Current approaches to the revegetation and reclamation of metalliferous mine wastes

Chemosphere 41 (2000) 219±228

Current approaches to the revegetation and reclamation of metalliferous mine wastes G.M. Tordo€, A.J.M. Baker *, A.J. Willis Department of Animal and Plant Sciences, University of Sheeld, Sheeld S10 2TN, UK

Abstract Abandoned metalliferous mine wastes can result in severe pollution and have aesthetic impacts on the local environment. Use of a vegetation cover gives a cost-e€ective and environmentally sustainable method of stabilising and reclaiming wastes such as mine-spoils and tailings. Many characteristics of metalliferous wastes are often inimical to successful vegetation establishment, most notably phytotoxic levels of residual heavy metals, low nutrient status and poor physical structure of the substratum. Current approaches to revegetation and reclamation involve both ameliorative and adaptive strategies to allow plant establishment and encourage subsequent vegetation development. Di€erent techniques of revegetation are available for temperate and arid, subtropical regions depending on the characteristics of the waste. These include direct seeding with commercially available plants, use of cover and barrier systems and the enhancement of natural revegetation processes. Ó 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Metalliferous mining associated with exploitation of non-ferrous metals has taken place on a massive global scale for several centuries. In the UK this mining activity peaked in the period 1750±1900 but is now much diminished. In many other countries ± particularly in the developing world ± metalliferous mining has only recently become a large-scale industry. An increasing volume and range of heavy metals have been exploited with the growth of world industry. In particular, lead, zinc and copper have been mined extensively. In the UK abandoned mine workings are typical features of the landscape in areas of central/N. Wales, SW England and the central/N. Pennines (Johnson et al., 1977). Pb and Zn ± occurring principally as the sulphide minerals galena (PbS) and zinc blende (ZnS) ± usually occur together and have been mined in all these regions. They have also been smelted in areas distant from mines, and

*

Corresponding author. Tel.: +44-114-2224626; fax: +44114-2220002. E-mail address: a.baker@sheeld.ac.uk (A.J.M. Baker).

this too has left a legacy of abandoned spoil heaps. Pb and Zn have been mined extensively in regions outside Europe ± including western USA, Canada, Australia and India. Many of the mines are in areas of arid climate which presents increased problems for the establishment of vegetation. Although heavy metal mining can bring much economic prosperity, large areas of industrial dereliction often result once mining has ceased. This dereliction includes a legacy of abandoned tips and tailings, which contain the waste products of both mining and oreprocessing operations. Such materials are often a major source of heavy metal pollution in the local environment owing to dust blow and from the leaching of the products of mineral weathering into nearby watercourses. This pollution may have serious detrimental e€ects upon crops and public health (Smith and Bradshaw, 1972). For example, in N. Wales a washout of tailings from Parc lead/zinc mine caused the despoilation of a large area of productive agricultural land owing to toxicity to arable crops (Firth et al., 1981). Metal mines also leave visual scars on the landscape. This problem is especially evident in the UK where mining has often been in National Parks and other areas of natural beauty. There is

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therefore a growing need to reclaim such sites to increase environmental quality after mining operations have ended. It is now a requirement in most countries that reclamation schemes are incorporated at the planning stage of mining proposals. Many older mines, however, have never been fully reclaimed. There is a need to improve reclamation technology in future mine abandonment, and also to ®nd economical and practical ways of reclaiming derelict mines. A range of reclamation techniques is available for metalliferous substrates but only through the use of vegetation to stabilise mine wastes can complete longterm rehabilitation be achieved. Successful revegetation can be a permanent and visually attractive solution and, at the same time, be relatively inexpensive. A vegetation cover can be e€ective in providing the necessary surface stability to prevent wind-blow of contaminated particulates, and in reducing water pollution by interception of a substantial proportion of incident precipitation. Although revegetation is desirable, metalliferous wastes are very unfavourable environments for plants because the presence of many growth-limiting factors ± particularly residual high levels of heavy metals, macronutrient de®ciencies and poor substrate structure. Such features result in most metal wastes being largely devoid of any natural vegetation, even many years after abandonment. Consequently, experimentation has been undertaken at mine sites to attempt to elucidate and overcome limitations to vegetation establishment, allowing large-scale revegetation schemes to be formulated. Although such schemes have often been successful at speci®c sites, their widespread application is limited owing to the great variation in physical, chemical and biological factors which exist between mine wastes. In this review, various approaches to vegetation establishment on metalliferous mines wastes are critically evaluated in the light of revegetation schemes practised in the UK and elsewhere. Valid generalisations concerning detailed work in different countries, and hence, di€erent climates, available plant species, waste conditions, etc., are, however, dicult to make (Down, 1975). This review can therefore only attempt to outline general features of revegetation problems and potential solutions. 2. Physical, chemical and biological stabilisation Two basic techniques other than the use of vegetation have been employed to stabilise metalliferous wastes. Physical stabilisation involves covering unstable mine waste with an innocuous material ± often the waste rock obtained from stripping operations ± to reduce wind and water erosion. The widespread application of this technique is limited, however, by the availability of suitable materials and the high costs of transportation (Johnson and Bradshaw, 1977). Physical stabilisation is also un-

