Mining and its environmental impact

164 Pages·1994·1.445 MB·xv, 164 p. : ill\164

by  R E Hester, Roy M Harrison, Royal Society of Chemistry (Great Britain)

Revegetation of Metalliferous Wastes and
Land After Metal Mining
M.S. JOHNSON, J.A. COOKE, AND J.K.W. STEVENSON
1 Introduction
Therehasbeenaprogressiveworldwideincreaseinmetalliferousmininginrecent
years underpinned by social, economic, and technological demand.1 The
ever-increasingneedformetals,andtheabilityofmodernminingandprocessing
methods to develop low grade ore-bodies economically, has placed increased
strainontheenvironmentatatimewhenthedemandsforhighenvironmental
standardsarealsoincreasing.2Sinceminingis,byitsverynature,adestructive
industry, attention has become focused on ways in which the environmental
impact may be reduced or rendered temporary in nature.
Ane⁄ectivedecommissioningplanforeventualmineclosurewhichprovides
for the reclamation of disused workings, waste-rock dumps, and tailings
impoundments, is an increasingly important factor when decisions are being
takenastowhethertograntplanningconsentforanewmine.3Thisconcernis
underpinnedbythelegacyofpastmininginmanypartsoftheworld.Continuing
environmentaldamagearisingfrompollutedwatersanddispersalofcontaminated
solidwaste is a feature of old mines in NorthAmerica, Australia,Europe,and
elsewhere.4,5 It is, therefore, becoming standard practice for reclamation
measurestobeconsideredasanintegralpartofmineplanningandoperations,
eventotheextentthat(cid:222)nancialprovisionsaremadeduringtheoperationallifeof
aminetoe⁄ectsuchreclamationmeasuresuponclosure.1Thehigherenvironmental
pro(cid:222)leattachedtomodernminingislinkednotonlytosocialacceptabilitybut
alsotolegalrequirementsinmanycountries.Thisrequiresattentiontobepaidto
the prevention of environmental damage from mining operations, waste
production, and site closure.
1 UnitedNations,(cid:212)EnvironmentalAspectsofSelectedNon-ferrousMetalsOreMining(cid:213),Technical
Report Series No. 5, United Nations Environment Programme/Industry and Environment
ActivityCentre,UNEP,Paris,1991,p.116.
2 UnitedNations,(cid:212)MiningandtheEnvironment:TheBerlinGuidelines(cid:213),UnitedNationsDepartment
for Technical Co-operation and German Foundation for International Development, Mining
JournalBooks,London,1992,p.180.
3 OntarioMinistryofMinesandNorthernDevelopment,(cid:212)RehabilitationofMines:Guidelinesfor
Proponents(cid:213),MinistryofMines,Sudbury,Ontario,Canada,1991,p.137.
4 G.M.Ritcey,(cid:212)TailingsManagement:ProblemsandSolutionsintheMiningIndustry(cid:213),Elsevier,
NewYork,1989,p.970.
5 N.J.Coppin,M.G.Sta⁄,andM.S.Johnson,in(cid:212)Minerals,MetalsandtheEnvironment(cid:213),Institute
ofMiningandMetallurgy,London,1992,p.104.
31
M.S. Johnson, J.A. Cooke, and J.K.W. Stevenson
2 Constraints Upon Revegetation
Althoughchemicalandphysicaltechniquesexistfordustcontrolandstabilization
againstwatererosion,4suchobjectivescanonlyberealisticallyachievedinthe
long-termbytheuseofvegetationasabasisforlandscaping,stabilization,and
pollutioncontrol.However,itiswidelyrecognizedthatwastesfrommetalliferous
mines,especiallyacidicones,areverydiƒcultmaterialsuponwhichtoestablish
vegetation.6
Thereasonsthatmetalminewastespresentdiƒcultiesforplantgrowthderive
from a combination of their physical, chemical, and biological properties. The
majorphysicalconstraintofminetailingsistheirsmallanduniformparticlesize
distributionwhichisdominatedbymaterialofsiltandclaydimensions.Physical
propertiesvarybothhorizontallyandverticallywithinanytailingsimpoundment
duetostrati(cid:222)cationduringdeposition,andalsototheparticlesizingobjectives
setforoptimumrecoveryofmetalsinthepriororemillingand(cid:223)otationprocess.
