www.elsevier.com/locate/scitotenv

Science of the Total Environm

Mechanisms of arsenic attenuation in acid mine drainage from

Mount Bischoff, western Tasmania

Andrew G. Gaulta,*, David R. Cookeb, Ashley T. Townsendc,

John M. Charnocka,d, David A. Polyaa

aSchool of Earth, Atmospheric and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science,

University of Manchester, Oxford Road, Manchester, M13 9PL, UKbCentre for Ore Deposit Research (CODES), University of Tasmania, Hobart, Tasmania 7001, Australia

cCentral Science Laboratory, University of Tasmania, Hobart, Tasmania 7001, AustraliadCLRC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK

Received 8 March 2004; received in revised form 13 October 2004; accepted 20 October 2004

Available online 25 January 2005

Abstract

There is a dearth of research concerning the geochemistry of arsenic in acid mine drainage (AMD) in western Tasmania. To

help address this, the controls on the mobility and fate of arsenic in AMD and its associated sediment at the Mount Bischoff

mine site in western Tasmania were investigated. AMD issuing from the adit mouth contained dissolved arsenic and iron

concentrations of 2.5 and 800 mg L�1, respectively. The aqueous concentration of both arsenic and iron decreased markedly

over a 150-m stretch from the adit mouth due to precipitation of hydrous ferric oxides (HFO) and jarosite, both of which are

effective scavengers of arsenic. Microwave-assisted digestion of the sediment collected at the adit mouth revealed that the

arsenic concentration exceeded 1%. Sequential extraction of this sediment showed that the bulk of arsenic was associated with

amorphous and crystalline hydrous oxides of Al and/or Fe. Extended X-ray absorption fine structure (EXAFS) analysis

indicated that the solid phase arsenic exists as As(V). EXAFS data were consistent with arsenate tetrahedra substituting for

sulphate in jarosite and with corner-sharing complexes adsorbed on ferric oxyhydroxide octahedra. Erosional transport of AMD

sediment downstream to higher pH waters may increase the mobility (and hence bioavailablity) of arsenic through dissolution

of As-rich jarosite.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Arsenic; Speciation; Acid mine drainage; EXAFS; Mount Bischoff; Tasmania

0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.scitotenv.2004.10.030

* Corresponding author. Fax: +44 161 275 3947.

E-mail address: andrew.gault@manchester.ac.uk (A.G. Gault).

1. Introduction

Western Tasmania is one of Australia’s most well-

endowed metallogenic regions, containing a remark-

able variety of ore deposits (Solomon, 1981). Mining

ent 345 (2005) 219–228

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228220

began in the area with the discovery of cassiterite at

Mount Bischoff in 1871 (Fig. 1). Over the next 3

decades, discoveries of important mineral deposits

such as the Mount Lyell copper deposit near Queens-

town (Fig. 1), Zeehan lead–silver, Renison tin and

Rosebery lead–zinc–silver–gold deposits resulted in

the development of western Tasmania as a major

mining district. In recent times, problems associated

with depressed base and precious metal prices and

increasing production costs have resulted in the

closure of many mines in western Tasmania, although

mining continues notably at Rosebery, Mount Lyell,

Henty and Savage River.

Over 100 years of exploitation of these sulphide-

rich mineral occurrences has resulted in significant

heavy metal pollution in many of the local rivers and

creeks. Most of the published research documenting

the nature and effects of such acid mine drainage

(AMD) has focused around the largest mining oper-

ation, Mount Lyell (Carpenter et al., 1991; Feather-

stone and O’Grady, 1997; Stauber et al., 2000). In over

a century of mining activities at Mount Lyell, some 100

million tonnes of sulphidic mine tailings and slag have

Fig. 1. Map of Tasmania showing the location of Mount Bischoff

and other selected mineral deposit sites in western Tasmania.

been discharged to the King and Queen River, forming

an extensive delta where the King River enters

Macquarie Harbour (Featherstone and O’Grady,

1997). Such gross contamination has significantly

impacted the local ecosystem. The bioavailability of

copper is a concern of a number of studies due to the

toxic nature of free dissolved Cu2+ (Featherstone and

O’Grady, 1997; Stauber et al., 2000; Eriksen et al.,

2001). Other recent work has used pollen and trace

element analysis of sections of sediment cores collected

from local lakes to assess the impact of mining on the

surrounding environment from the 1800s to the present

(Hodgson et al., 2000; Harle et al., 2002).