likely to result in a signi®cant decrease in the pollution of adjacent watercourses, as leaching of soluble heavy metals will still occur. Chemical stabilisation involves reacting a chemical agent ± such as lignin sulphonate or a resinous adhesive ± with the mine waste to provide a crust resistant to wind and water erosion. The use of chemical methods is restricted by their lack of permanency and the need for regular inspections. This technique is useful, however, for the temporary stabilisation of waste tips prior to revegetation. The other major limitation of these techniques is that they fail to enhance the unsightly nature of abandoned mine sites. 2.1. Advantages of vegetation in mine waste stabilisation It is now widely accepted that stabilisation by vegetation is far more desirable than by the above methods. A vegetation cover is very e€ective in reducing surface erosion because the roots bind the substrate. Also, vegetation can return a large proportion of percolating water to the atmosphere through transpiration, thus reducing the concentrations of soluble heavy metals entering watercourses. A vegetation cover also goes a long way towards reducing the visual scars in the landscape caused by large-scale mining operations. Successful revegetation may allow recreational use of the land, and even agriculture or forestry if conditions are favourable. A well-planned revegetation scheme should overcome the problems on a permanent basis. This requires a thorough site evaluation and the selection of the most appropriate revegetation technique with regard to local conditions (geology, climate, toxicity levels, etc.). Many of the successful schemes have been preceded by detailed analytical studies, by small-scale glasshouse pot trials followed by extensive ®eld trial programmes. In this way the major limitations to plant growth are identi®ed and appropriate remedial treatment can be formulated (Williamson and Johnson, 1981). 2.2. The nature of waste materials and problems of vegetation establishment Metalliferous mining produces waste of two distinct physical forms. Very coarse waste rock (diameter usually 2±20 cm) consists mainly of unmineralised overburden rock, excavated to uncover the ore-body, and rock from areas within the ore-body which contains metals at concentrations below the cut-o€ grade. Tailings, on the other hand, are ®ne-grained (<2 mm) deposits from the ®nal stage separators. Plant establishment on both these waste types is subject to severe physical restrictions. Spoils composed of waste rock have very poor waterholding capacity, and are usually subject to regular and prolonged surface drought. This makes direct vegetation establishment on waste rock almost impossible because of severe limitations to germination and seedling estab-

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lishment. The majority of waste tips, however, consist mainly of ®ne tailings which have a much better waterretention capacity. Tailings are often pumped into slimes dams, and are liable to surface compaction and even cementation processes which can combine to form an impenetrable surface barrier (Bradshaw et al., 1978). Even if tailings are not compacted, they are liable to wind and water erosion (Bradshaw and Chadwick, 1980), which can also limit plant establishment. Although physical factors may severely limit vegetation establishment, chemical properties of metalliferous wastes are regarded as the most inhibitory to plants. In particular, the toxic e€ects of residual quantities of target metals and the extremely low nutrient status of such wastes can strongly restrict plant growth. The mechanisms by which heavy metals cause phytotoxicity are poorly understood, but it is known that they inhibit root growth, which, among other problems, leads to increased susceptibility to drought. The residual concentration of heavy metals varies considerably between mine sites, and even within a single waste tip. Concentrations range from as low as 0.1±0.5% (w/w) in some modern tailings to > 5% (w/w) in the potentially more toxic spoils from long disused mines (Bradshaw et al., 1978), worked when ore bene®ciation processes were much less ecient than today. Elevated concentrations of associated non-target metals and metalloids ± such as arsenic, cadmium and silver ± may also be present (Bradshaw and Johnson, 1992). Total metal concentrations greater than 0.1% (w/w) area generally phytotoxic (Williamson and Johnson, 1981), although exact thresholds are in¯uenced by numerous factors ± such as pH ± which determine the availability of metals to plants. In addition, plant species vary in their level of susceptibility to heavy metal toxicity. Chemical analytical techniques are therefore of limited value in revegetation plans compared to phytometric assays. Metalliferous wastes are invariably de®cient in essential plant nutrients, particularly N and P. This is of major importance in revegetation and the problem is exacerbated by the lack of clay minerals and organic matter in mine spoil. These fractions provide cationexchange sites in normal soils and their absence can result in the rapid leaching of any added inorganic nutrients. One of the most important determinants of nutrient status and metal concentration at which phytotoxicity occurs is the associated gangue mineral. Plant uptake of heavy metals from calcareous materials is restricted by the formation of hydroxides and carbonates or calcium-heavy metal complexes (Bradshaw et al., 1978). High concentrations of calcium are also favourable for growth (Johnson and Bradshaw, 1977), but spoils derived from acidic rocks give much poorer growth. The solubility of Pb, Zn and Cu ± as well as accessory metals such as Mn and Al ± is increased at low pH. Extreme acidity may result from the weathering of

