Arsenic, copper and zinc occurrence at the Wangaloa coal mine, southeast Otago, New Zealand

Ž .International Journal of Coal Geology 45 2001 181–193www.elsevier.nlrlocaterijcoalgeo

Arsenic, copper and zinc occurrence at the Wangaloa coal mine,southeast Otago, New Zealand

Amanda Black), Dave CrawEnÕironmental Science Programme and Geology Department, UniÕersity of Otago, P.O. Box 56, Dunedin, New Zealand

Received 15 December 1999; accepted 15 May 2000

Abstract

Waste piles, created from open cast coal mining activities at the abandoned Wangaloa mine in SE Otago, have exposedŽ .pyrite FeS to atmospheric conditions. This has led to the acidification of the surface tailings and nearby drainage waters2

Ž . Ž . Ž . Ž .acid mine drainage, AMD . Mobilisation of trace metals arsenic As , copper Cu , and zinc Zn has occurred, partly as aŽ .result of the low pH levels ca. pH 2–4 , leading to elevated concentrations of these metals in receiving waters. Authigenic

pyrite deposited in a marginal marine coal-forming environment is enriched in As with levels reaching up to 100 ppm.Copper and Zn in solid solution are not elevated above background levels in either coal measures or associated pyrite.

Ž .Water discharges, sediments, waste rock and background samples were sampled and analysed during the driest summerŽ .and wettest winter seasons of 1998 and 1999. During the winter season, water discharging from the waste piles contained

Ž .up to 0.7 ppm mgrkg As, as measured in 1998. During the 1999 wettest season, no such levels of As were observed, withthe highest level attaining 0.07 ppm As. Copper and Zn were locally elevated in waters, with Zn concentrations reaching 1ppm. During the summer season of 1999, only one sampling site recorded elevated metal concentrations. Adverse effectsfrom the remnant waste piles appear to be highly localised due to downstream natural remediation processes occurring in awetland area.

The absence of strongly elevated metal concentrations during the drier season is a result of strongly depressed waterlevels within the waste piles. Flushing of acid and metals occurs when the water levels increase with the onset of the winterseason. During the summer season, pyrite within the waste piles has been readily decomposing from the increasedavailability and transport of atmospheric oxygen. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: arsenic; copper; zinc; wetland; acid mine drainage; coal

1. Introduction

1.1. Acid mine drainage in coal mines

Declining coal mine industries have caused theproblem of toxic discharges from abandoned coal

) Corresponding author.Ž .E-mail address: [email protected] A. Black .

mines to become increasingly prominent in recentyears, while the need to prevent or mitigate the

Ž .effects of acid mine drainage AMD has become amajor international concern. There have been several

Žrecent studies Dragovich and Patterson, 1994; Bankset al., 1997a,b; Gray, 1997; Foos, 1997; Schuring et

.al., 1997; Younger, 1997 describing the environ-mental effects and the mechanisms responsible forproducing AMD. This study describes an example

0166-5162r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0166-5162 00 00032-X

( )A. Black, D. Crawr International Journal of Coal Geology 45 2001 181–193182

from southern New Zealand in which AMD causeslocal mobilisation of metals that collect in down-stream wetlands.

Oxidation of pyrite, occasionally catalysed bymicro-organisms, is responsible for the production ofAMD. This process of oxidation is greatly enhancedby mining activities due to the increased exposure ofpotentially reactive surfaces to the atmosphere andcertain bacterial species. Pyrite is abundant as anaccessory mineral in many coal deposits and oftenexists in association with other metallic elements

Žsuch as As, Cd, Co, Cu, Pb and Zn Monterroso and.Macias, 1998 .

It is generally accepted that metal distribution ischiefly controlled by pH, with an increase in pHresulting in the co-precipitation andror adsorption ofmetals primarily with secondary Fe hydroxide miner-

Žals Boult, 1996; Banks et al., 1997a,b; Younger,.1997 . Co-precipitation andror adsorption may alter

the metal load and potentially allowing storage in aninsoluble form, within the receiving streams and

Ž .wetlands Boult, 1996 .