The paucity of research examining the trace

element geochemistry in AMD environments else-

where in western Tasmania, particularly with respect to

arsenic, formed the primary motivation for this snap-

shot study. Extended X-ray absorption fine structure

(EXAFS) spectroscopy, a technique only recently

applied to solid phase arsenic speciation analysis in

geological media (Foster et al., 1998; Savage et al.,

2000; Gault et al., 2003a,b; Paktunc et al., 2003), was

used to establish the coordination environment of

arsenic in AMD sediment collected from the aban-

doned Mount Bischoff tin mine. Based on data

obtained from EXAFS analysis, in addition to more

conventional sediment sequential extraction and water

trace element analyses, we were able to elucidate the

environmental behaviour and mobility of arsenic in

AMD from Mount Bischoff and ultimately predict its

fate.

2. Mount Bischoff background

Mount Bischoff is located in northwest Tasmania

(long 145831VE, lat 41826VS; Fig. 1), approximately

2 km north of the nearest township, Waratah. It was

discovered by James bPhilosopherQ Smith in 1871 and

was the first major ore discovery and mining

operation in Tasmania. The geology of the Mount

Bischoff tin deposit has been documented in detail by

Groves and Solomon (1964), Groves et al., (1972),

Wright and Kwak (1989) and Halley and Walshe

(1995). Briefly, Precambrian sedimentary rocks

(quartzite, shale and a 70-m thick bed of dolomite)

were intruded by numerous quartz–feldspar porphyry

dykes of Devonian age. Tin mineralisation formed

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228 221

during dyke emplacement, when magmatic-hydro-

thermal fluids caused massive replacement of dolo-

mite, greisenisation of the emplaced dykes and

deposition of polymetallic veins (Wright and Kwak,

1989; Halley and Walshe, 1995). Cassiterite is the

principal ore mineral at Mount Bischoff. It occurs

together with abundant pyrrhotite and lesser amounts

of pyrite, arsenopyrite, stannite, chalcopyrite and

sphalerite (Halley and Walshe, 1995).

Mining commenced in 1872 and continued inter-

mittently for more than 100 years, ceasing finally in

1978. During this period, approximately 62,000

tonnes of tin were produced (Halley and Walshe,

1995). There is an estimated resource of 4.72 million

tonnes remaining at around 0.59% Sn (Halley and

Walshe, 1995). Mining operations consisted of a

combination of open pit and underground mining

techniques. The site has not been rehabilitated, with

fresh sulphides exposed in surface mine workings.

Acid drainage is being generated by sulphide weath-

ering from the open pit and is discharging from the

main underground adit.

3. Sampling and analysis

3.1. Study site description and sample collection

A shallow (approximately 100 cm wide by 5–20 cm

deep) stream of acid mine drainage was issued at ca.

0.4–0.6 m s�1 from the main adit mouth on the

southern flank of Mount Bischoff (Fig. 2). The stream

flows down a narrow constructed channel over terraces

of orange-red sediment and over a small constructed

waterfall (~5 m high). The watercourse continues

through a series of shallow pools (approximately 5–

10 m long, 3–5 m wide and 1–2 m deep) into a

shallow-dipping trench that has been cemented by

terraces of orange-red sediments. Approximately

150 m downstream of the adit mouth, the acid drainage

passes over a steep waterfall into a final blood-red

coloured pool before discharging into the natural

stream, whereafter the drainage water meanders down

a densely forested hillside into the Waratah River, a

tributary of the Arthur River. A green algal-like film

was observed to coat the AMD sediment from the adit

mouth (but only in the sunlit area) until about 30 m

downstream. This is postulated to be composed of

acidophilic green algae, similar to that reported by

other workers studying AMD systems (e.g., Boult

et al., 1997; Brake et al., 2001); further micro-

biological examination, however, was not undertaken.

Water was sampled within 1 metre of the adit

mouth, with subsequent samples taken approximately

30, 40, 80 and 150 m downstream. Attempts to obtain

samples that were more distant were frustrated by the

relatively impenetrable vegetation through which the

AMD stream flowed. Approximately 100 ml of the

waters sampled were immediately filtered through a

0.45 Am filter (Whatman cellulose nitrate membrane)

into acid-washed polypropylene bottles (Azlon) and

acidified to 0.3–0.5% HNO3 (Mallinckrodt, Paris,

USA). Eh, pH, temperature and total dissolved solids

were measured on site using portable probes that had

been calibrated on the day of sampling. Samples of

loose surface sediment from the adit mouth and 40 m

downstream were collected using a plastic trowel and

double-wrapped in polythene bags. Care was taken to

collect the top 5–10 cm of sediment since that

represents the interface between the solid and aqueous

phases where arsenic uptake and/or release will occur.

