R E S T O R A T I O N O F NATIVE BIOTIC COMMUNITIES ON LAND DISTURBED BY M E T A L L I F E R O U S MINING M.S. JOHNSON, BSc., Ph.D. and P.D. PUTWAIN, BSc., Phi).
Department of Botany, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, U.K.
ABSTRACT The worldwide expansion of the metalliferous mining industry has caused increased attention to be directed at the reclamation of mine wastes. Until recently the majority of reclamation procedures were based on substrate amelioration with innocuous covering materials, the application of fertilisers and the sowing of non-indigenous species. Reinstatement of the native flora and fauna was rarely attempted. However, in several countries the mining legislation now requires reinstatement of at least the native vegetation as a reclamation objective. Many mines are located in natural ecosystems such as tropical and temperate forest, and arctic and alpine tundra where faithful reinstatement of the native flora and, if possible, the fauna is the prime objective. Despite the adverse physical and chemical properties of metal mine wastes, the requirements for the restoration of native floras and faunas are not impossible to meet. This paper reviews procedures for the restoration of native vegetation on land disturbed by metalliferous mining, and assesses the success of the procedures where appropriate evidence is available. Using these methods it has often been possible to create a permanent, seN-perpetuating and relatively maintenance-free native vegetation which provides a diverse mosaic of habitats for native fauna to colonise. However, the development of methods for reinstatement of native biological communities on metalliferous mine wastes is in its infancy. Proper criteria have not yet been established for the assessment of the success of reinstatement and there is a paucity of published qualitative information on the success of restoration of native plant communities, particularly information covering a sufficient period of time to be ecologically meaningful.
arctic and alpine tundras and xerophytic desert shrub communities. Reinstatement o f these ecosystems has often been viewed as b e y o n d the scope o f normal mining reclamation practice, partly on economic grounds but also because remnant pits, waste rock, processing residues and overburden can present severe physical and chemical limitations to plant growth. Moreover, the need to safeguard the quality of land and water resources through .rapid vegetative stabilisation o f mine residues often conflicts with the timescale o f reinstatement techniques required to restore the original, native vegetation. These factors have meant that on a world scale reclamation systems based on substrate amelioration with innocuous cover materials, fertiliser treatment, seeding and planting o f nonindigenous species have received much greater attention 9 . Until recently, environmental protection and, wherever possible, .provision for agriculture, commercial forestry, amenity improvement and recreation have been the major reclamation objectives o f the mining industry 4~ . The attitudes o f mining companies towards the ecological approach to land restoration are n o w changing, as are the social and legislative requirements for reinstatement o f semi-natural vegetation as a reclamation objective. F o r example, recognition that former lead-zinc, copper and alluvial tin workings in Britain have, with the passage o f time, TABLE I World Production of metalst*
INTRODUCTION The recent worldwide expansion o f the metalliferous mining industry (Table 1), responding to a combination o f social, economic and technological factors, has caused increased attention to be directed at the environmental consequences o f mining and processing o f mineral ores. Many nations have recognised that past experiences o f pollution episodes were caused b y inadequate planning for environmental protection after decommissioning o f a mine. These nations have evolved legislation that requires submission o f intended land reclamation plans before consent is given to development proposals. Hitherto, reclamation procedures for disused metalliferous workings have rarely involved the faithful restoration o f the original natural ecosystem, even though a high proportion o f new mines in the major production areas o f North- and South-America, South Africa, Australia and SouthEast Asia coincide with ecologically important and receding tropical rain forests, temperate forests,
1945
1953
1961
1969
1977
3.9
13.7
28.5
53.0
79.6
Chromium ore
1.1
3 ~5
4.1
5.3
9.2
Copper
2.1
2.8
4.2
5.8
8.6
Iron ores
157
336
499
697
869
Lead (metal in ore)
1.2
1.9
2.4
3.1
3.3
Manganese ore
4.0
10.9
13.8
18.6
23.7
Nickel (metal in ore)
,0.15
0.2
0,37
0,5
0.8
Tin (metal in ore)
0.09
0.18
0.16
0.2
0.18
Zinc (metal in ore)
1.6
2.6
3.3
4.8
5.7-
Aluminium "(as bauxite)
tAppropriate total production in Mt *Adapted from (18) and (73).
68
Restoration of Land Disturbed by Metalliferous Mining
acquired significance as wildlife habitats (Table 2) has stimulated interest in planning new operations with a view to fostering the development of wildlife interest when mining eventually ceases a2 ,3s ,4s. Since reworking of residues discarded by former mining operations is now a prominent feature of metalliferous mining in Britain, restoration in this case commonly refers not to the original agricultural environment but to the wilderness a r e a s of high conservation value that have fortuitously developed on wastes from mining activity that ceased more than one hundred years ago. This is in marked contrast to modern operations in North America, Australia and Africa, the majority o f which are exploiting ore deposits beneath ecosystems not previously disturbed b y man.
Although the scale o f modern mining operations makes the task o f restoration even more formidable than in the past, technological advances within the industry have enhanced the prospects for successful reinstatement o f the original vegetation. Pre-mining plans that cater for topsoil conservation and replacement, nursery production of native species and the development o f long-term management programmes have made a useful contribution. Another significant development has been recent improvements in mineral processing technology. The exploitation o f lower-grade orebodies and the ability to recover more o f the valuable minerals from crude ores has reduced a major constraint to revegetation, namely phytotoxicity caused b y high residual metal concentrations. Serious growth limitations of mining residues still apply in some situations, but in many cases techniques have been developed to make waste rock dumps and tailings more amenable to direct establishment o f ground cover vegetation and w o o d y species 2s ,s6,46,4s. FACTORS AFFECTING THE APPROACH TO RESTORATION Solid wastes produced by-metalliferous mining are still recognised internationally as particularly difficult substrates on which to establish a vegetation cover. Complex specifications are often necessary for vegetation establishment, especially for demanding land uses such as agriculture and forestry, because of the unfavourable physical and chemical properties o f mine waste materials. The severity o f plant growth limitations varies with the origin and composition of the orebody, recovery objectives and techniques, and the age o f the waste in relation to recent developments in mineral processing technology (Table 3). Proposals to simply stabilise or landscape tips o f development rock, sub-marginal ore, overburden or mill tailings must also counter other constraints, notably adverse climatic conditions, if the remedial measures are to be permanently successful. Mill railings, which are usually deposited in specially constructed impoundments, and tips of submarginal ore that contain insufficient metal to justify, extraction are the most formidable materials.
TABLE 2 List of Odonata recorded on Porkellis Moor and other alluvial tin deposits in Cornwall, Great Britain, and their relative abundance at local, regional and national levels (32).