1.2. Pyrite oxidation

ŽNumerous studies Singer and Stumm, 1970;Moses et al., 1987; Nicholson et al. 1988, 1989;

.Moses and Herman, 1991; Evangelou et al., 1998have investigated the mechanisms of pyrite oxida-tion, concentrating primarily on the rate of pyriteoxidation. Oxidation rates in a waste pile are largely

Ždominated by the rate of oxygen transport advec-. Žtion, convection and diffusion Nordstrom and

.Southam, 1997 . The overall oxidation of pyrite is acomplex process that can involve a number of reac-tants and products under varying conditions. Theoxidation of pyrite from the exposure to atmosphericconditions is well documented and can be sum-

Ž . Ž . Ž . Ž . Ž . Ž .marised by 1 , 2 , 3 and 4 . Reactions 1 and 2occur in acidic conditions with O as the primary2

Ž .oxidant, reaction 3 in basic conditions with O as2Ž .the primary oxidant and reaction 4 in acidic condi-

3q Žtions with Fe as the primary oxidant Moses et al.,1987; Webb and Sasowsky, 1994; Gray, 1997;

.Schuring et al. 1997; Kirby et al. 1999 :

2FeS q7r2O q2H O™2Fe2qq4SO2yq4Hq2 2 2 4

1Ž .

2Fe2qq1r2O q2Hq™2Fe3qqH O 2Ž .2 2

2FeS q15r2O q7H O2 2 2

™2Fe OH q4SO2yq4Hq 3Ž . Ž .Ž .3 s 4

FeS q14Fe3qq8H O™15Fe2qq2SO2y16Hq.2 2 4

4Ž .

Two possible oxidants for pyrite are available inmost mine scenarios, namely oxygen and ferric iron.The rate determining step in the oxidation of pyriteand the subsequent formation of acidity in receivingwaters is the rate at which Fe2q is oxidised by O ,2

Žwhich is shown to be a function of pH Singer and. 3qStumm, 1970 . However, in low pH conditions, Fe

Žoxidises pyrite more readily than O Moses et al.,2.1987; Moses and Herman, 1991 . Ferrous iron re-

Ž .leased in reaction 1 by the oxidative dissociation ofpyrite, is commonly referred to as the initiator se-

Ž .quence reaction Younger, 1997 . After this se-quence has begun, a cycle is then established inwhich Fe3q is subsequently reduced by pyrite, gen-

2q Ž 3qerating additional Fe oxidised by Fe , O and2. Ž . Žbacteria and acidity reaction 2 Moses and Her-

.man, 1991 .Oxygen transport to pyrite is assumed to occur by

diffusion from air-filled rock wall fractures into porespaces and then into the pyrite bearing waste piles

Ž .Fig. 1. Locality maps and stratigraphic setting. A Locality maps for the South Island, New Zealand and an insert of the Kaitangatacoalfields with reference to Dunedin city and the two main river catchments, the Taieri River and New Zealand’s largest river, the Clutha.Ž . Ž .B Summary stratigraphic column for the Kaitangata area. The thick sequence )1000 m of the Taratu Formation, a predominantly quartzrich fluvial sediment with occasional lenses of carbonate-rich mudstone, is the main coal-bearing unit. Otago Schist and greywackes form

Ž .the basement rock for the SE Otago region, and is the source of the sediment contained within the Taratu Formation. C Map view ofWangaloa open cast mine, indicating the position of the ponds, the main waste pile bounded at each side by cliffs of the Taratu Formation,

Ž .and wetland areas. A longitudinal sampling section is shown with transects in dashed lines. D An enlarged map view of the biggestwetland section immediately downstream from the main waste piles and tailings Pond 2. Crosses represent sampled areas and go all the wayto Pond 3 impounded behind a constructed dam.

( )A. Black, D. Crawr International Journal of Coal Geology 45 2001 181–193 183

Ž .Ritchie, 1993 . The assumption of atmospheric oxy-gen in the rock fractures accounts for all the trans-port mechanisms of oxygen into the wall rock.

Research at various abandoned mine sites in theŽ .UK Banks et al., 1997b; Younger, 1997 suggest

that acidity generated can be represented by two


Ž . Žcomponents: i a highly polluted first flush i.e. start. Ž . Žof wet season and ii prolonged acidity pyrite

.oxidation during seasonal water level fluctuations .When water levels are low within the waste piles,pyrite oxidation is facilitated by atmospheric oxygenŽ .Ritchie, 1993 . During periods of high rainfall andhence, higher water levels, pyrite oxidation by oxy-gen is restricted to aerated parts. In this study wehave collected field data in both dry and wet seasonsto compare results of these two stages of AMD.