Tailings also possess unfavourable porosity, aeration, water in(cid:222)ltration, and
percolation properties, along with a high bulk density and an absence of
structuralaggregates.Theresultisthatwaterandwinderosionofunprotected
disused tailings surfaces is a common hazard.7
In the case of old mines, often long abandoned, the processing technology
deployedtorecoverthetargetmetalswaselementaryandineƒcientascompared
withcontemporarymagnetic,gravity,and(cid:223)otationprocesses.Theconsequence
is that waste materials representing past eras of mining contain high residual
quantitiesof toxic metals, often in excess of 1% by weight of elements such as
lead, zinc, and/or copper (Table 1). These high metal values are frequently
accompanied by elevated levels of non-target elements such as arsenic and
cadmium from the original crude ore.8
Somemetals(e.g.copper,zinc)areessentialtraceelementsatlowconcentrations
buttoxictoplantsathighlevels.Itisnotpossibletostatespeci(cid:222)cconcentrations
fornormalmetabolismasopposedtotoxicityasthethresholdvarieswithother
soilvariablesandthespeciesconcerned.Also,somecombinationsofmetalsact
antagonisticallyorevensynergisticallyinsolution.Nickelandzinc,copperand
zinc, and copper and cadmium are more toxic than their individual toxicities
wouldsuggest.9Thepresenceofphosphateorcalciumcanreducethetoxicityof
lead, zinc, and copper through precipitation and ion competition reactions.
Othernon-essentialelements(e.g.lead,mercury)arelesstoxictovegetationbut
hazardoustolivestockthat maygrazevegetationwhichhasaccumulatedsuch
metals either via the roots or as surface dusts.10
Thesuiteofproblemsdescribediscompounded,inmanyinstances,bywhatis
6 M.S.JohnsonandA.M.Mortimer,in(cid:212)EnvironmentalAspectsofMetalliferousMining:ASelect
Bibliography(cid:213),TechnicalCommunications,Letchworth,UK,1987,p.212.
7 J.M.RingeandD.H.Graves,Reclam.Reveg.Res.,1987,6,121.
8 N.A. Williamson, M.S. Johnson, and A.D. Bradshaw, in (cid:212)Mine Waste Rehabilitation: the
EstablishmentofVegetationonMetalMineWaste(cid:213),MiningJournalBooks,London,1982,p.103.
9 C.G.DownandJ.Stocks,(cid:212)EnvironmentalImpactofMining(cid:213),AppliedScience,London,1977,p.371.
10 DepartmentofEnvironment,(cid:212)NotesontheRestorationandAftercareofMetalliferousMineSites
for Pasture and Grazing(cid:213), ICRCL Guidance Note 70/90, Department of Environment, UK
Government,London,1990,p.15.
32
Revegetation of Metalliferous Wastes and Land
Table1 Copper,lead,and Number Principalbasemetals
zincinspoilfrom
Mining ofsites
abandonedminesin
region Counties surveyed Cu Pb Zn
Britain*
S.W.England Devonand 16 65—6140 48—2070 26—1090
Cornwall
W.andN.W. Shropshire 12 15—7260 840—26000 980—21000
England andCheshire
N.Pennines N.Yorkshire 8 20—140 605—13000 470—28000
andDurham
S.Pennines Derbyshire 17 23—97 10800—76500 12700—42000
LakeDistrict Cumbria 7 77—3800 2070—7360 4690—7370
Mid-Wales Powysand 10 67—195 1670—54000 475—8000
Dyfed
N.Wales Clwydand 19 30—5750 6400—76000 11300—12700
Gwynedd
S.Scotland Dumfriesand 6 125—657 4730—28300 1600—31400
Galloway
Normalagriculturalsoil 2—100 2—200 10—300
*Allvaluesinmgkg~1airdriedsubstrate
probably the most intransigent revegetation problems faced by the mining
industry.Thepresenceofsigni(cid:222)cantquantitiesofironpyrites(FeS ),whichmay
2
not be removed during ore bene(cid:222)ciation,often leads to very acid waste as the
mineral is comminuted during processing and then undergoes weathering to
generatesulfuricacid.11Pyrite-bearingwastesdisposedofatneutralorslightly
alkalinepHcandegradewithinmonthsoryearstoproduceextremeacidity.12
Initialchemicalprocessesareprobablytheresultofnaturalweatheringbutthis
oxidation and hydrolysis is then assisted by ferrous ion oxidizing bacteria,
Thiobacillus ferrooxidans, which thrive at pH 1.5—3.0.13,14
Therateofoxidationofpyriteisin(cid:223)uencedbythesurfaceareaofthematerial
availableforweathering.15Otherfactorsalsoin(cid:223)uencetherateofproductionof
acid, namely the native carbonate content of the material, and the size,
morphology, and type of the pyrite present. If the wastes contain only small
quantities of native carbonates, acid regeneration may exceed the neutralizing
potential,resultinginasigni(cid:222)cantdeclineinpH.ThepHvaluesofminetailings
and waste-rock range from below 2 to more than 8, depending on the gangue
material,thepyritecontent,and,inthecaseof tailings,thechemicalsaddedin
themill.GoldminingwastesinSouthAfricahavepHvaluesaslowas1.5,and
11 M.U.Ahmed,in(cid:212)ExtractionofMineralsandEnergy:Today(cid:213)sDilemmas(cid:213),ed.R.A.Deju,Ann
ArborScience,AnnArbor,Michigan,1974,p.49.