ANISOPTERA (Dragonflies)
oo ,. ~
~ o
m ~
z<
Aeshna cyanea
C
C
C
CS
Aeshna juncea
C
C
C
C
Anax imperator
O
C
F
FS
Cordulegaster boltonii
VC
VC
C
CS
Orthetrum coerulescens
t
F
F
FS
Libellula depressa
F
F
F
FS
Libellula fulva
t
R
R
R
Libellula quadrimaculata
O
C
C
C
Sympetrum striolatum
C
C
C
C
Agrion splendens
F
F
F
CS
Agrion virgo
C
C
C
FS
Ceriagrion tenellum
C
LF
O
O
Coenagrion pueUa
F
C
C
C
Enallagma cyathigerum
t
C
C
C
Erythromma najas
R
VR
VR
LF S
Isctmura elegans
F
C
C
C
Lestes sponsa
C
C
C
C
Pyrrhosoma nymphula
O
C
C
C
ZYGOPTERA (Damselflies)
Key 1. Refers to the county of Cornwall 2. Refers to Cornwall, Devon, Somerset and Dorset 3. Refers to Great Britain and Ireland t Species not recorded on Porkellis Moor but present on other alluvial tin areas C common F frequent O occasional R rare V very L locally Subscript S refers to species with predominantly southern distribution in Great Britain
M.S. Johnson and P.D. Putwain TABLE 3 Changes in the chemical composition of lead/ zinc/fluorspar tailings (1963-80) at Cavendish Mill, Derbyshire, England. Site name
Active period
Pbt
Znt
Glebe
1941 - 62
38000
18500
Cupola
1933 - 59
34000
3050
No. 1 & 2
1963 - 73
11000
,2+020
No. 3
1971 - 79
9400
1"460
No. 4
1978 -
9240
1470
tValues in mg/kg oven-dried tailings
Physical and chemical properties o f mine wastes Mill tailings consist o f small, uniform, mostly sandand silt-sized particles and they characteristically have a high bulk density and deficiencies o f organic matter, clay minerals and the micro-organisms that t o g e t h e r control particle aggregation and soil structure 7+. Inactive tailings dams are prone to wind erosion and also waterlogging due to poor infiltration and slow percolation through the tailings profile. Stockpiles o f low grade ore are similarly deficient in organic matter but they also have a high void: solid ratio and are subject to moisture stress through drought (Table 4). Chemical constraints on plant growth are particularly evident on wastes from mining o f ferrous and non-ferrous metal sulphide overbodies where problems of metal phytotoxicity, acidity, salinity and nutrient deficiency occur to varying degrees, In general, total metal concentrations above 0.1%, W/w for individual elements (eg. Cu, Zn) prove phytotoxic, but thresholds vary between metals and according to their chemical form, release characteristics a n d the accessory minerals present 9. Modem extraction methods produce
TABLE 4
69
wastes with concentrations of' this order, and from a metal toxicity viewpoint these are m u c h less intractable than spoils produced by older operations for which values o f 1 - 3% W/w, and even higher levels, are n o t u n c o m m o n in certain parts o f the world 39 . In recent years the problem of sulphuric acid regeneration by oxidation and hydrolysis of unextracted iron sulphide (FeS2) has received at least equal attention to metal toxicity. Countering potential acidity and the hazards it presents to the persistence o f established vegetation is a particularly complex problem on sulphide-bearing tailings. Many factors including indigenous carbonate : pyrite ratios and the reactivity o f different crystal forms influence the degree to which potential acidity is expressed in practice. Several counter measures have been investigated in South Africa and Australia, including limestone neutralisation, lime injection and artificial irrigation systems 7 ,as. Interactions between the products of pyrite weathering and native carbonates (eg. MgCO3 ) can be a source o f high salt concentrations. High salinity is a c o m m o n feature o f m o d e m tailings and conductivities in the range 4 - 16mmhos/cm have been reported, due to chemical interactions, concentration o f naturally occurring salts in recycled process liquors or adjustment o f effluent pH prior to discharge ss . High salt concentrations in the surface layers o f tziliugs are "prominent in arid climates where t h e y are a major problem for plant establishment 22. In contrast to residual metals, acidity and salinity, all o f which occur in excessive concentrations, nutrient status is a factor in mine waste revegetation because o f deficiencies. Low levels o f nitrogen and phosphorus are universal featurds o f mine tailings and waste rock, and amelioration o f low fertility with respect t o these and sometimes other macronutrients such as potassium, calcium and magnesium, is an integral part o f restoration work 9 .
Major plant growth limitations of metal mine wastes
Texture and Structure
Stability
Water Supply
Nutrient Status
pH
Toxic Metals
salty
Overburden
1- 3
1- 2
1- 2
2- 3
1- 3
1
1
Waste rock
4
1- 2
4
4
1- 4
1- 4
1-3
Tailings
3
1- 4
2
4
1- 4
1- 4
1-4
Waste Material
Severity of Problem insignificant
,slight
moderate
Severe
1
2
3
4
70
Restoration of Land Disturbed by Metalliferous Mining
Topsoil conservation The broad spectrum of limitations to plant growth and landscape reconstruction, combined with the more exacting requirements of mining companies by development control agencies, has gradually led to changes in attitudes towards revegetation work. Nowadays restoration proposals are often built into the mining programme from the planning stage, and operational changes have paralleled this development. For example, identification, careful handling and stockpiling of non-toxic mine residues are now practised because these materials are often suitable for treatment of more hostile waste products. Application of a coveting layer of topsoil, subsoil or overburden (100-1000mm), with or without a barrier or isolating layer of clay or development rock (50-300mm) that prevents upward migration of acidity and metal salts, is a valuable technique for avoiding the growth limitations of toxic metalliferous wastes 2 . However, this ref'mement of earlier coveting techniques is comparatively expensive and not always practical, so future applications may be restricted to extreme situations such as the pyritic Pb-Zn-Cu railings of the Captain's Flat mining area near Canberra in New South Wales, Australia t6 Methods of excavating previously undisturbed soil and overburden overlying mineral deposits, together with stockpiling and replacement methods, have received attention for several reasons. The need to maintain the physical and chemical attributes of soil during handling operations is self-evident and elaborate precautions to ensure a minimum of deterioration in soil quality have been proposed ~7 . These have been developed mainly for shallow surface workings, strip mining and open pit developments in relation to agricultural or forestry afteruses. Double stripping of topsoil (0-150mm) and subsoil (150-300mm) layers, the exact depth of which must be confirmed by soil surveys based on horizon patterns and differentiation, is an expensive but important step in the mining programme. This avoids mixing of different layers and deterioration in topsoil quality, particularly if stock-piling time is minimised or even eliminated
by immediate replacement of the soil layers in reverse order on worked out areas or abandoned waste disposal sites. Even more sophisticated stripping and handling techniques may be crucial to the re-establishment of the original ecosystem in areas of semi-natural vegetation. In most cases over 90% of viable native seed and vegetative propagules occurs within the top 2 - 5cm of the soil prof'de 7~ and this layer must be removed separately to prevent deep burial and dilution by sub-surface horizons. Stripping of this seed-rich layer should be followed by immediate replacement elsewhere to avoid losses of viable propagules that occur due to physical and chemical alterations under stockpiling conditions. Trials have shown that natural regeneration of legumes and proteaceous species forming the shrub layer of jarrah (Eucalyptus marginata) forest overlying bauxite deposits in the Darling Range, near Perth, S.W. Australia, is greatly assisted by multiple stripping of topsoil layers. Seedling derisities recorded after eighteen m o n t h s 7~ distinguished clearly between the double-stripped and immediate replacement treatment (12.5/mZ), stockpiling for two years before replacement (1.5/m 2) and deepstripped material (3/m 2 ).
Native seed Topsoil conservation is important but on deep mining sites this can rarely be used as a preliminary step in replacing the original vegetation cover. In these circumstances greater reliance is placed on collection, treatment and reseeding of native species, and on transplanting of species that do not readily establish from seed. Novel methods of seed collection have been developed on the grounds that indigenous material will be adapted to the local climate, and to compensate for difficulties in obtaining native species from commercial seed suppliers. Sauer 6a advocates vacuuming of established plants, litter layers and surface mineral horizons using a vehicle-mounted collector; but the timing of seed harvesting is critical and the method must be related to the results of detailed sampling and analysis of litter and soil profiles (Table 5).
TABLE 5 Native seed collection techniques and their disadvantages Method
Examples
Disadvantages
Manual
(20) (61) (62)
Slow and expensive but often the only method available
Collection of seed-bearingshoots and branches
(47) (61)
The optimum time for seed collection is sometimes difficult to establish and varies between species; temporary visual damage may result
Conservation and replacement of topsoil with its indigenous seed bank
(70)
Multiple stripping is expensive; only some native species will regenerate readily
Vacuuming of plants and soil surfaces
(47) (63)
Difficult to execute and only suitable for certain species
M.S. Johnson and P.D. Putwain Seed harvesting is only the first stage in a sequence of events that precede resowing. Immediate sowing may be appropriate, but if storage is necessary, conditions of temperature and humidity are usually critical. Treatments may be required to improve rates of germination and establishment. These vary from extraction of encapsulated seed by exposure to sunlight or heat (eg. Eucalyptus. spp. and Banksia. spp), to dormancy breaking measures. Direct sowing increases the risk of damage, predation and dispersal in the interim period until natural breakage of dormancy occurs. Low temperature stratification is necessary, for example, in the case of most Juglandaceae and Pinaceae, .and must be synchronized to proposed seeding dates. Scarification to increase seed coat permeability may be carried out by mechanical methods, hot water or short term treatments with concentrated acids, and is essential for dormancy breaking in Acacias and many other legumes. Lewis47 provides an account of suitable practices for the Australian flora, and similar studies have been made of wild and cultivated species in the United States 64 7s and of indigenous and introduced trees and shrubs in Britain 1 .