All sampling was performed in September 2001.

3.2. Water analysis

Aqueous major and btraceQ elements were analysed

by ICP-OES (Iris, Thermo Jarrell Ash, USA) within 1

week of sample collection. External reference stand-

ards were used as quality check solutions to assess

analytical accuracy. Agreement was generally better

than 7% for the elements of interest. Arsenic concen-

trations determined by ICP-OES were also verified by

separate analysis by magnetic sector (or high resolu-

tion) ICP-MS (Element, Finnigan, Germany). This

technique allows interference-free measurement of

arsenic via increased spectral resolution (Townsend,

1999). The arsenic level in NIST 1643d bNaturalWaterQ reference material was found to be in good

agreement with the certified concentration using this

method (our analysis: 54.7F2.0 Ag L�1, certified:

56.0F0.6 Ag L�1).

3.3. Sediment characterisation

X-ray diffractometry (XRD) was used to determine

the mineralogy of the sediment collected. Each

Fig. 2. (a) Overview of Mount Bischoff sampling site showing the locations of the adit mouth and constructed waterfall (~5 m high); (b) adit

mouth showing green algal film on HFO precipitate; (c) closer view of algal coating; (d) series of shallow pools that the constructed waterfall

flows into; (e) final deep pool into which the AMD flows before discharging into the Waratah River. The reader is directed to the electronic

version of this paper, which contains a colour reproduction of this figure.

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228222

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228 223

sample, air-dried at room temperature, was ground

into fine slurry with the addition of a few drops of

amyl acetate, and the suspension was air-dried onto a

glass slide. The sample was then analysed using a

Philips 1730 diffractometer with Cu Ka radiation.

Subsamples (400–700 mg) of the two sediment

samples were dried overnight under helium at room

temperature. The surface area was then determined

using the N2-BET method (five point) with a Micro-

metrics Gemini 2360 gas adsorption BET analyser.

3.3.1. Sequential extraction

Sequential extractions procedures are often used to

assess the solid phase partitioning of a range of trace

elements, including arsenic (Keon et al., 2001; Wenzel

et al., 2001). Although the relative efficiency and

specificity of a particular reagent for its intended phase

may be subject to debate (Chao and Zhou, 1983;

Kostka and Luther, 1994; Hall et al., 1996; Smedley

and Kinniburgh, 2002), such techniques do provide

useful information regarding the operationally defined

chemical association of trace elements in sediments.

A six-stage sequential extraction, adapted from that

reported by Wenzel et al. (2001), was performed on

sediment collected from Mount Bischoff. A 0.05 M

(NH4)2SO4 solution was reacted with 1 g of sediment

for 4 h to remove easily exchangeable arsenic species.

Specifically sorbed surface-bound arsenic was

extracted with 0.05 M NH4H2PO4 for 16 h. Dis-

solution of amorphous and poorly crystalline hydrous

oxides of Fe and Al was effected by leaching the

sediment residue for 4 h in the dark with 0.2 M

ammonium oxalate buffer (pH 3.0). Well-crystallized

hydrous oxides of Fe and Al were targeted by reaction

with 0.2 M ammonium oxalate buffer (pH 3.0) and

0.1 M ascorbic acid for 30 min in light at 90 8C.Arsenic associated with sulphides and organics was

extracted through the addition of 1 g of potassium

chlorate and 20 ml concentrated HCl followed by

occasional shaking for 45 min then dilution with 15

ml deionised water. Finally, the sediment remaining

was dissolved using a microwave (MDS-2000, CEM,

USA)-assisted HF–H3BO3–HNO3 digestion. A single

digestion of the original, raw sediment was also

performed using the latter method to evaluate the

recovery of the sequential extraction procedure. All

extracts were analysed by ICP-OES (Horizon, VG

Elemental, UK).