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pyrite (FeS2 ) in spoil. Bacterial oxidation processes cause the release of free sulphuric acid from pyrite, and this can itself be phytotoxic. Metalliferous wastes in arid climates often also have phytotoxic salinity levels, as natural leaching of salts does not occur. Other environmental factors operating at mine sites ± including temperature extremes, wind scouring and slope instability ± have deleterious e€ects on vegetation. Biological factors, particularly levels of soil micro¯ora, also play a major role in plant establishment. Soil micro¯ora are an integral component of the functioning of all ecosystems, and their paucity in metal wastes can result in very slow carbon and nutrient cycling. Plants must, then, overcome a whole suite of physical, chemical and biological constraints ± some of which may act synergistically ± if they are to persist on metalliferous wastes. Nevertheless, even many hostile lead/zinc wastes do have isolated patches of sparse vegetation; these indicate that revegetation may be a realistic goal. 3. Approaches to revegetation Several di€erent methods for revegetation have been developed and implemented. All have advantages and drawbacks, and no single approach is universally applicable. The major approaches, and the circumstances in which they should be implemented, are discussed in detail, in relation to studies in the UK and elsewhere in the world. 3.1. Establishment directly onto mine waste 3.1.1. Direct seeding with commercially available plants Although metal wastes are often very unfavourable environments for plants, recent advances in processing technology have led to much decreased metal concentrations in modern tailings. In some situations the concentrations of plant-available heavy metals may be low enough to allow the direct establishment of commercially available plants onto tailings without resultant phytotoxicity. This is an attractive option as direct seeding with agricultural seed mixtures is a very economical revegetation technique. This approach usually entails seeding with a balanced grass/legume mixture combined with inorganic fertilizer treatment to alleviate nutrient de®ciencies. Organic matter (such as sewage sludge) would normally be applied as a thin surface covering to improve the physical structure of the substrate and to provide nutrients in a slow-release form ± to encourage sward establishment. Once established, legumes such as birdÕs-foot trefoil (Lotus corniculatus) and white clover (Trifolium repens) supply the sward with N by atmospheric ®xation. A self-perpetuating system can therefore develop although the application of P-rich fertilizer may be needed. This technique was used

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to establish vegetation on abandoned metalliferous ¯uorspar dams in Derbyshire, UK (Johnson et al., 1976). Residual levels of Pb and Zn were low, and so a commercial grass/legume mixture combined with slow-release fertilizer was applied by hydroseeding. An excellent vegetation cover developed within six months and showed no signs of deterioration after two years. White clover grew extremely well, reducing the need for repeated fertilizer applications. Revegetation has overcome pollution and aesthetic problems, illustrating that direct seeding can be successful on a calcareous substrate where heavy metals are relatively unavailable and acidity is not a problem. 3.1.2. Direct seeding/planting of native plant species The establishment of plants directly into tailings may also be possible in arid climates. Although less straightforward than in temperate regions, this can be achieved through the use of native species ± which are ecologically adapted to the prevailing climate ± in combination with irrigation techniques. Irrigation is valuable not only as a source of water, but also because it can be used to leach out accumulating salts and soluble compounds of heavy metals (Peterson and Nielson, 1978). Drip irrigation ± involving the application of water at a low rate over relatively long periods ± has been used on several arid metal wastes after the direct planting of grasses, shrubs and trees, to facilitate establishment. Bach (1973) has used drip irrigation in Arizona to revegetate non-metalliferous mine wastes successfully with several species. Irrigation leaches toxic materials out of tailings to produce a sterile medium devoid of nutrients. Water and nutrients can then be applied simultaneously using drip irrigation; this facilitates plant establishment (Bach, 1973) while using fairly small volumes of water. Direct revegetation has been very successful on tailings dumps at Broken Hill, NSW, Australia, by the use of drip irrigation with sewage sludge (Thorne and Hore-Lacy, 1979). Broken Hill has a very arid climate, and tailings contain Pb and Zn. Despite this, a ¯ourishing vegetation cover has developed after direct planting and subsequent irrigation with sewage sludge, which provides an inexpensive source of nutrients as well as water. Vegetation has shown no signs of regression even after the cessation of irrigation, and is likely to survive inde®nitely now a layer of humus has formed (Thorne and Hore-Lacy, 1979). The main advantage of direct vegetation establishment is that it is relatively inexpensive. The value of this approach is illustrated by the above revegetation schemes, all of which were on neutral or basic tailings of moderately low Pb/ Zn content. However, direct establishment of normal plant material is not feasible on metal wastes which have higher residual metal concentrations, and in some cases extreme acidity. Under these circumstances seedlings of commercial varieties persist for only a few weeks. More