TABLE 6 Advantagesand disadvantages of native vegetation establishment Advantages
Disadvantages
A self-perpetuating,low maintenance vegetation results: long term commitments to irrigation, fertiliser treatment etc., will be unnecessary
Commercialsupplies of native seed may be restrictive
The vegetation will be adapted to climatic conditions
Specialised ecologicalknowledge is required of the native flora and fauna
Aesthetic and landscape conflicts with the adjacent areas can be minirm'sed
Re-establishment of native vegetation can be a slow process. There is an increased risk of soil erosion. The visual impact on the landscape may be slow to disappear
The potential exists for wildlife recolonisation and faithful redevelopment of the original ecosystem
Soils disturbed by mining and mineral waste products present physical and chemicallimitations to growth
Economic returns on the Permanent destruction of capital outlay are impossible large areas of already declining and internationally important ecosystemswill be reduced
71
Cuttings and Transplants Establishment of woody species'from cuttings and rooted transplants is now a common landscaping practice, particularly in aesthetically sensitive areas. In non-toxic conditions, direct planting and fertiliser amelioration may be all that is required but in other situations planting into layers of imported cover material or pockets of topsoil is essential to establishment and long term s u r v i v a l 3 9 . However, growth rates in waste rock dumps and tailings rarely compare with normal situations because roots eventually penetrate to the interface between the planting medium and the underlying spoil. This applies particularly to wastes from sulphide-bearing ore bodies which provide, a marked contrast to the innocuous substrates of, for example, iron ore and bauxite mining operations 9 . Other large or slow-growing species that are difficult to establish from seed have been successfully transplanted on to wastes produced by modern mines, a particularly notable example being that of Cyprus-Pima copper mine in Arizona, U.S.A., where native cacti have been used to stabilise barren tailings slopes 48 . At this particular site the extra cost of transplanting 10cm 2 vegetative plugs of the grass Cynodon dactylon at 300mm centres has also been justified by subsequent performance, when compared to establishment from seed broadcasting at 10kg/ha. Similar large scale planting of native grasses is used to stabilise gold mine sands and slimes in South Africa 14 .
Practical considerations Increasing interest in locally indigenous species as a basis for mine waste revegetation can be ascribed in particular to t h e economic attractions of establishing a self-perpetuating and maintenancefree vegetation compatible with the climatic conditions. However, this approach has both advantages and disadvantages (Table 6). Since metalliferous mines occur in most parts of the world, covering extremes of temperature, moisture, daylength and exposure, these factors are vitally important considerations, particularly in arctic, alpine, arid and semi-arid regions. However, it is not always practical to rely entirely on native vegetation establishment for landscaping requirements and environmental protection. Many of the three hundred or more species native to the arctic tundra ecosystems of Alaska and other'northern regions of the United States, where more than 3400ha have been disturbed by mining, are very slow growing, poor or unpredictable ~seed producers and often reproduce entirely vegetatively. Only recently has transplanting and containerised production . developed o n a commercial scale 12 . Hitherto, reliance has been on substrate amelioration and seeding of commercial varieties of non-native grasses and clovers b u t the ability of native and naturalised species of subarctic landscapes to persist, despite adverse conditions of climate and frequent fires, is clearly essential to the long term success of revegetation schemes.
72
Restoration of Land Disturbed by Metalliferous Mining
Species native to the arid and semi-arid climates of the western United States and m u c h o f Australia possess adaptations that minimise water loss; for example, narrow leaves (eg. Tamarix spp.), absence of leaf blades (eg. Acacias), waxy surfaces (eg. evergreen Eucalypts); and root systems that enable moisture to be retrieved from depth (eg. Acacia raddiana and Prosopis ]uliflora). Even on waterretaining tailings, mesophytic vegetation may regress unless expensive artificial irrigation is maintained indefinitely, so in arid areas restoration with native xerophytes is economically sound as well as ecologically appropriate. The same principle applies for different reasons in tropical lowland climates; for example, in Papua New Guinea where tailings stabilisation using non-native grasses is impractical and planting of the native Eucalyptus deglupta and Terminalia brassii has been adopted instead 26 . Although one o f the aims o f using native species is the production o f a low maintenance vegetation, aids to establishment are commonly used in severe climates or for hostile wastes. Special irrigation measures are rarely necessary when total rainfall exceeds 750mm/annum, provided that the seasonal distribution is not too pronounced, but in arid zones innovative planting methods, surface manipulation or irrigation are often essential to germination and establishment. Evaporationretarding surface f'rims, condensation traps, supplemental root transplants and tubeling transplanting methods al have been shown to improve water relations for tree and shrub establishment, and both o~,erhead sprinklers and trickle irrigation have enhanced establishment and growth under moisture stress conditions s 7 (Fig. 1). Development of ground cover vegetation on steep surfaces that are prone to water erosion has
been accomplished by physical stabilisers such as brushwood matting overlays, windbreaks and various protective fabrics 5 ,a4, brushwood having the advantage o f acting as a seed source if collected in the right condition z4 . In South Africa, reed fences established as a network o f small paddocks have greatly reduced wind erosion and promoted seedling establishment as (Fig. 2). In recent years, mulches and stabilisers have been used to an increasing extent, particularly on erosion-prone surfaces where special techniques such as hydroseeding are employed. Mulching with chopped hay or straw, wood chips, bark or plant litter at 1-4t/ha reduces evaporation and leaf temperatures, and can improve germination and establishment. Soil stabilisers that reduce erosion losses o f mulch and seed in exposed conditions are commonly used additives for the steep, angular walls of railings impoundments but their performance varies and careful selection o f the most suitable product is essential. Inhibited germination and very short teroa stability have been reported for certain latex, bitumen and plastic compounds 6s . The feasibility o f reconstructing the original flora and fauna following mining varies, but there are still fundamental issues where disagreement exists on the approach to semi-natural vegetation establishment even when such an endpoint is theoretically possible. The most basic of these arguments concerns the need for reclamation to meet the dual objectives of environmental protection and faithful replacement of the native vegetation cover. In the severe conditions o f the metal mine environment, the timescale over which natural colonisation occurs is considerable, even allowing for changes in the pioneer role of different species that can accelerate development
Fig. 1. Overhead spray irrigation during establishment of a grass sward on the retaining wall of a gold slimes dam (from Bradshaw and Chadwick, 1980).
Fig. 2. Use of reed fences as an aid to plant establishment on sand dumps produced from gold mining (from Bradshaw and Chadwick, 1980).