12 A.C.Hartley,Aust.J.SoilRes.,1979,17,355.
13 N.V.Blesing,J.A.Lackey,andA.H.Spry,in(cid:212)MineralsandtheEnvironment(cid:213),ed.M.J.Jones,
InstituteofMiningandMetallurgy,London,1975,p.341.
14 J.R. Hawley, (cid:212)The Problem of Acid Mine in the Province of Ontario(cid:213), Ministry of the
Environment,Ontario,1977,p.32.
15 M.Kalin,Proceedingsofthe4thAnnualGeneralMeetingofBiominet,Sudbury,Canada,Special
Publication,CANMETNo.SP,87—10,1988.
33
M.S. Johnson, J.A. Cooke, and J.K.W. Stevenson
uranium tailings in Colorado have been reported with pH values of 8.0.16
Afurtherconsistentfeatureofmetalliferouswaste,whetherrockortailings,is
thelowconcentrationofessentialplantnutrients.Nitrogenlevelsareinvariably
inadequate for plant growth, phosphorus levels are generally very low, and
de(cid:222)cienciesofpotassium,calcium,andmagnesiumalsomayoccur.17Inwarmer
climatesthantemperateBritain,salinityoftailingscanalsoprovetroublesome.18
Itresultsfrom:(1)theinteractionoftheproductsofpyriteweatheringwithnative
carbonates;(2) the concentrationof naturally occurring saltsin tailings due to
recyclingofwater;(3)theadditionsmadetotailingsbymillpersonnelinorderto
adjuste§uentpH;and(4)excessiveevaporationfromthesurface.Theresulting
salinitylevelscanbehighenoughtopreventplantgrowth.Tailingsgenerallyare
morecomplexchemicallythanwaste-rock,duetotheadditionofreagentsand
pH modi(cid:222)ers during the metal extractionprocess.19
The basic combination of adverse physical and chemical characteristics
produces an environment in mine wastes that is hostile to plants. This is
accentuatedbytheabsenceoftheorganicfractionthatcomprises3—5%ofthe
surface horizons of most natural soils. Organic matter contributes to soil
structure, provides a reservoir of essential macronutrients, and a resource for
invertebrates and micro-organisms that support the decay processes which
underpinenergyandnutrientcycling.Thesterilenatureofminewasteshastobe
recti(cid:222)ed before adequate and sustainable growth of plants can be achieved.
3 Revegetation Objectives
Essentially,theobjectivesofvegetationestablishmentare:long-termstabilityof
thelandsurfacewhichensuresthatthereisnosurfaceerosionbywaterorwind;
reduction of leaching throughputs, lessening the amounts of potentially toxic
elementsreleasedintolocalwatercoursesandtogroundwaters;developmentofa
vegetatedlandscapeorecosysteminharmonywiththesurroundingenvironment;
andwithsomepositivevalueinanaesthetic,productivity,ornatureconservation
context.