re®ned techniques of vegetation establishment are therefore required. 3.1.3. Direct seeding with metal-tolerant plants As already mentioned, most Pb/Zn wastes with a moderately favourable physical structure do show some natural colonisation by plants, albeit sparse. In the UK these patches of vegetation are usually made up of a characteristic limited range of plant species, mainly grasses. On acid wastes Agrostis capillaris and Festuca ovina predominate, while on calcareous substrates the dominants are Agrostis stolonifera, Festuca rubra and Deschampsia cespitosa (Smith and Bradshaw, 1972). All these grass species also occur on normal soils, and are therefore termed pseudometallophytes. Analysis of these plants has shown that they are tolerant to the heavy metals present in the substrates on which they grow, and that this tolerance is highly heritable (e.g. Bradshaw and Snaydon, 1959; Urquhart, 1971; Gartside and McNeilly, 1974). Metal-tolerant populations are a result of evolution by natural selection (Bradshaw et al., 1978). It appears that genes for metal tolerance occur at a low frequency in populations of pseudometallophytes from normal sites. This results in the rapid evolution of heavy metal tolerance when coupled with the high selection pressures resulting from metalliferous wastes. There has been no case reported of a species that evolves tolerance but which does not possess variability for tolerance in its normal populations (Baker, 1987). Metal-tolerant grasses are also tolerant of other stresses present in the hostile mine waste environment. A. capillaris from an acidic lead mine has been found to show tolerance of low nutrient conditions owing to its relatively low growth rate compared to this plant growing in normal soil (Jowett, 1959). Low nutrient tolerance is just one of an integrated complex of adaptations present in tolerant plants enabling them to grow under the severe environmental stresses of mine sites (Bradshaw et al., 1978). Metal-tolerant plants were not utilised in revegetation until the 1960s, when their value in this respect was investigated in ®eld trials in the Lower Swansea Valley. This area has not been mined, but was a world centre for the smelting of many metals ± particularly Cu, Pb and Zn ± by 1800. Smelting had largely ceased by the beginning of this century, and the region became one of the largest areas of industrial dereliction in the UK, containing substantial areas of metal toxic wastes. Experiments by Gadgil (1969) on these wastes involved metal-tolerant plants obtained from several other mine sites in Wales and England. Good growth of this tolerant material when supplied with nutrients indicated the potential of this approach for large-scale revegetation. Subsequent seeding trials indicated that metal-tolerant A. capillaris grew well for several years on both Cu and Zn wastes, and showed no signs of sward regression (Gemmell and Goodman, 1978). Non-tolerant A. capil-