M.S. Johnson and P.D. Putwain
73
IRON and ALUMINIUM The economic benefits of a maitatenance-free, selfperpetuating vegetation cover of native species are gradually being appreciated; for example, in the contrasting iron-ore country of the arid Pilbara region of Western Australia and the bauxite mining area on the Weipa peninsula of Queensland, northern Australia. Neither of these operations produces chemically hostile waste materials nor physical landforms that preclude vegetation establishment. Restoration plans in the Pilbara region have evolved through co-ordination of expertise to develop ecologically appropriate rehabilitation methods compatible with the arid climatic conditions. Joint research on re-establishment of native vegetation has been undertaken by Hamersley Iron Ltd., Mt. Newman Mining Co. Ltd., and Texasgulf Australia Ltd., and particular attention has been given to the alternatives of OLDER METAL MINE WASTES immediate establishment of the native climax Attempts have been made to reproduce the plant vegetation or an earlier successional stage s~ The species composition of local ecosystems on older climax association of the semi-desert, savannah metalliferous mine wastes by direct seeding or grassland is dominated by spinifex (Triodia spp, planting without amelioration. This approach has and Plectrachne spp.); but the development of failed consistently on Pb-Zn mine waste deposited spinifex communities is a very slow process even on the limestone grasslands of temperate Britain 37 , though the mine waste xerosere is similar to that of and on untreated Cu - Ag - Pb - Zn residues in natural talus slopes with sub-climax herbs and the sparse Eucalyptus brevifolia woodlar/d grasses, such as buffel (Cenchrus ciliaris) and surrounding Mount Isa mine in monsoonal, semi- birdwood (C. setigerus), being the main pioneers. Since an accelerated succession is basic to dust arid Queensland, Australia 22 . However, a compromise on direct establishment of native control, in that the climax community fulfils this species in toxic situations has been achieved. role most effectively, studies have been made of In Britain, for example, populations of the germination, establishment and survival of Triodia grasses Festuca rubra and Agrostis tenuis naturally spp. introduced by direct seeding and by the colonising Pb-Zn or Cu dumps have evolved replacement of topsoil that contains viable seed on tolerance to the metals in their native spoil, and mine waste and overburden. This approach overthey also possess adaptations to low nutrient comes the climatic constraints on agricultural land status and moisture stress 33,66. This phenomenon uses and provides a revegetation endpoint in the surrounding landscape. has been exploited through the development of harmony with metal-tolerant cultivars for restoration work. Intra- Alternative methods using species introduced from specific variation has been used for similar homoclimes or irrigation maintenance of tropical purposes in Zimbabwe where tolerance to salinity and temperate grasses and legumes have been as well as Cu-As has been reported in Acacia discarded because they can become invasive plant karoo 29, Cynodon dactylon and other grasses28 . pests and the latter have high maintenance In north-west Queensland seed of native species commitments and are incompatible with the colonising mineralised outcrops and old mine surrounding vegetation. Even though the skeletal soils of this mining dumps has been considered for the r'evegetation of the more hostile of the modern non-ferrous metal area are typical of the arid tropics by being difficult to separate and stockpile before mining, tailings dams and waste rock dumps 62 . Tolerances to these and other metals also occur they are valuable sources of native seed and organic in tropical and sub-tropical species yet the mulch (eg. broken woody material and spinifex probable development of further metal-tolerant straw). Soil conservation is therefore an important varieties by plant breeders can provide only a prelude to later restoration work 6 . Regrading, deep partial solution to replacement of the original ripping to improve water inf'dtration and root semi-natural vegetation. Very few species have the penetration, and surface cultivation of the mine genetic basis for evolving metal-tolerant ecotypes waste must precede topsoil placement, seeding and so the opportunities for faithful reproduction of fertilising. Seed collected by manual or mechanical complex grassland, scrub and woodland ecosystems methods, and seed-laden branches of woody on long-since disused metal mine sites are negligible. species, are used to supplement or substitute for From this viewpoint it is fortunate that on a world the topsoil propagule and native seed bank. scale mining operations with a large land take have Further studies are investigating the effects of mainly been commissioned or expanded during the fertilisers on natural succession. However, there is last twenty years during which ore processing evidence that for direct re-establishment of the climax spinifex grassland and woody species such methods have advanced considerably. of the vegetation towards a climax. Seeding or planting of native species cannot always achieve the rapid stabilisation required to overcome dust dispersal and water erosion. Consequently, a combined agronomic-ecological approach has often been used on the assumption that swards of agricultural and often non-indigenous grasses and legumes will undergo natural invasion by native species according to the subsequent management programme. This can be successful but native species are not always aggressive invaders and changes in the species composition of the vegetation cover can be slow to occur. This approach is particularly questionable where the objective is to provide habitats for native wildlife including sedentary species with specific foodplant requirements19
Restoration o f Land Disturbed by Metalliferous Mining
74
TABLE 7 Summary of the mining sequence and revegetation programme at the Weipa bauxite mines in N. Queensland, Australia t" (a) Mining sequence (i) Native vegetation is pushed over, windrowed and burnt (ii) Topsoil overburden is scraped off to a depth of <60cm, usually by single stripping, and then relaid on adjacent mined out areas (iii) Exposed bauxite ore is mined down to 1-3 metres and hauled to the processing plant by road (iv) The bauxite is graded and washed prior to shipment, and the tailings wastes pumped to lagoons on the Weipa peninsula (v) Relain overburden is carefully ripped at 3m intervals to a depth of 2m to improve drainage and root development (vi) Re-establishment of vegetation commences at the start of the wet season using nursery.raised stock or direct seeding techniques. (b) Final uses of restored land (March 1980) - hectares Native, open forest woodland Plantation timber Tropical pastures Agricultural crops Amenity planting Research trials Roads, township, mine plant and tailings dams Total area mined
985 490 209 102 134 205 931 3056
(c) Native and locally indigenous species planted or seeded to re-establish forest woodland t
( Casuarina littoralis ) Black Oak ( Casuarina equisettfolia ) Beach She Oak Acacia crassicarpa) Golden Wattle (Acacia aulococarpa) Black Wattle (EucaLyptus tetrodonta ) *Darwin Stringybark (Eucalyptus nesophila ) *Melville Island Bloodwood (Eucalyptus polycarpa) *Large Fruited Bloodwood ( Grevillea glauca) Bushrnans Peg Weeping Tea Tree (Melaleuca leucadendron ) (Melaleuca dealbata) Fine-seeded Tea Tree (Melaleuca quinquenervia ) Broadleaf Tea Tree (Melaleuca viridiflora) Dwarf Tea Tree (Alstonia actinophylla ) Milkwood ( Wrightia milliga) Milky Bean ( Callitris collumelaris) Cypress Pine (Erythrophleum chlorostachys ) *Cooktown Ironwood Swamp Mahogany [ Tristania sauverleins) *Principal species of the native woodland tAdapted from (3) and (51)
as Acacia spp. and Cassia spp. fertilisers are important substitutes for the ameliorative effect on soil nitrogen status of the numerous legume pioneers that would intervene if natural succession had been allowed to occur. The bauxite mining lease of the Commonwealth Aluminium Corporation Ltd., at Weipa on the North-west coast of Cape York peninsula in far north Queensland covers some 2560km 2 , and it is predicted that 6000 ha o f land will be mined by 1988. The rich bauxite ore contains 52 - 60% alumina and occurs as flat or gently-dipping lateritic d e p o s i t s of variable thickness (1 - 9 m ) less than 60cm from the soil surface. The native vegetation is open forest dominated by Darwin stringybark (Eucalyptus tetrodonta) and other eucalypts 6s , together with small areas o f closed rain forest around freshwater swamps and streambeds. Mining laws and agreements accompanying the Weipa operations involve a commitment to revegetation that has developed on a multiple land use system covering plantation timber, tropical pastures for livestock and arable land; but predominantly replacement o f the original native forest (Table 7). Reinstatement o f native forestwoodland is compatible with the adverse soil conditions and seasonal changes in the tropical climate, but this approach is only partly based on vegetation ecology. Cultural customs are also a major consideration in that the open woodlands are hunting areas for aboriginal tribes because they provide important habitats for native wildlife. The multiple land use approach to revegetation has been developed partly to protect the intrinsic value o f the native forest, b u t also to introduce a secondary agricultural industry that will provide security for the long term residents of Weipa as . Faithful replacement of the forest ecosystem is difficult to achieve but the policy is to re-create the woodland cover as closely as possible by a combination o f topsoil conservation and replacement, seeding and planting of native species. The sequence of operations from mining to revegetation (Table 7) includes (a) stripping o f topsoil overburden to a depth o f 50-60cm in order to conserve buried seed and vegetative propagules, and (b) supplementary mechanical seeding and planting of indigenous trees using equipment modified or specially constructed for the project. The scale o f native woodland regeneration (some 214ha were re-planted in 1979-80) has been accomplished partly through nursery production o f over 100,000 tube seedlings a year o f some forty species, sixteen o f which are native and mostly locally indigenous trees (Table 7). In other bauxite mining areas of Australia, such as the Gove peninsula in Northern Territory, greater reliance is placed on the native topsoil seed bank supplemented where dilution by underlying soil has occurred, by broadcasting o f seed collected manually from surrounding woodlands 3~ In this situation rhodes grass (Chloris gayana) is sown immediately after the earthmoving operations, as a stabilising cover crop that does not inhibit subsequent development of the indigenous flora.