The (cid:222)rstobjectiveis achievableby a continuousvegetationcover, especially
wherethecoverisatleast100%andofrelativelylowgrowth.Withlowercover
valueserosionmaybegintooccur.20Thedegreetowhichthesecondobjectiveis
met by a vegetation cover depends on the ambient climate. Vegetation will
intercept and return rainfall to the atmosphere by evapotranspiration. In
temperateclimatestheamountinterceptedandreturnedwillbeupto50%ofthe
total,in the wet tropics less than 25%, and in the dry tropics more than 75%.
However,thesevaluesarealsoa⁄ectedbythedistributionoftherainfallandthe
16 H.B.PetersonandR.F.Nielson,in(cid:212)EcologyandReclamationofDevastatedLand(cid:213),Gordonand
Breach,NewYork,1973,Vol.1,p.15.
17 A.D. Bradshaw and M.S. Johnson, in (cid:212)Minerals, Metals and the Environment(cid:213), Institute of
MiningandMetallurgy,London,1992,p.481.
18 K.L.LudekeandA.D.Day,Trans.Soc.Min.Eng.AIME,1985,278,1807.
19 L.C.BellandM.Evans,Reclam.Rev.,1980,3,113.
20 A.D.BradshawandM.J.Chadwick,(cid:212)TheRestorationofLand(cid:213),BlackwellScienti(cid:222)cPublishers
Ltd.,Oxford,1980,p.297.
34
Revegetation of Metalliferous Wastes and Land
typeofrainevent;itismorediƒcultforvegetationtointerceptandre-evaporate
precipitationifit fallsintheformofintermittent,heavystorms.Thethird,and
ultimatelymostdemanding,objectiverepresentsthesuccessfulintegrationofall
thefactorsthatcanin(cid:223)uencearevegetationschemeandassuchitistheultimate
test of the approach or the philosophy of the programme in practice.
4 Philosophies of Revegetation
Theapproachestorevegetationcanbedescribedintermsofthreedi⁄erentbasic
philosophies: (1) ameliorative, (2) adaptive, and (3) agricultural.
(1) The ameliorative approach relies on achieving optimum conditions for
plant growth by improving the physical and chemical nature of mine
wastesusingorganicmatter,fertilizer,and/orlime.Alternatively,mixing
orcovermaterialsmaybedeployed.Themostsuitablespeciesavailable
commerciallyaresownontothewasteswhoseedaphicpropertieshave
been modi(cid:222)ed in accordance with the land use objectives and type of
vegetation to be introduced. This approach is commonly used in
preference to the adaptive because it is quicker, requires less forward
planning, and is less labour-intensive.
(2) Theadaptiveapproachemphasizesselectionofthemostsuitablespecies,
sub-species, cultivars, and ecotypes to meet the rigours of the extreme
conditions. In addition, but not necessarily, the mine wastes may be
improved using amendments to achieve optimum establishment and
long-term growth. This approach is simple but is constrained by the
availabilityofsuitablepropagulesinsomeareasandbythelonglead-time
in producing commercial seed from promising natural or arti(cid:222)cially
selected plant material.
(3) Theagriculturalandforestryapproachhasbeenuseddirectlyonlesstoxic
mediasuchasironstoneandbauxitewastes,andonwastescoveredover
withdeeplayersofsoiloroverburden.Agriculturalcropsorlivestockor
woodland and/or scrub species are established using conventional or
specialized techniques.
Inpractice,itiscombinationsoftheaboveapproachesbasedonsite-speci(cid:222)c
considerationsthatproducethe(cid:222)nalrevegetationstrategy.Anextensiontothe
above philosophies, and their combination, is the (cid:212)ecological approach(cid:213) which
placesemphasisonthe importanceofestablishingbiologicalprocessessuchas
nitrogen(cid:222)xation,decomposition,nutrientcyclingandretention,andimportant
biotic interactions (e.g. pollination). It is these that indicate proper ecosystem
functioning, which is as important as the careful selection of plant species in
providingtheprimaryvegetationstructure.21Whetherthereclamationgoalsare
torestoretheoriginalnaturalecosystemortoproduceanacceptablealternative,
ecological principles should underlie all good reclamation schemes.
21 A.D. Bradshaw, in (cid:212)Restoration Ecology(cid:213), ed. W.R. Jordan, M.E. Gilpin, and J.D. Aber,
CambridgeUniversityPress,Cambridge,1990,p.53.