3.3.2. EXAFS analysis

The local coordination environment of arsenic

associated with the Bischoff sediment was probed

using extended X-ray absorption fine structure

(EXAFS) spectroscopy. Arsenic K-edge X-ray

absorption spectra were obtained on Station 16.5 at

the UK CLRC Daresbury Synchrotron Radiation

Source operating at 2 GeV with a beam current of

between 130 and 210 mA. A Si (220) double crystal

monochromator was employed, with harmonic con-

tamination of the beam removed by a focusing mirror.

A thin layer of finely ground sediment was mounted

in an aluminium sample holder with Sellotape

windows. Fluorescence spectra were collected at

ambient temperature using an Ortec 30-element solid

state Ge detector.

After background subtraction, the data were

analysed in EXCURV98 using full-curved wave

theory (Gurman et al., 1984; Binsted, 1998), with

phase shifts calculated ab initio using Hedin–Lundqv-

ist potentials and von Barth ground states (Hedin and

Lundqvist, 1969). The experimental data were fitted

by defining a theoretical model and comparing the

calculated EXAFS spectrum with the experimental

data. Shells of backscatterers were added around the

central arsenic atom, and the As-scatterer distance (r),

Fermi energy and Debye–Waller factor (2r2) were

refined until a least squares residual (the R factor

(Binsted et al., 1992)) was minimised. For each shell

of scatterers around the arsenic, the number of atoms

in the shell was constrained as the integer or half

integer to give the best fit but was not further refined.

Additional shells of scatterers were only considered

justified if they improved the final fit of the data

significantly.

4. Results and discussion

4.1. AMD chemistry

The highly acidic waters that discharged from the

adit mouth contained elevated concentrations of a

range of major and btraceQ elements (Table 1),

including arsenic (2.5 mg L�1). Upon travelling

downstream, a general reduction in the total dissolved

solids was observed across all elements considered.

This is likely to be due to precipitation of and

Table 1

Concentration (mg L�1) of selected elements in acid mine drainage at Mount Bischoff and variation with distance from adit mouth

Distancea (m) pH As Cr Cu Pb Zn Fe S

0 2.3 2.5 1.2 2.7 1.0 12 810 720

30 2.4 2.4 1.2 2.7 1.0 12 790 730

40 2.5 2.4 1.2 2.7 0.9 12 790 730

80 2.5 1.4 0.8 1.8 0.6 8.0 550 510

150 2.5 0.8 0.5 1.3 0.5 5.6 310 350

RSDb – 5–10% 0.5–2% 0.5–2% 1–2.5% 0.2–1% 0.3–1.5% 0.2–1.5%

Values shown determined by ICP-OES.a Approximate distance downstream of adit mouth.b Typical relative standard deviations (RSDs) for each element.

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228224

subsequent trace element scavenging by hydrous ferric

oxide (HFO) as evidenced by the widespread ochreous

sediment terraces that underlay the AMD stream. Iron

and sulphur may also be lost from solution due to

jarosite (KFe3(SO4)2(OH)6) precipitation, which was

identified (together with a minor amount of quartz) at

Mount Bischoff by XRD analysis. Jarosite can act as a

sink for trace elements via adsorption/substitution/

coprecipitation processes (Dutrizac and Dinardo, 1983;

Scott, 1987; Levy et al., 1997; McGregor et al., 1998).

It should be noted, however, that amorphous HFO was

present in the analysed sediments, and the lack of long-

range order precludes their identification by XRD.

4.2. Arsenic association in sediment

The sediment samples collected from the adit mouth

and 40 m downstream had surface areas of 68.7 and

22.2 m2 g�1, respectively. This might seem counter-

intuitive since the finer suspended sediment material

would be expected to be deposited further downstream,

Table 2

Partitioning of arsenic (Ag g�1 dry weight) in Mount Bischoff sediment c

Extractant Operationally defined phase

0.05 M (NH4)2SO4 Non-specifically sorbed, exchange

0.05 M NH4H2PO4 Specifically sorbed

0.2 M NH4-oxalate Amorphous and poorly crystalline

oxides of Fe and Al

0.2 M NH4-oxalate+0.1 M

ascorbic acid

Well-crystallized hydrous oxides o

KClO4+HCl Organics and sulphides

HF–HNO3–H3BO3 ResidualP

Asa

HF–HNO3–H3BO3 Total digest

Arsenic concentrations determined by ICP-OES.a P

As denotes the sum of the arsenic concentration of the six fractions

rounding errors.

hence increasing the surface area of such sediment. The

sluggish waters flowing from the adit mouth form an

ideal setting for fine-grained clastic and chemical

sediment deposition. In contrast, the sampling site

40 m downstream is located a few metres beyond the

base of a small (~5 m height) waterfall. This higher

energy flow regime can transport larger detrital sedi-

ment particles, explaining the lower surface area of the

sediment collected in this location.