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laris, however, regressed after growth for a few years. Following this promising start, Bradshaw and coworkers at the University of Liverpool investigated the potential of tolerant plants in the revegetation of many metalliferous sites in England and Wales (e.g. Smith and Bradshaw, 1972, 1979). Tolerant grasses from a large number of populations were shown to grow successfully on a wide range of metalliferous wastes provided sucient fertilizer was applied. These grasses root very well in the toxic substrate and are therefore much less prone to drought, which appears to be the main cause of death in non-tolerant plants. Preliminary glasshouse experiments followed by small-scale ®eld trials showed the e€ectiveness of seed-grown tolerant plants ± even under conditions of extreme drought ± which continued to grow nine years after establishment (Smith and Bradshaw, 1979). Three cultivars have since been developed commercially, enabling large-scale direct seeding of metalliferous wastes. Two of these were developed speci®cally for growth on Pb/Zn wastes: Festuca rubra ÔMerlinÕ and Agrostis capillaris ÔGoginanÕ. These are intended for growth on calcareous and acidic wastes, respectively. Theoretically, therefore, any Pb/Zn waste in a temperate climate, and with a moderate physical structure, can be revegetated by these tolerant cultivars. The third cultivar Agrostis capillaris ÔParysÕ was selected from pyritic Cu/Zn wastes in N. Wales, and is therefore appropriate for more acidic mine wastes. On newer wastes ± with low residual metal concentrations ± the use of tolerant plants may be unnecessary. Direct seeding with tolerant plants has many distinct advantages on wastes too toxic to be seeded directly with non-tolerant plants. The tolerant plant approach is very economical, requiring only direct seeding of the substrate with the addition of standard NPK fertilizer. Tolerant plants also have a great advantage in that they are already adapted to other stress factors of mine sites, particularly nutrient limitation and drought. A complete vegetation cover may be achieved in time. As the roots of tolerant grasses penetrate and bind the substrate effectively, pollution risks via wind-blown dust may be minimal. Although the landscape can never be entirely restored to its original appearance, the aesthetics of a waste revegetated with metal-tolerant grasses should allow it to harmonise with the surrounding countryside. Despite the economic and practical bene®ts of this approach, however, the use of metal-tolerant cultivars in large-scale revegetation schemes has not become as widespread as predicted since the development of commercial varieties in the late 1970s. Palmer (1990a) attributes the little use of tolerant plants to continuing pollution factors. As spoil material remains at the surface, the run-o€ from the site will still contain heavy metals. In addition, if grass establishment is slow ± as is often so owing to the slow-growing nature of tolerant cultivars ± erosion can occur prior to development of a

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complete vegetation cover. Sward establishment is also limited by the absence of metal-tolerant populations of legumes (Bradshaw et al., 1978), which would maintain a supply of nitrogen through symbiotic ®xation. Phosphate de®ciency may also develop because of the formation of insoluble heavy metal-phosphate complexes. There thus exists the possibility of sward regression although tolerant cultivars are adapted to low nutrient conditions. An additional constraint is the speci®city of metal tolerance systems in plants. This means that a particular cultivar can be established only on wastes containing only the metals to which it has evolved tolerance. This speci®city may limit the use of metaltolerant cultivars on heterogeneous wastes containing elevated concentrations of several heavy metals. Constraints on ®nal land use are also imposed by the metal tolerance approach. Although tolerant plants translocate less heavy metals into their aerial parts than do non-tolerant plants under the same conditions (Smith and Bradshaw, 1972) ± owing to immobilisation in the roots ± elevated concentrations are invariably found in the shoots. These concentrations, often reaching 5000 mg kgÿ1 Pb and 1000 mg kgÿ1 Zn (Smith and Bradshaw, 1979), are far too high for continuous livestock grazing and revegetated areas should therefore be fenced. A metal-tolerant grass cover also has low trampling resistance (Bradshaw and Johnson, 1992), reducing possibilities for recreational use. Revegetated areas therefore persist as low-grade ungrazed grassland, which renders the unproductive nature of the sward largely insigni®cant. The research on and development of tolerant cultivars have taken place almost exclusively in the UK. Consequently, cultivars have been developed from temperate species, and although suitable for a range of physical and chemical conditions, these would be of little use in tropical and arid regions where climatic conditions are markedly di€erent. There is consequently the need to develop similar tolerant material ± adapted to the climate of these regions ± if this approach to revegetation is to be adopted on a global scale. Natural metal-tolerant populations are known to exist in arid and tropical regions (Smith and Bradshaw, 1972) and it should be fairly easy to develop commercial strains of these (Bradshaw and Johnson, 1992). Some species have the added bene®t of possessing salt tolerance, and being capable of N-®xation (Bradshaw and Chadwick, 1980). Field surveys of natural colonists of tailings can identify suitable species, although in some areas (e.g. the western USA) tolerant populations have not been found as plants have not been subject to toxic waste conditions for long enough (Peterson and Nielson, 1978). This scenario may be common in areas where metal mining has become widespread only recently. In such situations in arid climates, the use of widespread tolerant species from other arid areas, such as Cynodon dactylon, is an attractive option. At Zawar Pb/Zn mines, India, stolons