Oxidation of pyrite in the environment is essen-tially an abiotic process, upon which several factorscan impact, mainly seasonal climatic effects andmicrobial activity. Mechanisms involved in definingthe magnitude of the environmental impacts of pyriteoxidation in waste piles have time scales of years to

Ž .tens of years Ritchie, 1993; Sengupta, 1994 . Thisstudy examines sources of metals during pyrite oxi-dation in a coal mine abandoned tens of years priorto the initial pilot study of 1998.

2. Site description

Wangaloa open cast mine is located within theKaitangata coalfield, approximately 80 km southwest

Ž .of the city, Dunedin Fig. 1A . The climate is charac-Ž . Ž .terised by seasons of high winter and low summer

Žrainfall and moderate temperatures average annual.temperaturef128C . The mine was opened in 1945,

and was included in a larger area that was purchasedby the New Zealand Government in 1946. From thenuntil the late 1980s, it operated as a major open castcoal mine. At the peak of its operation, some 23,000tonnes of coal were produced annually. Output de-clined after the 1950s, when new mines further south

Ž .were opened Harrington, 1958 . Wangaloa opencast mine was decommissioned in 1989. To date,remediation of the site has been indirect and limitedto establishment of commercial pine plantations.

There are three distinctive lithologies within theŽ . Ž .Wangaloa area Fig. 1B : i Basement rock metased-

Ž .iments, ii Taratu Formation, containing coal mea-Ž .sures, and iii Wangaloa Formation, a marine sedi-

mentary unit found along the coastal sections ofsoutheast Otago, but absent from Wangaloa mine asa result of tectonic uplift and subsequent erosion.

Basement rock at Wangaloa comprises low gradeschist and greywacke sandstone, belonging to the

Ž .larger Otago Schist group Haast Schist TerraneŽ .Fig. 1B . This quartzofeldspathic rock is the sourceof the fluvially derived Taratu Formation. The TaratuFormation unconformably overlies the basement rockand is the main coal bearing unit in southeast Otago.It consists predominantly of quartz gravels and sands,with horizons of coal and carbonaceous mudstone.The Formation is late Cretaceous to early Tertiary in

Žage and formed in a marginal marine setting Fig..1B . Coal horizons vary in thickness from 2 and 10

m and are interbedded with thick quartz conglomer-Ž .ate sequences f30 m . Pyrite at Wangaloa occurs

in the coal bearing horizons as pyritic cement, fram-boidal pyrite and more rarely cubic pyrite on thesurface of coal cleats and fractures.

Since the abandonment of the Wangaloa minedewatering scheme, two mine depressions im-pounded by waste piles are now filled with waterfrom groundwater seepage and rainfall. These twoponds are situated between the exposed cliffs of the

Ž .hillside and large waste piles Fig. 1C . A third pondis situated downstream of the largest water filled

Ž .pond, immediately beyond a wetland Fig. 1D . An-Ž .other wetland lies in a separate catchment stream 1

Ž .on the south side of the main waste pile Fig. 1C .

3. Methods

3.1. Sampling techniques

Solid and water samples were gathered in themonths of summer and winter of 1999. A pilot studywas undertaken in July 1998 to assess the extent ofaquatic degradation. Solid samples were collectedfrom depths of 30–40 cm in sterile plastic bags atdistances of 50 m apart down a longitudinal section,with sample transects, perpendicular to the sectionŽ .Fig. 1C . Water samples were gathered in plasticacid washed bottles from the three ponds, nearbysurrounding streams and wetlands, and some drainageand surface water features. Contamination was min-imised by standing upstream during sampling. Carewas taken not to disturb sediments in streams withlow flow rates and stagnant waters. Water fromtributaries of the Clutha River and sites of Lake


Table 1Ž .Whole rock geochemical analyses of the Wangaloa area in wt.%

Sample SiO Al O Total Fe MgO2 2 5

as Fe O2 3

Schist 59.95 16.21 7.57 3.17Schist 69.75 14.90 4.36 1.68Schist 68.73 15.55 4.18 1.42Schist 68.65 14.47 3.82 1.35Schist 75.27 12.42 1.87 0.81Schist 55.97 19.33 6.59 2.67Schist 73.66 13.63 2.48 0.88Schist 58.39 17.57 7.74 3.43Schist 52.17 18.83 8.83 4.25Pyrite 1 56.74 3.79 17.81 0.15Pyrite 2 56.75 3.8 17.82 0.06Pyrite 3 57.34 3.88 18.08 0.06Pyrite 4 57.34 3.88 54.6 0.06Pond 1 45 12.75 1.52 0.5Pond 2 44.59 12.63 1.53 0.48Pond 2b 44.92 12.67 1.4 0.59Waste pile 1 50.35 6.94 1.26 1.36Waste pile 2 77.53 4.88 0.83 0.17Waste pile 3 89.27 5.27 0.68 0.15Waste pile 4 78.87 5.45 0.76 0.13Waste pile 5 90.08 5.89 0.59 0.08Wetland 1 52.12 20.14 3.22 1.02Wetland 2 68.97 14.17 1.72 0.48Wetland 3 53.22 20.61 3.14 1.1Wetland 4 70.17 14.49 1.6 0.5