35
M.S. Johnson, J.A. Cooke, and J.K.W. Stevenson
5 Revegetation Techniques and Land Use
Identi(cid:222)cationandtreatmentoftheproblemspreventingplantgrowth,coupled
withcarefulselectionofspeciesandappropriatelong-termmanagement,is the
basisofsuccessfulrevegetation.Varioustechniqueshavebeendevelopedtosuit
particularwasteproblems,rangingfromcultivationwithconventionalagricultural
machineryfollowedbyfertilizationanddirectseedingforinnocuouswastes,to
specialist proceduressuch as placementof a barrier layer or deep coveringsof
non-toxicmaterialforverytoxicsites.Therangeofspeci(cid:222)crevegetationoptions
togetherwith theirlimitationsareoutlinedinTable 2,and arebased uponthe
degree of toxicity, salinity, and acidity of the waste material or site.
Manyfactorshavetobeconsideredinthechoiceofplantmaterials,andtheir
method of establishment, in particular the nature of the spoil, the prevailing
climate,andtheeventuallanduse(Table3).Examplesofotherlocalfactorstobe
considered, including pest and disease incidence and availability, are given in
(Table 4).
6 Direct Seeding
Normal Species
Unfortunately, straightforward direct seeding with conventional species and
fertilizersisoftenunsuccessfulasarevegetationmeasure,atleastonoldermine
tailings, because of the toxic residual levels of metals—often with an acidity
problemaswell.Underthesecircumstancesgrassandotherseedlingspersistfor
onlyafewweeks.However,itremainsanattractiveoptioninprinciplebecause
directseedingis muchcheaperthananyothermethod.Insituationswherethe
waste has little residual metal, or where the metal is not available to plants,
normal species can be established directly with the assistance of fertilizer.22
Becausethelong-termgrowthofvegetationdependsonanadequatesupplyof
nitrogen, legumes such as white clover (Trifolium repens) or birdsfoot trefoil
(Lotuscorniculatus)areanimportantcomponentoftheseedmixture,sincethey
have the capacity to supply nitrogen by (cid:222)xation of atmospheric sources.8
Metal Tolerance
A close examination of even the most toxic waste from old metal mines
representingeras whenprocessingtechnologywas crudeandineƒcient,nearly
alwaysrevealsasparsenaturalvegetationcover.Sometimesthisislimitedtoonly
afewplantsbut of acharacteristicandnarrow rangeof species.Suppliedwith
fertilizer, these plants are apparently able to tolerate and indeed thrive under
conditionswherenon-tolerantplantmaterialdiesinashorttime(seeFigure1).It
isnowknownthatthesenaturalcolonizersarespecialmetal-tolerantpopulations
of normal species that have become genetically adapted to thrive on metal-
contaminated sites.
Metaltoleranceisawidelyrecognizedphenomenoninhigherplantsandhas
22 M.S.Johnson,A.D.Bradshaw,andJ.F.Handley,Trans.Inst.Min.Metall.,1976,81,A32.
36
Revegetation of Metalliferous Wastes and Land
Table2 Approachesto Wastecharacteristics Revegetationtechnique Problemsencountered
revegetation
Lowmetaltoxicity. (1) AmeliorationandDirect Probablecommitmentto
Nomajoracidityor SeedingwithAgriculturalor long-termmaintenance.Grazing
alkalinityproblems. AmenityGrassesandLegumes mustbestrictlymonitoredand
ApplylimeifpH\6.Add controlledinsomesituationsdue
organicmatterifphysical tomovementoftoxicmetalsinto
ameliorationrequired.Otherwise vegetation.
applynutrientsasgranular
compoundfertilizers.Seedusing
agriculturalorhydraulic
techniques(e.g.Pb/Zn/CaF
2
tailings,Derbyshire,UK).22
Lowmetaltoxicityand (2) AmeliorationandDirect Irrigationoftennecessaryduring
climaticlimitations. SeedingwithNativespecies establishmentinaridclimates.
Nomajoracidityor Seedortransplantadaptednative Expertiserequiredonthe
alkalinityproblems. speciesusingamelioration selectionofnative(cid:223)ora.Labour
Extremesoftemperature, treatments(e.g.lime,fertilizer) intensive.
rainfall,etc. whereappropriate(e.g.Cu
tailings,Arizona,USA).23Use
thinlayerofnativesoilasseed
inoculum.