M.S. Johnson and P.D. Putwain Land preparation for woodland regeneration in these areas has been researched since 1957 and although plant growth is not subject to the severe toxicity problems of m a n y mine wastes, the nutrient-poor lateritic red earths require careful management to achieve a satisfactory performance s~ . Regrading o f the topsoil overburden on mined areas is followed by deep ripping which extends into the underlying ironstone and then by ploughing with off-set discs followed by other surface cultivations to improve the physical conditions. Care is sometimes taken to ensure a minimum of admixing between topsoil ( < 5cm) and subsoil (5 - 60cm) material and to replace these in the correct sequence in order to maximise conservation o f the seed bank a~ . Establishment o f trees from seed, which is favoured on economic grounds, is accomplished by (a) direct sowing and simultaneous fertiliser application, (b) mechanical broadcasting by dispensing and spinning pelleted or untreated seed and fertiliser from a large hopper, or (c) by pneumatic planting a . Fertiliser treatment to overcome nutrient deficiencies and induced fixation o f phosphorus involves application o f 400g of general fertillser (5% N:7%P:5%K) around the base o f each tree at planting. This is supplemented by simultaneous applications o f the micronutrients copper and zinc s2 . The influence o f fertiliser treatments on re-establishment and the composition o f the native ground vegetation is largely unknown but ecological studies currently being undertaken are examining this important aspect and the extent to which the native reptiles, mammals and birds o f the undisturbed woodlands are re-colonising the reinstated landscapes s~ . Very recent investigations o f wildlife in rehabilitated areas of Jarrah forest, based on mark-release-recapture studies o f mammals, mist netting o f birds and both TABLE 8 Constraints on agricultural and commercial forestry land uses* at Groote Eylandt manganese mine in N. Territory, Australia. (a) Pastures for beef cattle 0) Fire hazards during the dry season (ii) Invasion of native species unless intensive fertiliser management is continued (iii) Uncertain economics of beef production (iv) Only small areas (10ha/annum) are suited to pasture establishment (v) Hygiene problems in slaughtering (vi) Cost of establishing watering points, handling yards
75
observation and opportunity collecting, have shown that recolonisation b y , n a t i v e vertebrate fauna is closely related to the stage o f vegetational development and to the availability of food, shelter and suitable breeding areas 44 MANGANESE and NICKEL The manganese mining scheme o f Groote Eylandt on the Gulf o f Carpentaria in Northern Territory, Australia illustrates an important yet for technical reasons u n c o m m o n development in the restoration o f semi-natural vegetation on areas disturbed by mineral working. This site is within the Arnhem Land Aboriginal Reserve and the c o m m i t m e n t to re-establishment o f the native flora encompasses the lagoons o f metalliferous tailings as well as the mining areas themselves. The native vegetation o f this hot, humid region is tall, open forest with smaller areas of monsoon forest and the edaphic complexes o f freshwater swamp, quartzite sandstone and coastal dunes 6a . T h e open forest formation consists mainly o f E. tetrodonta, though this can be co-dominant with Darwin woollybutt (E. miniata), and an understorey o f Acacias, wild plum (Buchanania obovata) and Red Kurrajong (Brachychiton paradoxum). Various legume species and grasses dominate the ground cover vegetation, particularly in the wet season. The initial approach t o restoration was the establishment of beef cattle and forestiny enterprises but because o f the problems encountered (Table 8) the objective has recently changed to reinstatement o f the native vegetation. Climatic constraints on the growth of non-native species have led to a policy of topsoil conservation and supplementary seeding as a basis for regenerating native vegetation in disturbed areas. A double-stripping system for topsoil (0 - 20cm) and overburden is used, and restoration is continuous. As the mining development advances topsoil stockpiling is virtually eliminated and m a x i m u m re-establishment occurs from buried seed and vegetative fragments. This source o f propagules is supplemented by broadcasting manually collected seed o f the major open forest species at the beginning of the wet season. The restoration sequence for mined land, as opposed to tailings deposition areas, is outlined in Table 9. TABLE 9 Summary of the mining sequence and native, open-forest* revegetation programme at Groote Eylandt manganese mine in N. Territory, Australia
(b) Commercial forestry (i) Poor establishment and growth of introduced plantation species (ii) Invasion of plantations by Acacias (iii) Fire hazards (iv) Destruction of introduced species by termites (v) Uncertain cost-benefit economics (vi) Liability to damage by tropical cyclones
(i) Clearance of open forest woodland overlying ore body (ii) Separate stripping of topsoil (10-30cm) and overburden, usually for immediate replacement on a mined-out area backf'flled with clays, laterite and coarse tailings. The surface is ripped to a depth of lm before topsoil replacement. ('tii) Late in the dry season seed of the major (canopy) species of the native open forest is collected for broadcasting at the start of the wet season (November). Oversow with rhodes grass {Chlorisgayana) at 5 - 8kg/ha and apply NPK fertiliser at < 200kgpaa
*Adapted from (21).
*Adapted from (21).
76
Restoration of Land Disturbed by Metalliferous Mining
Restoration o f disused tailings dams is at an early - for chalc.opyrite (CuFeS2) and precious metals that stage b'ut t h e revegetation potential of this waste occur in association with b o m i t e and pyrite. material, which is innocuous when compared' with Under the terms of the mining agreement, waste many non-ferrous metal railings, is already rock is deposited against the valley walls in the apparent. Many locally indigenous grasses (eg. 'Upper Kawerong and railings are discharged in~o Schizachryium fragile and Eriachne schultziana) the Kawerong River, a tributary of the Jaba .ri~er and Acacias (eg. A. oncinocarpa and A. latescens) that flows into Empress Augusta Bay or~ the west are volunteer colonisers o f inactive areas of the coast of t h e i s l a n d . During dispersal the tailings lagoon, a n d there is clear evidence that natural or segregate and form titally washed fiats which assisted regeneration of native open forest will be already cover some 900ha 27 . feasible without t h e complex amelioration The 8100ha lease area involved in waste techniques commonly required on railings disposal, comprises lowland tropical sWamp forest surfaces 2 ~. of open-canopied Campnosperma-Terminalia, Invasion of native trees and shrubs into success- together with Leersia-Hanguana or Phragmitesful as opposed to deteriorating non-indigenous Saccharum open swarnp2S. The revegetation swards has also been responsible for changes in strategy, tailored to the needs of this unusual waste revegetation policies. Tailings restoration work disposal system, involves forestry development carried out by the International Nickel Co., at compatible with the surrounding swamp ecosystem. Copper Cliff, Ontario, Canada illustrates this Despite elevated copper (400 - 1400mg/kg) and development. Initially, abandoned tailLngs surfaces sulphur (1000 - 3000mg/kg) concentrations in the were limed, fertilised a n d b r o a d c a s t with an tailings, and the problem of acidification from the agricultural grass mixture of species able to persist weathering of sulphides which produces a stable under local conditions and suited to management pH of 3.5 - 4.5 after 3 - 4 years if left by cutting for hay (eg. Poa compressa, Agrostis undisturbed and aerated, considerable progress has alba and Festuca arundinacea). Subsequent been made in re-establishing native swamp-forest invasion by indigenous trees, invertebrates, birds trees on the more stable zones of the delta. This and mammals, coupled with the attractions of success is probably due to the absence of a developing a maintenance-free vegetation, have significant acidification problem in the constantly changed the management emphasis away from hay waterlogged conditions. production towards the development of wildlife Growth of self-sown seedlings of the mangrove, interest s s ,s 9 Sonneratia caseolaris, a species tolerant of freshRecords indicate that volunteer invasion by water and saline conditions, has been far more locally indigenous plant species is a precursor for rapid than that of other native but not locally the re-establishment of a diverse invertebrate fauna indigenous species including Bruguiera gymnorrhiza and that the range and numbers of the resident and Rhizophora apiculata. Moreover, some 250ha avian and mammal fauna, migrating waterfowl and of the total 385ha comprising the main delta have shorebirds have responded accordingly s9 . Wildlife been invaded by the native salt water couch colonisation is being encouraged by developing the (Paspalum vaginatum) and Phragmites spp. Revegesite as a "Wildlife Management Area" in tation of the mine access road margins has been conjunction with the Ontario Ministry of Natural even more successful with a diverse, indigenous Resources, and by the appointment of a company flora dominated by the nitrogen-fixing pioneer biologist who is responsible for breeding bird Casuarina gracillimum developing unaided within census work, ringing programmes and experimental five years of construction through seed dispersal investigations into modified restoration techniques from the adjacent forest. There is clear evidence of compatible with wildlife conservation. In the strict succession towards a forest vegetation, seedling sense this case history differs from those described trees having established within the dense ground previously, in that no real attempt has been made cover of grasses and vines. This recolonisation of to faithfully restore the native vegetation cover. potentially hostile wastes is unusual in that it is However, the current intention to maximise the proceeding rapidly and largely unaided by human wildlife value of disused tailings dams, which are intervention. usually in remote and undisturbed areas, suggests Revegetation of wastes produced by mining of that restoration of semi-natural vegetation may be non-ferrous metal ores has been equally successful a primary objective in the future. at the Pima mine, south-west of Tucson in Arizona, U.S.A. This mine in the arid Arizona-Sonora desert COPPER, COBALT, MOLYBDENUM region is jointly owned by Cyprus Mines Corp. and and URANIUM Union Oil of California and Utah International Equally successful restoration of semi-naturai Inc., and strenuous efforts have been made to vegetation has been carried out on waste rock and restore native vegetation on disturbed areas. tailings generated by m o d e m , non-ferrous metal However, the principal successes have notably been metal mines; but in spite of advanced ore processing achieved on the relatively innocuous wastes methods adverse chemical conditions for plant comprising the retaining wails of the copper growth still exist to varying degrees and compound tailings dams. Pressures to maintain the natural splendour of climatic and ecological constraints. The BougainviUe Copper mine at Panguna on Bougainville Island, the low desert environment have greatly increased Papua New Guinea operates an open-cut working since the mine was commissioned in 1957, partly
M.S. Johnson and P.D~ Putwain
17:7
Fig. 3. Establishment of native species on the retaining walls of a copper tailings dam at Cyprus Pima Mine, Arizona, U.S.A. (from Bradshaw and Chadwick, 1980). because of several expansions in the development, and encroachment towards the inter-state highway linking Tucson with Nogales. Vegetative stabilisation of the railings dam walls against :erosion is a difficult task in the arid, desert environment and emphasis has been placed on re-establishment of native trees, shrubs, cacti and grasses. Trials have also been conducted with species from homoclimes in Africa, South America and Australia 48 . Stabilisation has been accomplished by seeding, transplanting and amelioration of salinity and plant nutrient deficiencies. The hydroseeding technique, in which seed, fertiliser, mulch and stabiliser are applied simultaneously, under pressure, from a water-filled, agitated tank t h r o u g h a narrow-bore nozzle, has proved particularly useful in this situation. Use of heavy machinery on tailings dam walls is restricted and hydroseeding has provided the access necessary to preYent wind erosion. Although both introduced and indigenous species were sown initially, the success of native shrubs such as desert broom (Baccharis sarothroides), creosote bush (Larrea tridentata) and hopseed bush (Dodonaea viscosa), as well as weeping lovegrass (Eragrostis curvula) and bermuda grass (Cynodon dactylon), has been prominent (Fig. 3).