Table 2 displays the distribution of arsenic between

the operationally defined phases of sediment collected

from both locations. The total arsenic concentration in

sediment taken from the adit mouth and 40 m

downstream was 1.4% and 0.3%, respectively. The

high arsenic concentrations measured in the sedi-

ments demonstrates the efficiency of AMD precip-

itates in sequestering such trace elements due in no

small part to their large surface area. The green

coating on the AMD sediment, thought to be

comprised of acidophilic green algae and their

associated biofilm, may also play a role in attenuating

ollected at the adit mouth and 40 m downstream

Adit mouth 40 m downstream

able b0.2 b0.2

370F40 60F10

hydrous 4900F1000 1300F600

f Fe and Al 6800F1400 1200F600

590F60 510F50

70F40 b0.5

12700F1700 3100F900

13700F1400 3000F800

of the sequential extraction; inconsistency in summed total is due to

Table 3

Parameters obtained from fitting EXAFS As K-edge spectra for

Mount Bischoff AMD sediment collected at the adit mouth and 40 m

downstreama

Sample Atom

type

N r (2) 2r2 (22) R factor

Adit mouth O 4.0 1.69 0.005 26.7

MS 12.0 3.06 – 25.1

Fe 2.0 3.30 0.015 21.9

40 m

downstream

O 4.0 1.69 0.003 27.2

MS 12.0 3.06 – 26.4

Fe 2.0 3.28 0.010 21.5

a N is the coordination number (F25%), r is the interatomic

distance (F0.022 for the first shell,F0.052 for more distant shells),

2r2 is the Debye–Waller factor (F25%), and the R factor is the

goodness of fit. A multiple scattering (MS) contribution to the outer

shell is included in both fits (As–O–OV–As, path length equivalent

3.06 2).

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228 225

dissolved elements from solution (Boult et al., 1997),

including arsenic (Farag et al., 1998). Both sediments

showed good agreement (better than 8%) between the

sum of the arsenic concentrations for each stage of the

sequential extraction procedure and that obtained

from a single digestion. Recently, some studies have

shown that HF-based digestion procedures can lead to

diminished recoveries for arsenic (Krachler et al.,

2001, 2002); however, such work has involved

material with a considerably lower arsenic content

than the sediments studied herein, which may explain

the relatively good agreement observed.

Fig. 3. Normalized As K-edge EXAFS spectra (a) and radial distribution fu

mouth and 40 m downstream. Solid lines represent experimental data, and da

The majority of the sediment-hosted arsenic was

found to be associated with the amorphous and

crystalline hydrous oxides fraction (81–92%; Table 2),

with lesser amounts found in the sulphide and

specifically sorbed fractions. Arsenic present in the

extractant supernatant after the third extraction step is

likely to be released from the dissolution of amorphous

ferric oxides and partial dissolution of secondary

jarosite by the acid ammonium oxalate reagent used

(Dold, 2003a,b). That a considerable amount of arsenic

remains associated with the more recalcitrant opera-

tionally defined crystalline fraction suggests that the

remaining jarosite may harbour much of the sedimen-

tary arsenic. Although jarosite was the only Fe-bearing

phase identified by XRD, it should be noted that

crystalline iron oxides present at levels below the limit

of XRD phase detection (~5%) may also hold some

arsenic.

Table 3 lists the parameters that were found to yield

the best fit of the EXAFS spectra (Fig. 3a) for the adit

mouth and downstream sediments. The oscillations

giving rise to the first peak in the Fourier transform

(Fig. 3b) of both sediments were best fitted with a shell

of four oxygen backscatterers at 1.69 2. The coordi-

nation number, As–O bond distance and position of the

absorption edges all indicate that the bulk of the arsenic

present was in its highest oxidation state, As(V)

(Farquhar et al., 2002). A second shell interaction

between arsenic and iron was fitted at 3.28–3.302. The

nction (b) of Mount Bischoff AMD sediments collected from the adit

shed lines the least squares best fit using parameters listed in Table 3.