Mediumtohighmetal (3) AmeliorationandDirect Possiblecommitmenttoregular
toxicity. SeedingwithTolerant fertilizerapplications.Relatively
Highsalinity. Ecotypes fewspecieshaveevolvedtolerant
Sowmetaland/orsaltand/or populations.Grazinginadvisable.
acidtolerantseed.Applylime, Veryfewspeciesareavailable
fertilizer,andorganicmatter,as commerciallyastolerantvarieties.
necessary,beforeseeding(e.g.
Pb/ZnwasteWales,UK).24
(4) SurfaceTreatmentandSeeding Regressionwilloccurifshallow
withAgriculturalorAmenity depthsofamendmentareapplied
GrassesandLegumes orifupwardmovementofmetals
Ameliorationwith10—50cmof occurs.Availabilityandtransport
innocuousmineralwaste(e.g. costsofsurfaceamendmentsmay
overburden).Applylime,fertilizer belimiting.
andorganicmatterasnecessary
(e.g.Pb/Znwaste,Wales,UK).25
Extremetoxicity. (5) BarrierLayer Susceptibilitytodrought
Veryhightoxicmetal Surfacetreatmentwith accordingtothenatureand
content. 30—100cmofinnocuousbarrier depthofthesurfacecovering.
Intensesalinityoracidity. material(e.g.unmineralizedrock) Highcostandpotentiallimitation
andsurfacecoveringwitha ofavailabilityofbarriermaterial.
suitablerootingmedium(e.g. Integrityofbarrierlayermaybe
subsoil).Applylimeandfertilizer a⁄ectedbyrootpenetration.
asnecessary(e.g.Pb/Zn/Cu
wastes,NewSouthWales,
Australia).26
23 K.L.Ludeke,in(cid:212)TailingsDisposalToday(cid:213),ed.C.L.AplinandG.O.Argall,MillerFreeman,San
Francisco,1973,p.606.
24 R.A.H.SmithandA.D.Bradshaw,J.Appl.Ecol.,1979,16,595.
25 M.S.Johnson,T.McNeilly,andP.D.Putwain,Environ.Pollut.,1977,12,261.
26 B.Craze,J.SoilConserv.Serv.,1977,33(2),98.
37
M.S. Johnson, J.A. Cooke, and J.K.W. Stevenson
Table3 Primary Primary considerations Plant types selected
considerationsinselectionof
plants Nature of Spoil
Toxic metals at high concentrations Metal-tolerantcultivars
Natural invaders of mineralized
outcrops
Toxic metals moving into herbage Unpalatable species
Spiny shrubs around site perimeter
Extreme acidity/alkalinity Natural invaders of acidic or alkaline
conditions
High levels of salts Salt-tolerant species
Natural invaders of salty areas
Drought conditions Drought-tolerant species
Certain metal-tolerant cultivars
Poor nutrient status Legumes or other nitrogen-(cid:222)xers
Species that grow in nutrient poor
areas
Climate
Extreme cold with short growing Native or naturalized species
season Species that grow and develop rapidly
Arid or semi-arid Native or naturalized species
Transplants or cuttings of slow
growing species
Temperate Agricultural, forestry, or other
commercial species depending on land
use
Eventual Land Use
For rapid stabilizing cover and high Agricultural species
productivity
For wildlife Variety of native and naturalized
species that provide seeds, fruits,
palatable herbage, nesting sites, etc.
For aboriginal or tribal use Native species
Timber, medicinal, or food crops
Species that regenerate after practices
such as burning of forests
For amenity and recreation Wear-tolerant cultivars as developed
for sportsground turf
Low productivity
38
Revegetation of Metalliferous Wastes and Land
Table4 Other (1) Insect resistance (2) Disease resistance
considerationsinselectionof
Riverredgum(Eucalyptuscamaldulen- Diseaseresistanceis becomingan im-
plants
sis)hasproventobeamostpromising portantfactorinselectionofspeciesfor
reclamation tree worldwide. Its per- reclamationofbauxite-minedlandsin
formanceatGrooteEylandtmanganese theJarrahforestsoftheDarlingRange,
mineinNorthernTerritory,Australia, Australia, where Phytophthora cinna-
hasalsobeenpromisingbutitslackof momi,asoilbornefungalpathogen,is
resistance to termites could impose causing large-scale dieback of vegeta-
considerable limitations on its timber tion.28
value, a prime consideration in this
reclamation scheme.27
(3) Intrusion by man (4) Landscape planting
ThicketsofspinyshrubssuchasArnot TreeswithrapidgrowthsuchasBlack
Bristly Locust (Robinia fertilis), an Locust (Robinia pseudoacacia) can be
acid-tolerant,nitrogen-(cid:222)xingshrub,may e⁄ectiveinvisualscreenplantationsfor
be e⁄ectively positioned around per- tailings ponds, waste heaps, and mine
imetersofopenpitsandradioactiveor buildings.
hazardous wastes to restrict public
access.