Attention has also been given to the re-establishment of cacti and other perennials of the desert environment. To overcome technical difficulties of establishment from seed, this has been achieved by transplanting from within the disposal lagoon where established plants would otherwise have been destroyed by encroaching tailings. Many species including ocotillo (Fouguieria splendens)~ barrel cactus (Echinocactus grusonii), giant yucca Yucca thornberi) and of particular importance saguaro (Carnegiea gigantea) - t h e ,giant native cactus that carries the Arizona state flower - have been re-established on barren tzil~ngs slopes. It is clear that under such hostile conditions reestablishment of, the native flora is the only, logical approach to revegetation unless arti,ficial irrigation and regular fertiliser treatment s are utilised. Moreover, the economic benefits o f a maintenancefree, native vegetation coincide with the aesthetic requirement to maintain the natural .character of this desert region. However, t h e experience o n the relatively innocuous tailings slopes at Pima.hasnot been repeated o n the more hostile m ~ taillrtgs o f copper mines elsewhere in the arid zones of the United States, Pronounced salinity, lack of major . nutrients, metal toxicity and in some eases acidit3/
78
Restoration of Land Disturbed by Metalliferous Mining
limit the establishment of species native to these western desert and semi-desert climatess 6. An increasingly common feature of mines exploiting mixed sulphide ores is an approach to native species re-establishment that recognises the constraints of different types of mine waste produced by a single operation. Extremely hostile railings and waste rock with high residual metal concentrations or a marked potential for acid regeneration may be buried or otherwise treated differently from, for example, development rock and innocuous overburden. At Blackbird mine, a copper-cobalt development at an elevation of 2000-2500m in the Salmon National Forest in Idaho, U.S.A. the intention is to re-create the local natural vegetation cover of lodgepole pine (Pinus contorta) with an understorey of huckleberry (Vaccinium spp.), spirea (Spiraea spp.) and pinegrass (Calamagrostis rubescens) on innocuous overburden20. In order to revegetate the 1 x 106m 3 of overburden generated by this now inactive mine, trials were initiated in 1972 in which the performance of mixtures of native seed, introduced species and a combination of the two are being compared under the major treatments of irrigation and topsoil amelioration (20cm)against untreated but seeded overburden. Measurements taken over a relatively short period of two years indicated a significantly greater initial dry matter production for the introduced and combined seed mixtures, but then a noticeable reduction in species diversity and plant density with the decline of the poorly adapted introduced species. The value of revegetating with native species under adverse conditions of soil and cimate is apparent, particularly in the absence of permanent artificial irrigation. Definite conclusions cannot be drawn from such a limited monitoring period but the topsoil replacement and native species mixture treatment, which produced a sward dominated by squirreltail (Sitanion hystrix), western yarrow (Achillea rnillefolium) and cheatgrass (Bromus tectorum), appears to have greater potential both as a method of stabilisation and as a means of initiating the development of the understorey and canopy species in the long term. Studies that are similar in principle have been conducted at the even higher altitutde (31503450m) molybdenum mines of Amax Inc., at Climax in Colorado where revegetation problems have been experienced in the sub-alpine conditions. Trials on mine tailings ameliorated with sewage sludge, wood chips or waste development rock have identified the latter as a suitable substrate but there are difficulties in collecting native seed and obtaining nursery-grown transplants of native trees and shrubs adapted to the harsh climate at this altitude. Transplanting of lodgepole pine, engelman spruce and aspen is being undertaken together with glasshouse production of rooted transplants from cuttings I 0,i In new mining ventures which produce nontoxic waste materials in less hostile climates, there is now a tendency to plan the restoration of semi-
natural vegetation at the outset, rather than as a secondary measure when alternative approaches have either failed or proved too expensive. Pancontinental Mining Ltd., who are proposing an open-pit mine for uranium oxide (UaOa) in the monsoonal Alligator River Region of Northern Territory, Australia has initiated research into native species propagation and establishment methods for the 350 x 106t of sandstone overburden and 170 x 106t of weakly mineralised schist that will be discarded during the life of the mine. The uranium ore contains fairly low levels of copper, zinc and cadmium sulphides (< 0.5% W/w in total) and the prospects for establishment of the original vegetation are encouraging54 . LEAD, ZINC and MULTIPLE METAL ORES Surface treatment of metalliferous wastes with innocuous amendments is a common practice where agriculture, forestry or intensive recreational afteruses are envisaged; but this approach can also be used as a means of encouraging native vegetation establishment. In phytotoxic situations it may be an essential preliminary treatment for establishing any type of ground cover vegetation but in other cases it may be simply a convenient way of disposing of innocuous mine residues or other industrial wastes produced in the immediate vicinity. This approach is the basis for native plant establishment on tailings now produced from Cu-Ag-Pb-Zn sulphide ores by Mount Isa Mines Ltd., on the Leichhardt River in far north-west Queensland. Arid woodland is predominant on uplands surrounding the railings dams, and the community consists largely of E. brevifolia and whitewood (Atalaya hemiglauca) in the canopy layer, and Triodia spp. in the understorey. A distinct shrub layer is usually absent 22. Successful reinstatement of this woodland on tailings dam surfaces is not easily achieved, particularly as mining is underground and does not generate potential ameliorants such as overburden and topsoil. Voluntary colonisation of indigenous species does not occur on inactive lagoon surfaces with the exception of Typha. spp in ponded areas. This is not surprising as the material has a high bulk density (up to 7.5g/cm 3), a low infiltration rate (12 25mm/hr), high conductivity (5.2mmhos/cm) and 300-1000mg/kg of copper, lead and zinc 23. To overcome these constraints locally available ameliorants are used including the comparatively fertile pulverised fuel ash from coal combustion and fine siltstone wastes (< 100mm) from quarrying for underground fill. This innocuous waste is applied as cover material before seed broadcasting and planting of native trees and shrubs is undertaken (Table 10). Although restoration is orientated towards native species, through a continuous seed collection campaign during the dry season, faithful reproduction of the local flora has been inhibited by the siting of the dams within the maximum impact zone of adjacent smelter plumes and the resulting
M.S. Johnson and P.D, Putwain sulphur dioxide fumigation. Although the restoration programme has been successful in developing vegetation of similar appearance and density to the surrounding, less affected hillsides, the relative abundance of different species is not comparable because selection for sulphur dioxide tolerance occurs. Thus species o f Acacia, Cassia, Ptilotus and A talaya hemiglauca, most of which are occasional or rare in the native vegetation, are particularly prominent in the introduced woodland. A similar approach based on deliberate selection of species according to tailings properties and climatic factors was proposed following detailed surveys of old tailings dumps and open-cuts o f the still active base metal mines at Broken Hill, New South Wales 61 . Revegetation work on Pb-Zn tailings at Broken Hill now concentrates on the use of temporary irrigation with sewage effluent as a means o f establishing a permanent ground cover vegetation o f native grasses (eg. Cynodon dactylon and Pennisetum clandestinum) and woody species (eg. A triplex mimmularia and Eucalyptus spp.)7, 2 .