Tuakitoto were chosen as reference points for back-ground levels in the area, as these were unlikely tohave been influenced by mining activities.

3.2. Solid geochemical analyses

All solid geochemical analyses were obtained us-ing a Phillips PW-2400 Automated Sequential X-ray

Ž .Fluorescence Spectrometer XRF , from the GeologyDepartment, University of Otago.

Each solid sample was dried at approximately1008C for 8 h to remove all surface water. Afterdrying, approximately 50 g of material were thencrushed in a Tungsten-carbide swing-mill for 30 suntil a homogenised fine powder was achieved.

Samples analysed for trace elements follow theŽ .procedures outlined by Norrish and Chappell 1967 .

Approximately 5 g of homogenised powder isweighed out into a beaker, to which 0.5 ml of

Ž .Molwiol 10–98 material binder is added. Sample isthen placed into a cylinder mould and pressed for 10

s under 5 tonnes of pressure using a 30-tonne pressM-30 series, number 1107. A pressed disc is pro-duced, which is then dried under a heat source for 10min before being transferred to an oven set at 1108Cfor 8 h.

Ž .Trace element data -0.1% of rock compositionwas determined for all solid samples. Pressed discswere analysed by XRF which was calibrated using aset of international standards for the trace elementsAs, Cu, Pb and Zn. Consistency was checked using a

Žsubset of the international standards Govindavaju,.1994 as ‘unknowns’ during routine analyses. XRF

analyses have a precision of "2% relative error insamples with concentrations of 500 ppm and below.

Table 2ŽTrace element geochemical analyses of the Wangaloa area in

.ppm or mgrkg

Sample As Cu Pb Zn

Schist 5 39 10 88Schist 7 13 15 63Schist 8 13 10 77Schist 2 6 15 53Schist 9 9 15 30Schist 11 28 27 98Schist 3 6 13 48Schist 11 75 12 102Schist 6 33 9 106Waste pile 1 17 15 14Waste pile 1 16 71 33Waste pile nd 13 38 24Waste pile nd 10 44 28Waste pile 1 8 15 17Waste pile 1 10 7 11Waste pile nd 5 6 21Waste pile nd 3 6 12Waste pile nd nd 5 15Waste pile 1 18 5 10Waste pile 1 3 22 9Waste pile 2 13 17 22Wetland 3 35 43 32Wetland 4 15 55 47Wetland 1 24 12 26Wetland 1 14 34 33Wetland 1 5 7 9Wetland 5 13 71 51Wetland nd 15 78 53Pyrite 98 10 6 14Pyrite 44 16 19 17Pyrite 2 6 13 17Pyrite 10 7 5 22

ndsnot detectable.


Major element composition, or whole rock analy-Ž .ses )0.5% of SiO , Al O , Fe O , and MgO for2 2 3 2 3

selected samples for each substrate, was determinedusing XRF spectrometry. Major element analyses

Ž .were performed on fused beads glass discs pro-

duced in PtrAu crucibles fitted in an AutomaticFusion Technology Phoenix 4000 automatic castingmachine at approximately 11008C. A sample rock to

Žflux ratio 0.64000 to 6.80000 g with 1.0 g of.NH NO was used for all samples. Flux is used4 3

Table 3Ž .Water analyses of the Wangaloa area in ppm, or mlrl . Water samples are in approximate order with respect to distances in Fig. 5

Sample Arsenic Copper Zinc pH

BackgroundClutha River a1 0.003 0.02 nd 7.86

Clutha River a2 nd nd nd 7.12Clutha River a3 nd nd nd 6.9Clutha River a4 0.003 0.02 nd 7.2Clutha River a5 0.001 0.03 nd 6.8