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of this grass transplanted from other tailings grew very well compared to those from uncontaminated soil (Aery et al., 1987). So far, however, the commercial development of tolerant plants from tropical and arid regions has apparently not been initiated. It is hoped that this development will soon occur, giving an economic and practical approach to mine waste revegetation as in temperate regions. 4. The use of covering systems Direct seeding with metal-tolerant cultivars is essentially an adaptive approach to revegetation, the principle being to combat the toxicity of the waste. This is one of the two major approaches to the revegetation of metalliferous wastes too toxic to be directly seeded with normal plant material. The other is essentially ameliorative, toxicity being avoided or diluted, rather than tolerated, by using some form of covering system. This approach has been widely used and subject to considerable research (Palmer, 1990a). There are essentially two types of covering material: ameliorants and inert amendments. Either or both of these components may be used in any particular situation. 4.1. Ameliorants These are usually low-cost organic amendments used principally to dilute the toxic e€ects of the waste, or to produce some kind of a barrier to isolate vegetation from it. Ameliorants improve substrate conditions to allow the relatively rapid establishment of non-tolerant plants from seed. Organic materials such as sewage sludge, domestic refuse, peat and topsoil are typically used. These play three major roles if applied directly to mine waste: (1) improvement of the physical nature of the rooting medium, especially increasing water and nutrient-holding capacity. The latter is especially improved owing to the presence of cation-exchange sites; (2) provision of plant nutrients in a slow-release form, facilitating vegetation establishment; (3) complexing of heavy metals, so reducing phytotoxicity. As well as amendments, the addition of lime ± usually as ground limestone ± is customary on acidic wastes. Not only does this raise pH, but lime also renders metals insoluble, reducing their availability to plants (Down, 1975). Standard NPK fertilizers are also added in most cases. After sowing ameliorated waste with a conventional agricultural seed mixture, including a legume component, a productive sward is usually obtained fairly quickly. This is, however, often temporary. Following this promising start, sward regression usually occurs within a few years. Declining nutrient levels and the gradual lowering of organic matter status (Goodman et al., 1973) may be the cause. Regular fertilizer dress-

ings are required to counter this, but even these measures prolong vegetation persistence by only a few years rather than permanently. Regression is more frequent owing to the reappearance of heavy metal toxicity. This may result from the upward movement of soluble metal salts (if net water balance is unfavourable), the remobilization of metals when organic residues decompose (Johnson and Bradshaw, 1977), or root penetration of the toxic underlying substrate. Even if roots are restricted to the amendment layer, problems arise as the vegetation in e€ect forms only a ÔskinÕ over the waste and is thus subject to erosion if disturbance occurs. The problems of sward regression received much attention in relation to the revegetation of metal wastes in the Lower Swansea Valley. As well as the use of metal-tolerant grasses, as described earlier, much revegetation work has incorporated ameliorative techniques. Amendment trials were initiated (Weston et al., 1965); preliminary results involving Agrostis capillaris and other grasses were encouraging. This initially good establishment was followed by regression owing to heavy metal toxicity (UK Department of the Environment, 1994). Although vegetation establishment was only temporary, it succeeded in its aim of attracting new business development to the area. Some parts were, however, required to remain under permanent grassland, so a longer term experimental approach was adopted (Gemmell and Goodman, 1978). It was concluded from this that standard grass mixtures could be employed only if regular applications of organic matter were made after establishment. Otherwise the use of metal-tolerant grasses would be required, especially on the Zn tips which had a very high residual metal content. Sward regression owing to metal toxicity is usually terminal to the revegetation scheme, and so amendments alone should be employed on highly toxic metal tips only if metal-tolerant cultivars are used, or if only a temporary covering is required. 4.2. Inert covering materials Following the realisation that organic and topsoil amendments provide only a temporary solution, subsequent research has concentrated on the use of inert waste materials for the surface treatment of metalliferous wastes. In the UK this work was pioneered by the experiments of Johnson et al. (1977) at two Welsh Pb/Zn mines, Minera and Y-Fan. Experimental plot trials using a variety of amendments indicated that inert substrates, themselves de®cient in macronutrients, were more suitable for long-term vegetative stabilisation than amendments of inherent high fertility. The latter substrates support agricultural yields in the short-term, but regression is accelerated as roots rapidly penetrate the toxic substrate. In contrast, inert amendments may allow permanent reinstatement if a shallow-rooting unproductive sward with an appropriate grass/legume