TABLE 10 Summary of the methods used to establish native, semi-arid* zone woodland on mine taillngs at Mr. Isa, Queensland, Australia. (a) Restoration objectives (i) Costsmust be kept to a minimum 9 (ii) Other waste materials that must be dumped within the lease area must be utilised as far as possible (iii) Nativevegetation should be established in line with local climatic conditions (iv) A permanent, low-maintenance, self-perpetuating vegetation is required. (b) Edaphic constraints (i) Extreme seasonal fluctuations in rainfall and temperature (ii) Adverse physical conditions of high bulk density, low inFdtration and surface crusting of the tailings 9 (iii) Deficiencies of plant macronutrients (iv) High salt concentrations and the presence of toxic metals (v) Windblasting (c) Restoration sequence (i) Apply lm of undersized reject or f'me Kennedy Siltstone using 45 tonne dump trucks and D-9 bulldozers for regrading (ii) Apply 4-5cm of pulverised fuel ash from a low coal-fired power station and incorporate into the overburden. (iii) Manually broadcast or hydroseed native shrub and grass seed (iv) Plant glasshouse-raised then hardened native tree and-shrub seedlings at a density of 100-500/ha, giving particular attention to SO2 tolerance in species selection. *Adapted from (22) and (23)
79
Less hostile non-ferrous metal tailings similar to those at Cyprus-Pima mine but occurring in more amenable temperate climates are amongst the most versatile from a restoration viewpoint. However, emphasis is often placed not on recreating the predevelopment vegetation cover o f these sites but on capitalising on the diversity o f habitats and opportunities that mining has created. The potential o f these areas can be considerable but the concept o f deliberately encouraging wildlife development once mining has ceased is relatively new.
Progress in the restoration o f Pb-Zn-CaF2 mine tailings produced by Laporte Industries Ltd., in the Peak District National Park, Derbyshire, England illustrates the possibilities that exist. Tailings produced by these operations in the 1960's contained sufficient residual lead and zinc to prevent recolonisation by indigenous species other than metaUophytes such as vernal sandwort (Minuartia verna) and species that have evolved metal-tolerant populations 42 . However, improvements in processing technology together with changes in the source o f raw mineral ore are reflected in the modern railings which contain m u c h lower levels of phytotoxic metals (Table 3). The significance o f this reduction is shown by the invasion o f a wide range o f locally indigenous species when tailings disposal ceases, even temporarily. However, in order t o - c o m p l y with planning conditions, revegetation work must commence soon after tailings dams cease to function. Apart from this legal requirement, inactive sites are prone to wind dispersal o f tailings and deposition on to adjacent agricultural pastures. In order to counter these problems revegetation work has been Orientated towards development of agricultural or amenity grassland and deciduous woodland, rather than the species-rich limestone grassland communities characteristic o f the Peak District. Even with seeding and transplanting o f locally indigenous species and importing o f cut turfs as supplementary sources o f native seed and vegetative propagules, invasion o f railings surfaces is too slow and unpredictable to meet the planning and environmental protection requirements. However, in this instance establishment o f pastures by a modified m e t h o d o f hydroseeding42 and extensive planting o f indigenous trees and shrubs (Figs. 4 and 5) has not prevented invasion by herbaceous species typical o f the surrounding limestone grasslands. Annual surveys o f the flora and fauna o f a railings dam restored in 1975 have shown a remarkable transformation in recent years. Even though influx o f plant species to the open tree and shrub plantations is inhibited by the surrounding agricultural pastures, which are species-poor compared to m a n y o f the limestone grasslands o f the Peak District dales, several characteristic limestone species have become established. Particularly significant colonisers are mountain pansy (Viola lutea), c o m m o n spotted orchid (Dactylorhiza fuchsii) and pyramidal orchid
80
Restoration o f Land Disturbed by Metalliferous Mining
Fig. 4. A disused fluorspar tailings darn in Derbyshire,U.K., shortly after abandonment.
(Anacamptis pyramidalis), and there is clear evidence that movements of livestock and birds as well as the more obvious agents of seed dispersal are contributing to the overall diversification of the flora4 I. The increasing species diversity is providing a genuine nature conservation interest. Marshy areas of the railings surface and adjacent settlement ponds support several species of dragonfly and damselfly which are locally uncommon due to the lack of suitable semi-natural habitat, and surveys of the vertebrate fauna have shown a remarkably rapid re-colonisation by rodents and insectivorous small mammals41. Moreover, mark-releaserecapture studies have shown that the predatory mustelids - weasel (Mustela nivalis) and stoat (M. erminea) - are resident on the dam surface in numbers that reflect the abundance of their small mammal prey and are therefore higher than in the surrounding landscape (Table 11). Common bird censuses have confirmed the breeding of several locally uncommon species (Table 12) and surveys of passage migrants visiting adjacent partially active lagoons, retained in a semi-open condition for emergency use, suggest that an even greater ornithological potential exists if current revegetation methods are modified to include some permanent open water areas. DISCUSSION
Land-use planning and rehabilitation o f native biotic communities Rehabilitation of metalliferous mine wastes frequently takes place within the framework of
central government and state government legislation for the protection of wild life and the environment. Legislation encompassing the use of Environmental Impact Assessment, for example, the National Environmental Policy Act (1970) of the United States of America, has had important effects on the formulation of mining rehabilitation proposals at the planning stage, and this has assisted in the proper planning of procedures for the rehabilitation of native biotic communities. In Australia the Commonwealth Environment Protection (Impact of Proposals) Act, 1974, has provided a framework within which the various State Mining Acts (eg. the Mining Act, 1978 of Western Australia) operate to apply particular rehabilitation conditions to a mining lease. Often, an approved plan will include proposals for restoration of native vegetation and possibly associated animal communities which should be sustainable in perpetuity. Within the framework of a range of potential land uses for restored mine wastes (eg. industrial and housing development, commercial forestry, pastoral and arable agriculture, recreation, conservation and wildlife preservation), the main immediate objective of rehabilitation in the past has been erosion control. Stabilisation of mine residues has minimised wind and water erosion and sometimes superficial slope failure and has prevented the pollution of water courses and wind blow on to surrounding land. Often the longer term objective of enhancement of the aesthetic qualities of an area and restoration of native biotic communities has been ignored or at best been given a low priority. Thus the approach to restoration involved rapid establishment of mixtures of fast
M.S. Johnson and P.D. Putwain
81
Fig. 5. Native tree and shrub plantations and ground cover vegetation on a fluorspar tailings dam in Derbyshire, U.K., fix years after abandonment.
growing, often non-indigenous species Which would persist in the local climate. Nevertheless, ecologists employed by various mining companies have recently undertaken research which is directed at providing a basis for the re-estabhshment o f native biotic communities, with the added incentive of environmental protection legislation giving more urgency to this type o f research. Reinstatement of native biotas on metalliferous mining wastes tends to be difficult, requiring a fundamental knowledge of the disturbed ecosystems. In addition mining companies have to organise special procedures for collecting, storing and replacing topsoil, for reestablishing the full spectrum o f native plant species and monitoring faunal colonisation o f a restored area. Research on such reinstatement is clearly long term. It is thus not surprising that there is httle pubhshed evidence of successful restoration of native plant communities (criteria for establish'rag what constitutes 'successful' reinstatement are discussed later). Moreover, ecologically sound quantitative data on recolonisation by vertebrate and invertebrate fauna and soil microbial populations, is scarce, as is information on mineral nutrient retention and cycling. Despite this, Farrel122, Martinick s~ Tacey and Glossop 71 and others s have provided encouraging indications that it should be possible to combine surface stabilisation o f many metalliferous mine wastes with faithful restoration of indigenous vegetation and ultimately perhaps,
TABLE 11 Estimated small mammal and predator densities on a revegetated* lead/zinc/fluorspar tallings dam at Cavendish Mill, Derbyshire, England Density (nos/ha) Tailings dam Nearby pastures /scrub 1977 1 9 7 9 1 9 7 7 1979 Field vole {Microtis agrestis}
2.3
3.6
2.1
2.4
Woodmouse (Apodemus sylvaticus}
6.8
9.7
2.2
2.9
Common Shrew [Sorex araneus)
3.2
11.2
3.4
5.4
Bank Vole {Clethrionomys glareolus}
0.2
0.7
1.7
2.9
Weasel {Mustela nivalis)
0.3
0.8
0.1
0.2
0
0.3
<0.1
<0.1
Stoat (Mustela erminea )
*Surveyed in September: figures are combined totals for grassland and scrub. The adjacent trapping area was selected to give a similar grassland: scrub area ratio to that on the tailings surface.