Ž .Wangaloa coast inland 0.005 0.02 nd 7.43Lake Tuakitoto nd nd nd 7.23

Wangaloa samples S W S W S W S W

Pilot study 1998Near wetland 0.31 0.07 0.6 nmPond 2 0.7 0.02 0.57 nmEdge of pond 2 0.41 0.1 1.08 nm

Study 1999Pond 1 0.002 0.002 0.02 0.02 0.005 0.005 3.73 4.24Pond 1b 0.001 0.001 0.02 0.02 0.022 0.745 4.28 4.19Top of pond 2 0.003 0.001 0.02 0.02 0.005 0.13 4.68 4.64Waste pile 0.002 0.001 0.02 0.02 0.005 0.01 4.45 3.88Edge of pond 2 nd 0.003 nd 0.02 nd 0.067 4.6 4.57Waste pile 0.046 0.002 0.17 0.02 0.005 0.201 4.83 3.79Waste pile nd 0.002 nd 0.03 nd 0.171 5.29 4.23Waste pile 0.001 0.002 0.02 0.02 0.005 0.201 4.78 5.3Waste pile nd 0.001 nd 0.02 nd 0.13 4.06 3.88Waste pile 0.002 0.001 0.03 0.02 0.005 0.273 4.85 3.63Waste pile 0.006 0.001 0.01 0.02 0.029 0.011 4.02 4.63Pond 2 0.001 0.001 0.03 0.03 0.012 0.015 5.28 4.46Waste pile 0.005 0.001 0.02 0.02 0.005 0.005 5.02 4.19Waste pile nd 0.001 nd 0.02 nd 0.005 4.68 4.51Wetland 0.006 0.001 0.01 0.02 0.029 0.051 3.95 4.49

Wangaloa samples 1999 S W S W S W S W

Wetland nd 0.001 nd 0.03 nd 0.012 3.92 3.47Wetland 0.002 0.001 0.03 0.03 0.005 0.017 1.59 4.66Wetland 0.046 0.008 0.17 0.04 0.466 0.054 5.91 4.14Wetland 0.006 0.001 0.01 0.02 0.005 0.201 4.28 4.66Wetland nd 0.001 nd 0.02 nd 0.097 5.83 5.4Wetland nd 0.002 nd 0.02 nd 0.012 5.91 5.32Wetland nd 0.001 nd 0.02 nd 0.005 3.92 6.24Wetland 0.003 0.001 0.02 0.03 0.005 0.005 4.44 4.52Wetland 0.001 0.051 0.02 0.15 0.022 0.897 2.36 5.32Wetland nd 0.002 nd 0.02 nd 0.169 6.16 5.24Wetland nd 0.002 nd 0.02 nd 0.005 6.8 7.06

ndsnot detectable, nmsnot measured, Sssummer and Wswinter.


homogenate the whole rock and to lower its originalmelting point temperature. The flux used is SIGMA

Žbrand XRF Flux, Type 12:22 Norrish and Hutton,.1969 composed of 35.3% lithium tetraborate

Ž . Ž .Li B O and 64.7% lithium metaborate LiBO .2 4 7 2

Major element matrix effects were calculated on-lineŽ .using the Philips model Alphas from the software

Ž .XRF control package Super Q 1996 .Ž .Loss on ignition LOI is calculated from the

Ž .incineration approximately 11008C of 2 g of sam-ple. After heating for 30 min, samples are thenre-weighed, and loss of material is calculated as apercentage of the original weight. For coal and or-

Ž .ganic rich samples with loss on ignition LOI per-centage greater than 20%, analyses were performed

Ž .on the ‘ashed’ contents Tables 1 and 2 .

3.3. Water analyses

Water samples were gathered in acid washedplastic 250 ml bottles, acidified, and analysed fortotal recoverable As, Cu and Zn. Samples wereunfiltered and solid suspension particles were re-moved by acid digestion. Care was taken in the fieldto minimise the amount of solid suspension in thesamples.

Hydride generation atomic absorption spectrome-Ž .try HAAS method was used to obtain total recover-

able As amounts, while flame atomic absorptionŽ .spectrometry FAAS was used to obtain the total

Ž .recoverable amounts of Cu and Zn APHA, 1989 .Detection limits for each method were 0.001, 0.02and 0.005 ppm, for As, Cu and Zn, respectively. ThepH was measured in the field using a portable Oak-ton WD-35615 meter, calibrated with standard pH

Ž .solution and Zo-Bells solution Nordstrom, 1977Ž .Table 3 .