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mixture is attained. This may initially require regular application of P-rich fertilizer to encourage legume growth. If a shallow-rooting sward is obtained, roots will not contact the underlying toxic spoil to any great degree. Also, coarse, granular inert materials act as a capillary break, which reduces upward migration of soluble heavy metal ions. The chance of heavy metalinduced phytotoxicity is therefore much reduced compared with organic amendments. Inert amendments must be locally available and of low cost, as they are required in very substantial quantities for large-scale schemes. Wastes from mining and quarrying operations ± such as non-pyritic colliery shale, slate quarry waste and limestone chippings ± are especially suitable. This approach therefore has the great bene®t of using one category of waste to overcome the problems of another form of waste (Bradshaw and Johnson, 1992). One drawback of this approach is that coarse materials may increase downward and lateral movement of heavy metal-laden water, and this has in one instance resulted in increased pollution of an adjacent watercourse (Palmer, 1990a). Such pollution may be reduced by covering the waste with polythene sheeting prior to application of inert material. The Minera and Y-Fan trials were reviewed 12 years after establishment, and a largescale revegetation scheme was implemented at Minera on the strength of their success. This involved capping large areas of Pb/Zn spoil with polythene membrane covered with a 500 mm layer of locally available colliery shale. Sowing with a standard grass/clover mix has resulted in very successful establishment, a vegetation cover of over 90% being achieved (UK Department of the Environment, 1994). Similar schemes have been implemented at other Welsh sites, including Goginan Pb mine where capping with a 300 mm layer of granular material and 200 mm subsoil has been undertaken (Palmer, 1990b). The inert covering approach may be even more valuable in arid climates ± where the potential for upward metal movement is much greater over longer periods (Palmer, 1990a) ± and on extremely phytotoxic tailings where even metal-tolerant plants cannot persist. Both inert amendments and soil have been used in the rehabilitation of tailings dumps at Captains Flat, NSW, which were the cause of extreme pollution problems. Direct establishment of a vegetation cover was thought to be impossible owing to high residual levels of Cu, Zn, Pb and Fe, coupled with extreme acidity (pH  2.8) and salinity (Craze, 1979). Tailings were sealed with a compacted clay covering, and 450 mm of shale and 300 mm of soil were then added. Although the expense of this scheme was considerable, vegetation establishment has been largely successful and the chronic pollution of a nearby river is now much reduced. The main drawback of the inert covering approach is the optimum depth of amendment, which may be higher than for other mate-

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rials, resulting in high application costs (Johnson and Bradshaw, 1977). This may be counter-balanced by the low cost of inert materials and their local availability. If such materials are not readily available, however, this technique is likely to be considered one of last resort ± as in eastern Canada (Watkin and Watkin, 1982) ± to be used only if other more economical techniques are inappropriate. 5. Combined approaches In some situations a combined approach ± involving a covering material sown with metal-tolerant cultivars ± may have advantages over any single conventional approach. A good example is the revegetation scheme at Parc Mine, North Wales. Pb/Zn mining had left a large tailings spoil tip, which became badly eroded after mine abandonment and caused severe pollution of the River Conwy and adjacent agricultural land. No single reclamation approach was suitable; covering material was too expensive to be used in large quantities, and direct establishment of tolerant material posed too great a risk ± as even small areas of failure would be likely to cause erosion centres (Firth et al., 1981). As a compromise, 75±100 mm of quarry shale was used to cap the tailings, before seeding with a mixture containing mainly Festuca rubra ÔMerlinÕ, with white clover as a legume component. An excellent vegetation cover developed following fertilisation, and still persisted in 1992 (Bradshaw and Johnson, 1992). Roots of the tolerant ÔMerlinÕ have penetrated the underlying spoil and e€ectively bound the shale amendment to the underlying spoil beneath. A visually acceptable vegetation cover has been created using this unconventional approach, and water pollution owing to Pb and Zn is now much reduced. Experiments  by Bergholm and Steen (1989) at Ammeberg Zn mine in central Sweden indicate that F. rubra ÔMerlinÕ produces a persistent sward in combination with the application of amendments such as topsoil, sewage sludge and coarse sand. After 10 years, ÔMerlinÕ thrived in all trial plots where regular NPK fertilizer had been applied. This success, undoubtedly aided by the high pH (7.3) of the waste, may allow large-scale revegetation of this site using a combined approach. 6. The ecological approach This is essentially an extension of the previously discussed procedures. It involves the application of ecological principles to enable successful revegetation. This approach is strongly advocated by Je€rey et al. (1975) who emphasise the importance of biological processes ± such as N-®xation, nutrient cycling, decomposition and other microbial processes ± in the

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Table 1 Approaches to revegetation (adapted from Williamson and Johnson, 1981) Waste characteristics

Reclamation technique

Problems encountered

Low toxicity. Total metal content: <0.1%. No major acidity or alkalinity problems

Amelioration and direct seeding with agricultural or amenity grasses and legumes. Apply lime if pH < 6. Add organic matter if physical and chemical amelioration required. Otherwise apply nutrients as granular compound fertilisers. Seed using traditional agricultural or specialised techniques

Probable commitment to a medium/ long-term maintenance programme. Grazing management must be strictly monitored and excluded in some situations

Low toxicity and climatic limitations. Toxic metal content: <0.1%. No major acidity or alkalinity problems. Extremes of temperature, rainfall, etc.