82
Restoration o f L a n d Disturbed by Metalliferous Mining
TABLE 12 Breedingbird census of a restored lead/zinc/ fluorspar tailings dam (1974-80) at CavendishMill, Derbyshire, England Density* (Breedingpairs/ha)
Common Partridge
1975
1977
0
0
1979 0.24 (2)
~Perdix perdix }
Curlew 'Numenius arquata) Meadow Pipit ~An thus pratensis) Pied Wagtain CMotacilla alba) Skylark ~Alauda arvensis)
0
0
0.12 (1) 0.48 (4) 0
0
0.24 (2) 0.60 (5)
Wheatear 'Oenan the oenan the)
0
Whinchat ~Saxicola mbetra )
0
Yellow Wagtail 'Motacilla tiara}
0
0.12 (1) 0 0.12 (1)
0.12 (1) 0.60(5) 0.24 (2) 1.08(9) 0.24(2) 0.12 (1) 0.36(3)
*Values in parenthesesindicate the total number of breeding pairs on the railings dam surface reinstatement of native faunas. Clearly there are some wastes which are just too hostile to permit this, combining chemical and physical properties which are a severe restraint on the restoration of native floras and faunas. The need f o r reinstatement o f native floras and faunas Faithful restoration of a flora and fauna may be important for wildlife conservation if individual species in the disrupted communities are of international or national importance or if the whole ecosystem is significant. Aesthetic values and the visual quality of the landscape may also be reasons for attempting to reinstate the original native biota. The locations of mines in relation to centres of population, areas of recreation and faunas and floras of special conservation interest are important in determining whether faithful restoration is attempted or whether some other long term land use is developed. It has been argued s3 that reinstatement of native biotic communities is not always the wisest use of f'mancial resources, since costs of reinstatement are fairly high. However, many mines axe situated in remote areas where commercial agriculture or forestry are not feasible alternative land uses. The objective of establishing a permanent, self-regenerating native vegetation requiring zero or minimal maintenance is then most attractive.
A difficulty of achieving this objective is that there is rarely adequate fundamental knowledge concerning a particular ecosystem ,to e.nable rational decisions to be made about the most efficient techniques to be used to effect the restoration of native species. Thus research on the restoration of native biotic communities and practical reinstatement by land managers have been, and are, proceeding in parallel. Approaches to reinstatement and objectives The first objective is to establish a vegetation cover which is adequate to stabilize mine wastes. Various options are available for restoration of native vegetation to achieve this objective. It may be possible initially to establish a vegetation that includes some of the characteristics of the original flora by planting native species which are known to establish successfully and fairly rapidly. For example, the restoration of lead/zinc/fluorspar tailings dams by Laporte Industries, Derbyshire, U.K., involved initially establishing a sown grassland vegetation composed of pasture species together with plantings of several species of indigenous trees and shrubs 42 . This vegetation, although not a faithful restoration of local limestone grassland, has provided an undisturbed habitat for many species of birds and mammals and has stabilised the tailings surface whilst the flora diversifies in the longer term 41. A similar approach to reinstatement of metalliferous tailings includes situations where limited numbers of native species have been sown and then unintentional colonisation of additional native species has subsequently occurred. For example, tailings restoration by the International Nickel Co.Sa, s9 involved this approach. However, the development of a native plant community will be unpredictable unless properly planned reinstatement procedures are used. These should be an improvement in comparison with the vagaries of natural colonisation. Similarly, early attempts to rehabilitate Jarrah forest in bauxite mining areas in Western Australia involved establishment of a few species of Eucalyptus after mining was completed and topsoil had been returned to restored areas. The resulting vegetation was poor in native understorey species 7 i. Where mine wastes remain toxic after processing (eg. many Cu, Zn, Pb, Ni wastes) usually only a few native species which have evolved metal tolerant populations are actually available for revegetation. In this situation it will normally be impossible to restore native plant communities unless extensive use is made of innocuous covering materials. This is rarely possible for economic reasons since a reasonable depth of material (> 50cm) would be required, together with a barrier or a break layer (10 - 100cm) to prevent upward movement of metals, even to establish a grassland vegetation. Tree and shrub species are necessarily excluded from the reinstatement programme. Where innocuous materials are unavailable in close proximity to a mine or are not
M.S. Johnson and P.D. Putwain '
a by-product of mining then use of metal-tolerant commercial cultivars such as Festuca rubra cv. 'Merlin' (Pb-Zn tolerant) or Agrostis tenuis cv. 'Parys' (Cu tolerant) are the only feasible methods of establishing a satisfactory vegetation cover. In a few metalliferous mining operations, the declared objective is the reinstatement of the original flora and fauna as faithfully as is practically possible. Reinstatement procedures have been intensively developed to enable this goal to be achieved. The bauxite mines at Weipa in north Queensland, Australia and Alcoa's mines in Western Australia, are excellent examples of how this approach is being applied. The reinstatement techniques which have been used at.Weipa have enabled a reasonably diverse flora to become established within 6 - 7 years a,s~. At Alcoa's mines the level of species diversity achieved by reinstatement is considerably greater than that attained by allowing natural colonisation of the Jarrah forest to proceed unhindered over the same period of time44, v~ . This is a relatively common experience and there are examples where reinstated areas of mature alpine grassland, prairie grassland and coastal marsh grassland have developed similar characteristics to the original vegetation much more rapidly than occurs with natural colonisation and succession ~a There is also evidence at Alcoa's mines44 that a reasonable proportion of the vertebrate fauna (> 50% of species) have been observed feeding, breeding or rest.ing in reinstated areas of native Jarrah forest which vary in age up to' twenty-five years. However, this is one of very few published studies which include quantitative data based on ecologically sound techniques of sampling and monitoring of the fauna. Quantitative or qualitative studies of invertebrate fauna are minimal49 and studies of the development of soil microbial populations and fungal mycorrhizas appear to have been completely disregarded. With such a lack of quantitative data concerning the establishment of native floras and faunas on reinstated areas of metalliferous mining, it is clearly impossible to generalise about the relative success of the techniques which have been used. Criteria for the evaluation o f recovery Criteria for evaluating the success of reinstatement of native biotic communities and for monitoring recovery have not been clearly established in the literature. However, quantification of the success of reinstatement will be important where mining companies are legally responsible for the reinstatement of native biotic communities. It will be necessary to regularly monitor reinstated areas using ecologically sound quantitative methods. The period of monitoring will be dependent on the
83
type of biotic community which is being reinstated. For example, 8 -~ 10 years may be adequate for short-grass prairie ts , 10 - 15 years for Eucalyptus forest 7~ and up to 25 years for arctic grass and sedge communities 12 . No single criterion is likely to be adequate as a measure of the success of reinstatement. Measurement of plant productivity and the biomass of fauna and flora may provide an indication of the rate of recovery and whether surface stabilization of waste materials is satisfactory. Measurement of plant and animal species diversity using well established diversity indices6~ may indicate the extent to which reinstated biotic communities approach the pre-disturbance condition. There may be difficulties in the interpretation of species diversity indicators since some climax communities are less diverse than at certain points in the reinstatement succession. There are many well established methods for sampling plant and animal populations 4,67. Quantitative data is readily obtained on the species composition of a fauna or flora and on the number of animals or cover of a plant species in some def'med sample size. Multivariate analysis of this data using principal co-ordinate analysis8 provides an excellent method of testing how biotic communities are developing and whether the trend in the composition of t h e species assemblage is towards the original undisturbed communities. Supplementary research will also be necessary to establish whether natural regeneration of individual plant species is occurring and whether vertebrate and invertebrate fauna are breeding successfully in an area. Monitoring of the development of soil microbial populations and fungal mycorrhizas should also be included in a programme of monitoring to provide maximum information about the redevelopment of reinstated communities. It will not always be possible to faithfully reinstate an ecosystem back to the original premining situation. This implies not only faithful reinstatement of flora and fauna but also of the soil to its original condition. In many metalliferous mining situations changes in the characteristics of substrates are so immense that it is not realistic to expect complete restoration of soil microbial populations, soil fungi, soil organic matter type and content and soil mineral nutrient cycling. Despite the latter comment, this paper has reviewed some encouraging evidence which shows that it may be possible to reinstate native plant and animal communities to a state, close to the original condition, in at least some areas of metalliferous mining, where it may be difficult to visually distinguish a reinstated area from an original preserved one.
84
Restoration o f Land Disturbed by Metalliferous Mining
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