4. Results

4.1. Major constituents of solids

Taratu Formation material from the waste pileswas collected throughout the area and these consti-tute the principal material of the area. RedepositedTaratu material dominates the sediments in activewatercourses. In addition, framboidal pyrite, pyritic

cement and cubic pyrite, was sampled from KaiŽ .Point Coal working mine at Kaitangata Fig. 1A ,

Wangaloa open cast mine and Wangaloa beach forŽ .comparison. Basement rock data Otago Schist was

obtained from a University of Otago database, forcomparison.

These rock suites have distinctive chemical signa-Ž .tures Fig. 2A, B . Basement rock contains more

alumina than other material, because of the highmica and intermediate silica content, characteristic ofthe quartzofeldspathic provenance. Relatively highMgO in basement rocks represents a significant chlo-rite and phengite component. The Taratu Formationis predominantly derived from the basement rocks,

Ž .but is chemically more mature Fig. 2A, B andconsists mainly of quartz, with a minor alumina

Žcomponent as a result of the kaolinite matrix Fig..2A . Pyrite from Wangaloa mine and surrounding

areas varies in composition but unsurprisingly has

Fig. 2. Whole rock major element geochemical data for theprincipal rock types of the Wangaloa area relevant to this study.Ž . Ž . Ž .A Silica vs. alumina. B Total Fe as Fe O vs. MgO.2 3


Fig. 3. Whole rock trace element geochemical data for the princi-Ž .pal rock types of the Wangaloa area relevant to this study. A

Ž .Arsenic and zinc concentrations. B Lead and copper concentra-tions.

Žconsiderably more iron total iron, calculated as.Fe O than the two other rock types.2 3

4.2. Trace elements in solids

Trace metal plots of As, Zn and Cu, Pb for thethree rock units are shown in Fig. 3A and B. Re-gional background values for these metals are de-fined by the range of basement rock fields, whilelocal background values at Wangaloa are defined by

Ž .the Taratu Formation Fig. 3A, B . Arsenic is stronglyanomalous in the pyrite material, but Cu, Pb and Znare present in amounts similar to the defined regionaland local background levels.

4.3. Water chemistry

Water samples were taken at the same sites assolid samples above, wherever free water was pre-sent. General pH levels for different water sourcesŽ .not influenced by mining activities of the Wanga-loa area range from 6.5 to 8. Wangaloa mine catch-

ment has lower pH values than background ranges.The lower pH observations occur in close proximityto seeps from the main waste piles, and are assumedto be a result of localised AMD. The pH levelsduring summer are more variable than those mea-sured in winter, with a significant lowering of pH inthe main wetland portion. However, there is a returnto circumneutral pH at Pond 3. Winter pH measure-ments are more consistent and gradually increasewith distance from the AMD source, again reachingcircumneutral values at Pond 3.

Fig. 4. Metal concentrations of solids and co-existing waters fromŽ . Ž .the Wangaloa coal mine sampling transect Fig. 1C . A Arsenic

Ž .in sediments and waters. B Copper in sediments and watersŽ . Ž . Žnote log scales . C Zinc in sediments and waters note log

.scales .


Ž . Ž . Ž .Fig. 5. A generalised cross-section base of diagram of Wangaloa open cast mine, from west inland to east coast . Water samples weregathered along the longitudinal section and analysed for As, Cu and Zn, with the pH also measured in the field at the same sites. Sampleswere collected in two periods, during the summer and winter periods. Samples were gathered at distances of 50 m, with the exception of themain wetland section where samples were taken at 25-m intervals.


Comparisons of water metal contents in contactwith sediments are shown in Fig. 4A, B, C. Thelatter two plots use log scales to depict the widerange of measured compositions. There is no appar-ent relationship between amount of As in water andthe As content of associated sediment, with As water

Žconcentrations near or below the detection limit Fig.. Ž .4A . Higher As concentrations up to 50 ppb, Fig. 5

occur locally and transiently, with the highest dis-Ž .solved As content 0.7 ppm recorded in the 1998

pilot study from a water seepage entering Pond 1directly sourced from the main waste pile. Copperconcentrations in the sediments and surrounding wa-

Ž .ter Fig. 4B , also show no relationship between thetwo parameters. Zinc content of water is locallyelevated, although the associated Zn levels in thesediments are similar to background schist levels andthere is a weak positive relationship between Zn insediment and Zn in adjacent water.