Amelioration and direct seeding with native species. Seed or transplant ecologically adapted native species using amelioration treatments where appropriate

Irrigation often necessary at establishment. Expertise required on the characteristics of native ¯ora

High toxicity. Toxic metal content: >0.1%. High salinity in some cases

(1) Amelioration and direct seeding with tolerant ecotypes. Sow metal- and/or salt-tolerant ecotypes. Apply lime, fertiliser and organic matter, as necessary, before seeding

Possible commitment to regular fertiliser applications. Relatively few species have evolved tolerant populations, and of those that have very few are available commercially. Grazing management not possible

(2) Surface treatment and seeding with agricultural or amenity grasses and legumes. Amelioration with 10±50 cm of innocuous mineral waste and/or organic material. Apply lime and fertiliser as necessary

Regression will occur if depths of amendment are shallow or if upward movement of metals occurs. Availability and transport costs may be limiting

Isolation. Surface treatment with 30±100 cm of innocuous barrier material and surface binding with 10±30 cm of a suitable rooting medium. Apply lime and fertiliser as necessary

Susceptibility to drought according to the nature and depth of amendments. High cost and potential limitations of material availability

Extreme toxicity. Very high toxic metal content. Intense salinity or acidity

development of a fully functional ecosystem on toxic mine waste. However, the previously discussed approaches also consider ecological factors, so this approach cannot really be considered as a new philosophy to revegetation. Certainly, the development of biological processes, especially nutrient cycling, is an integral component of all revegetation schemes which aim to achieve a low-maintenance vegetation cover. This reduces the need for repeated fertilizer applications and organic matter amendments. In the last few years the in¯uence of mycorrhizal symbioses on plants growing on metalliferous wastes has received attention. Mycorrhizal fungi may contribute to plant establishment on metal mine wastes by supplementing the nutrient absorption capacity of root systems and improving soil structure. These fungi may also contribute directly to plant establishment by binding metals to fungal hyphae, thus reducing translocation to shoots of infected plants. Mycorrhizae and other soil micro¯ora are absent from many mine spoils, however, owing to the lack of vegetation (Shetty et al., 1994). Revegetation may therefore be aided by a combined plant/microbe treatment. Experiments by Shetty et al.

(1994) in southeast Kansas showed that two grass species, Andropogon gerardii and Festuca arundinacea, failed to establish on Zn tailings whether or not mycorrhizae were present. It was thought that the low nutrient status of the tailings precluded contribution from the symbiosis. Concurrent research by Hetrick et al. (1994) at the same site indicated that both the above species had increased chance of survival and growth if organic matter was applied in combination with mycorrhizal fungi. The synergistic e€ects of this combined treatment allow maximum revegetation success. Although work in the ®eld is still in its infancy, the bene®cial e€ects of mycorrhizae may make inoculation an important component of future revegetation schemes. 7. Conclusions Public awareness of environmental issues is growing at an ever faster rate. The need to reclaim metalliferous mines to a high standard, particularly to meet pollution problems, is therefore even more critical now than in the past. Revegetation o€ers the best method for stabilising

G.M. Tordo€ et al. / Chemosphere 41 (2000) 219±228

waste tips and tailings dams, and is the only method capable of healing visual scars. Thorough planning is essential for successful revegetation, including physical and chemical analyses, bioassays and ®eld trials. The main approaches to revegetation are summarised in Table 1. The waste characteristics of a site determine which approach is most suitable. For wastes of high toxicity, however, two alternative questions must be posed: should the waste be seeded directly with tolerant cultivars or should a covering system be used and seeded with an agricultural mixture? If only a temporary but rapid vegetation cover is needed, covering the waste with organic material and seeding with productive species is recommended. If a permanent and low maintenance vegetation cover is required, direct seeding with metaltolerant cultivars may provide an economical and practical solution, and provide an e€ective pollution control. Unfortunately this approach leads to a restricted ®nal land use. If a waste is to be used for amenity or agricultural purposes then covering with an inert material may be more appropriate. This approach is especially attractive if inert materials are locally available. Although processing technologies are becoming increasingly ecient, with wastes less toxic, there is still the need for further research in revegetation science to give a more complete knowledge of the techniques available. At present, tolerant cultivars are available for use only in temperate regions. Their commercial development in arid and tropical regions is of paramount importance, as it is in developing countries with little ®nance for revegetation schemes that metal-tolerant plants may be of greatest bene®t.

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