(5) Growth habit (6) Competition
Ideally, material should be easily Species should be chosen that grow
propagated,quicktoestablish,bemat- favourably with other components of
forming with (cid:222)brous root systems or themixture;forexample,establishment
rhizomes.Selectionofdeeprootedplants of young trees in a lush ground cover
such as alfalfas and trefoils may be of vegetation may be adversely a⁄ected
valueinbreakingupacompactedsoil. by competition.
Herbaceousorperennialplantsshould
befavouredratherthanannuals,which (7) Availability
su⁄erproblemswithre-establishment. If possible, species should be selected
that are available commercially. If
companies can foresee requirements,
and order from reputable commercial
nurseriesgivingatleastayear(cid:213)snotice,
unusualrequirementscanoftenbemet.
Iffullreinstatementofadiversenative
(cid:223)ora is to be carried out, then the
companyshouldconsiderestablishing
its own nursery facilities.
27 P.LangkampandM.Dalling,BHPJ.,1977,1,42.
28 J.R. Bartle and S.R. Shea, Proceedings of the Australian Mining Industry Council (1979),
EnvironmentalWorkshop,Bunbury,Australia,1979.
39
M.S. Johnson, J.A. Cooke, and J.K.W. Stevenson
Figure1 Growthof
lead/zinctolerantand
non-tolerantpopulations
on lead/zincminewaste
aftersevenmonthsgrowth
beenreportedinrespectofcadmium,29arsenic,30andnickel31aswellasinthe
more widely publicized cases of copper, lead, and zinc.32 Recognizing the
revegetationpotentialoftheseplants,abreedingprogrammewasinitiatedinthe
1970s in order to produce commercial cultivars of certain grasses bearing the
same genetic characteristics as the (cid:212)natural(cid:213) plant material. As a result of this,
three cultivars of temperate grasses are now available commercially enabling
direct seeding of toxic areas. These three, and their associated tolerances, are:
Festucarubracv.(cid:212)Merlin(cid:213)(lead—zinc),Agrostiscapillariscv.(cid:212)Parys(cid:213)(copper),and
Agrostis capillaris cv. (cid:212)Goginan(cid:213) (lead—zinc). The value of these cultivars for
seedingunstableandtoxictipsisconsiderablesincerevegetationcanbeachieved
by direct sowing and treatment with simple inorganic fertilizers.24 They are
particularly useful for revegetation of older sites, though may prove to be less
essentialonnewerwastematerialsastheimprovedmineralprocessingtechnologies
of today leave less residual metal in the tailings. Under these circumstances,
normal plant material is able to thrive (see Figure 2).
Overall, revegetation costs are low using the tolerance route but there are
limitations. For example, the tolerance mechanism is speci(cid:222)c and though
cross-tolerance exists between related metals,32 this is at a low level so a
particularcultivarisunlikelytothriveonminewastecontainingtoxicquantities
ofmetalsotherthanthosetowhichithasevolvedanadaptation.Furthermore,
grazing of the sward for agricultural purposes is not usually possible and the
recreational or trampling resistance of the ground cover comprising tolerant
plantmaterialislow.Also,long-termfertilizerinputisusuallyrequiredandfrom
an ecologicalviewpointit is importantthatsuch schemesusing metal-tolerant
plantscanleadeventuallytothesitebecomingavailableforcolonizationbyother
tolerant but also non-tolerant species.
29 P.J.CoughtreyandM.H.Martin,NewPhytol.,1978,81,147.
30 J.R.C.Hill,Trans.Inst.Min.Metall.,1977,86A,98.
31 R.R.Brooks,J.Lee,R.D.Reeves,andT.Ja⁄re,J.Geochem.Explor.,1977,7,49.
32 A.Baker,NewPhytol.,1987,106,93.
40