Summer and winter As, Cu and Zn water concen-trations are shown directly below the pH measure-ments in Fig. 5. Arsenic levels were near or belowthe detection limit for both sampling periods with theexception of two high values observed in the mainwetland area. Background levels for As in the watersmostly ranged from slightly above, at or below thedetection limit. Summer and winter concentrations ofCu were similar to those of As, with most samples ator below the detection limit. One high peak for eachperiod was recorded, with both occurring in the mainwetland area. Zinc concentrations appeared the mostvariable of the three, with high peaks occurring inPonds 1 and 2, and in the main wetland sectionduring winter.

5. Discussion and conclusions

5.1. General obserÕations on AMD occurrence atWangaloa

The fundamental geochemical processes that areoccurring at Wangaloa mine appear to be the reason-ably similar to and consistent with overseas studies.Zinc is typically abundant in the catchments andexhibits a positive relationship with associated sedi-ment content. Arsenic and Cu are much less abun-dant, which probably reflect the different controls on

metal mobility and solubility. Recent studies haveshown that Zn adsorption and desorption in sec-ondary Fe minerals are primarily regulated by pH.Copper removal seems to be chiefly co-precipitatedwith iron and strongly related to Fe solubility and the

Žpresence of dissolved organic matter Herr and Gray,.1996: Kalbitz and Wennrich, 1998 . Arsenic solubil-

ity differs from Cu and Zn, with respect to pHsolubility controls, generally becoming more solubleas pH levels increase. Metal distribution is partlycontrolled by pH, with an increase generally causingthe co-precipitation and adsorption of Cu, Zn andoccasionally As under these conditions, primarily

Žwith secondary Fe minerals Winland et al., 1991;Boult, 1996; Yu, 1996; Banks et al., 1997a,b;

.Younger, 1997 .Copper was near or below the detection limits,

while As water content varied with maximumrecorded amount of 0.07 ppm, but mostly valueswere at or below detection limits. No relationshipwas observed between As and Cu water content andassociated As and Cu sediment content.

A correlation of metal content in the water andsediment metal content probably reflect two factors:Ž . Ž .i pH related solubility controls and, ii the increasein substrate–water interactions from protons released

Ž .by the oxidation of pyrite Banks et al., 1997a .Pyrite within the Taratu coal measures is assumed tobe the primary cause of AMD at Wangaloa and it issuggested that the rate at which oxidation occurs

Žmay be seasonally influenced ca. hydrological and.microbial factors as demonstrated in previous inter-Žnational studies Ritchie, 1993; Sengupta, 1994;

Blowes et al. 1995; Banks et al. 1997a; Klubek et al..1997; Edwards et al. 1998 . These studies also sug-

gested that influence of hydrological factors withrespect to As, Cu and Zn precipitation was impor-tant, with metal concentration in water dischargesdependent on rainfall.

5.2. Sources of pollution at Wangaloa

The waste piles are geochemically heterogeneouswith three distinctive groups present. Pyrite locatedwithin the waste piles at Wangaloa is locally en-riched in As with levels up to 100 ppm. Copper andZn are not significantly enriched above regional andlocal background levels in the waste pile con-


stituents. However, Zn is locally elevated in thewater catchments. Elevated As concentrations in thewater are probably derived from the oxidising pyrite.Dissolved As concentrations reach as high as 0.7ppm in the wetland area. However, the amount ofdissolved As decreases rapidly downstream due to acombination of factors such as, precipitation andadsorption with secondary Fe minerals, as well asdilution from increased rainfall andror mixing withother water sources. The waste rock is not signifi-cantly enriched in Cu, Pb or Zn in either the Taratusediments or the pyrite. Nevertheless, elevated dis-solved Zn concentrations are common in the mixingzones of AMD. It is suggested that elevated Znconcentrations arise due to the acid leaching of Zn in

Ž .trace amounts f1 ppm, Fig. 5 from the waste pileconstituents. This process affects Cu to a lesser

Ž .extent than Zn, but is still locally significant Fig. 5 .The pH levels of AMD and receiving waters at

Wangaloa are generally controlled by waste rockŽ .interactions between: i amount and transport mech-

Ž .anisms of atmospheric oxygen, ii rates of pyriteŽ .oxidation, iii presence of acid neutralising miner-

Ž .als, and iv dilution by mixing with other sources ofŽ .water Banks et al., 1997a; Younger, 1997 .

