 = 50 kg N ha-1 The amount of N required per 1000 kg growth is 20 kg 1000 kg"1, so the potential growth is: 50 kg N ha Y20 kg N 1000 kg-1 = 2500 kg ha-1 However, organic matter usually contains <2% nitrogen (Table 1), so the amount of organic matter 6 Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  in the soil to provide a pool of 3200 kg N ha-1 would be at least: 3200 kg N ha-1/0.02 N = 160,000 kg ha-1. Theoretically, if the initial organic matter at the site is zero and the site produces 2500 kg ha-1 of organic matter per year, of which none is broken down, the time to achieve a sustainable nutrient supply through the biodégradation of organic matter in most of Canada would be: 160,000 kg ha-1/2500 kg ha-1 yr-1 = 64 years If the amount of production required is reduced, then the amount of nitrogen is reduced proportionately. However, the amount of organic matter added to the pool in the soil each year is also reduced. Therefore the time to achieve a sustainable nutrient supply remains the same. Such a time frame is impractical for all rehabilitation projects. A target time of 10 years to a fully sustainable system is more acceptable in land rehabilitation, with restoration of the land continuing over the next few years. Other ways of either increasing the organic matter content of the soil or of naturally providing nitrogen must, therefore, be considered. Adding a mulch or other organic material to the site will decrease this time but one of the most practical ways of adding nitrogen to the soil is by using nitrogen fixing plants. These range from alder through to clover, vetch, lupine and peas. By using nitrogen fixing plants in a rehabilitation plan, the supply of nitrogen needed for the vegetation is reduced but the biomass is maintained. The dead plants also add high levels of nitrogen to the soil so decreasing the C:N ratio and allowing biodégradation to occur more rapidly. In temperate climates, the use of clover in a sward can add up to 160 kg N ha"1. In establishing a rehabilitation plan, one of the aims should be to accumulate a pool of organic matter in the soil similar to that required for the final restoration plan. The rehabilitation plan can make use of aggressive plants that produce large amounts of biomass and need large amounts of fertilizer. When the soil biomass approaches that required, the fertilizer additions can be stopped, which will allow the plants required to complete the restoration plan to gain the competitive advantage as the supply of nutrients is reduced. The mulch plants can then be gradually removed from the ecosystem as they have now completed the required task. In this way the restoration of the land can be completed in less time than by leaving the land to natural colonization and subsequent succession. Mining drastically disturbs land over a very restricted area. When a mine closes, this disturbed land must be rehabilitated to provide the fundamental building blocks for the desired ecosystem. The exotic species present at the site can then be removed and the desired species encouraged to restore the land to its former state. To achieve this, knowledge of the limitations of the site for plant growth and the interactions of the components of the ecosystem to be restored at the site are required. One of the primary components driving the evolution of the ecosystem is the condition of the soil, especially the organic matter pool and the cycling of nutrients to and from that pool. To establish a sustainable system, all the nutrient requirements in the system must be met from within the system or from natural sources. If external influences are required then changes will occur in the system once those influences are removed. This can be used to advantage if one regime is established to rehabilitate the land and then an "old field succession" is used to provide the final ecosystem at the site for the current climatic regime. 7 Proceedings of the 15th Annual British Columbia Mine Reclamation Symposium in Kamloops, BC, 1991. The Technical and Research Committee on Reclamation  Bradshaw, A.D. 1987. The reclamation of derelict land and the ecology of ecosystems. In: Jordan, W.R., M.E. Gilpin and J.D. Aber. Restoration ecology. A synthetic approach to ecological research. Cambridge University Press, Cambridge. Bradshaw, A.D. and MJ. Chadwick, 1980. The restoration of land. Blackwell Scientific Publications, Oxford. Bradshaw, A.D. and T. M0NeUIy. 1981. Evolution and Pollution. Edward Arnold (Publishers) Limited, London. Crocker, R.L and J. Major, 1955. Soil development in relation to vegetation and surface age at Glacier Bay, Alaska. Journal of Ecology 43, 427-448. Eleizalde, B., M. Sanz and L. Heras. 1981. Effects of sulphuric acid, iron sulphate, gypsum, compost and manure on the hydraulic conductivity of a saline soil. In: Welte, E. (ed). Proceedings of 3rd Symposium of CIEC, pp 117-121. CIEC Editorial Board, Gottinggen, FDR. Ferguson, K. and P. Erickson. 1987. Overview of AMD Prediction Methodologies. In: Proceedings of "Acid Mine Drainage Seminar/Workshop". Minister of Supply and Services Canada. Cat. No. En. 40-11-7/1987E. Leopold, A. 1934. The Arboretum and the University. Park and Recreation 18, 59-60. Marrs, R.H., R.D. Roberts, R.A. Skeffington and A.D. Bradshaw. 1983. Nitrogen and the development of ecosystems. In: Lee, J.A., S. M0NCiIl and LH. Rorinson (eds). Nitrogen as an ecological indicator. Blackwell, Oxford. Merrill, S.D., EJ. Doering, J.F. Power and P.M. Sandoval. 1983. Sodium movement in soil-minespoil profiles: diffusion and convection. Soil Science 136, 308-316. Sartori, G., G.A. Ferrari and M.Pagliai. 1985. Changes in soil porosity and surface shrinkage in a remoulded, saline, clay soil treated with compost. Soil Science 139, 523-529. Thorpe, M.B. 1989. Rehabilitation studies on saline land caused by potash mining activity. Ph.D. Thesis. Submitted to the University of Saskatchewan, Saskatoon. 8


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