Calcite is absent from the Taratu Formation at theWangaloa site. Authigenic calcite does occur in thestratigraphically overlying marine Wangaloa Forma-

Ž .tion Fig. 1B , but this unit is absent from this studyarea. Hence, AMD generated by the oxidation ofpyrite in the waste rock piles is not neutralised bycalcite. Minor acidrrock reactions may occur withother carbonate or silicate minerals, but these areinsufficient to raise the pH.

5.3. Downstream metal concentration and mobilityat Wangaloa

Patterns of metal distribution in the southeastOtago coal mines probably reflect the influences thatdynamic water saturation levels have on mobilisingany available metals in the surrounding waste piles.Pyrite oxidation in dry periods is facilitated by en-hanced oxygen access, releasing arsenic, heavy met-als and sulphate mineral encrustations. During peri-ods of high rainfall, As, heavy metals and acid areflushed from the waste pile to the wetland. Passage

of the AMD through the waste pile area leaches As,Cu and Zn from the Taratu sediments and coalbearing horizons, which is probably facilitated by thepotentially, highly reactive surface area of the wastepile.

Dissolved metal concentrations are generallyhighest in the wetland sections of the catchment,particularly for Cu and As. Zinc concentrations werealso locally high within the wetland but high Znlevels are not exclusive to the wetland area and thehighest values were recorded at Pond 2.

Summer and winter variations of As, Cu and Znshow similar distributions for both periods. Duringsummer, water availability is the lowest with themetal distribution and residency times within watercontaining areas reflecting this. In the wettest season,metal distribution is more varied and the highestconcentrations of As and Zn were recorded duringthis period. Copper averaged higher concentrationsduring this time, but the highest peak recorded wasin the driest period.

This pattern of metal occurrence probably reflectsinfluences that changing degrees of saturation haveon mobilising available metals in the surrounding

Žwaste piles Galbraith et al., 1972; Boult et al., 1994;Boult, 1996; Gerke et al., 1998; Monterrosso and

.Macias, 1998 . During these periods of high rainfall,As, Cu, Zn and acid are flushed from the waste pilesinto the wetland, which are then immobilised ontosolid particles and diluted with the addition of differ-ent water sources. The presence of a wetland envi-ronment is advantageous as it provides an importantsite for metal dissolution and retention in this catch-ment.

6. Conclusions

The fundamental geochemical processes occurringin coal mines appear to be similar. The final pH andchemistry of AMD waters is controlled by interac-tions between the availability of oxygen, rates of

Ž .pyrite oxidation, trace metal content enrichmentand the availability of neutralising minerals.

Waste piles at Wangaloa are not significantlyenriched in Cu, Pb or Zn in either Taratu sedimentsor associated pyrite. However, Zn is locally and


transiently elevated in the receiving catchments. Ele-vated Zn concentrations in the water arise as a resultof acidic leaching of background Zn contents. Thisprocess affects As and Cu to a lesser extent than Zn.During summer, water availability is at its lowestwith the metal mobility and occurrence reflectingthis. During the winter period, metal distribution ismore varied and the highest concentrations of As andZn were recorded then. Copper occurs generally at orbelow detection limits.

Dissolved metal concentrations are generallyhighest in the wetland sections of the catchment,particularly for As and Cu. Zinc concentrations werealso locally high within the wetland, but high Znlevels were not exclusive to the wetland area and thehighest values were recorded at Pond 2. Clearly, thewetland environment provides an important site formetal dissolution and retention in this area, possiblydue to the change in redox conditions and the longresidence time for passing water.

Metals in the water are only marginally elevatedand in their present concentration pose no immediateenvironmental hazard with respect to the water qual-ity guidelines. The amount of dissolved As and Zn,which are locally and transiently high, rapidly de-crease downstream probably as a result of a combi-nation factors including co-precipitation and adsorp-tion onto secondary Fe minerals, and dilution fromincrease in rainfall.

Acknowledgements

This research was partially financed by the NewŽZealand Public Good Science Fund Contract

.UOO620 , as part of a regional analysis of theenvironmental effects of the mining industry. Addi-tional financial and logistical support was providedby University of Otago, Geology Department re-search funds. The first author wishes to gratefullyacknowledge financial support from the New ZealandBranch of the Australasian Institute of Mining andMetallurgy. Water analyses were provided by CitilabDunedin, under the guidance of Dr. Frank Ho. XRFresults were provided from the Geology Department,University of Otago, under the guidance of DamianWalls.

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Arsenic, copper and zinc occurrence at the Wangaloa coal mine, southeast Otago, New Zealand

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