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228226

EXAFS fit was further improved by including a

multiple scattering contribution from the arsenate

tetrahedron, equivalent to a shell at 3.06 2 in the

Fourier transform. Similar observations of such multi-

ple scattering have been reported in other work

(Paktunc et al., 2003; Paktunc and Dutrizac, 2003;

Sherman and Randall, 2003).

The fitted As–Fe distances do not allow us to un-

ambiguously identify the host Fe-bearing phase that

harbours the arsenic. EXAFS analyses of the local

coordination environment of arsenate in jarosite-rich

materials have revealed an As–Fe interaction at 3.26–

3.32 2 (Savage et al., 2000; Savage et al., in press;

Paktunc and Dutrizac, 2003). Given that jarosite was

the only Fe-bearingmineral identified byXRD analysis

of the AMD sediments, the data are compatible with

arsenate substitution for sulphate in the jarosite

structure. Solid solution is extensive in alunite–jarosite

minerals (Scott, 1987), and the attendant charge

imbalance is likely to be compensated by protonation

of the SO4 and AsO4 (Paktunc and Dutrizac, 2003).

Alternatively, EXAFS studies concerned with arsenate

adsorption on the ferric oxyhydroxides goethite and

lepidocrocite have also obtained an As–Fe distance of

3.29–3.32 2 (Waychunas et al., 1993; Foster et al.,

1998; Randall et al., 2001; Farquhar et al., 2002). This

has been attributed to the presence of inner sphere

bidentate arsenate complexes attached to adjacent

apices of iron oxyhydroxide octahedra. Hence, the

arsenic present in the AMD sediments may be (i)

present as inner sphere complexes adsorbed on XRD—

amorphous HFOs and ferric oxyhydroxides, the latter

present at concentrations below the limit of XRD

detection; and/or (ii) incorporated into jarosite via

sulphate substitution.

4.3. Mobility of arsenic

AMD emanating from Mount Bischoff is likely to

be the primary source of arsenic to the Waratah River

downstream. Indeed, the arsenic concentration of a

water sample collected from the Waratah reservoir (0.2

Ag L�1), which sources the Waratah River, was

consistent with natural baseline levels of arsenic in

freshwater environments (b2 Ag L�1; Smedley and

Kinniburgh, 2002). Arsenic concentrations in the adit

drainage water decreased from 2.5 to 0.8 mg L�1 over a

distance of 150 m (Table 1); thus, it appears that

sorption by the AMD sediment is improving drainage

water quality prior to its entry to the Waratah River

under the base flow regime sampled during the current

study. No water sample could be collected downstream

of the confluence of the Waratah River and Mount

Bischoff acid drainage due to access problems; how-

ever, the dissolved arsenic concentration determined in

the Waratah River during a recent survey was less than

20 Ag L�1 (Mineral Resources Tasmania, 2003).

No arsenic was detected in the exchangeable

fraction of either sediment (Table 2). Although arsenic

may be released from the sediment through desorption

of specifically sorbed species, it is likely to be read-

sorbed by Fe-rich sediment a short distance down-

stream. Jarosite is stable under acidic conditions, thus

any arsenic that is substituted (as arsenate) for sulphate

within its structure is unlikely to be released in situ.

Western Tasmania experiences high levels of preci-

pitation (mean annual precipitation between 2400 and

3200 mm; Harle et al., 2002). Stormflow events are

common during the wet season. Such episodes may

result in the erosional transport of As-bearing sediment

downstream to waters of higher pH, leading to jarosite

dissolution and the concomitant release of its arsenic

burden. Future studies at Mount Bischoff need to in-

corporate sampling under stormflow conditions to test

this hypothesis. Dissolved iron released from jarosite

dissolution would be expected to precipitate relatively

rapidly as HFO in the oxic higher pH waters encount-

ered downstream. Thus, it seems likely that any arsenic

liberated from jarosite dissolution would ultimately be

removed from solution via adsorption on HFO-bearing

suspended particulate matter. This hypothesis is in line

with the findings of Azcue et al. (1994) who noted that

lakes impacted bymine tailings showed little difference

in dissolved arsenic concentration compared to those

unaffected by mining effluent. They concluded that

arsenic introduced to the lake was efficiently scavenged

by or coprecipitated with iron oxyhydroxides, a process

that has been utilized recently in the treatment of

arsenic-contaminated mine tailings and drainage (Kim

et al., 2003; Wang et al., 2003).

Acknowledgements

The authors acknowledge financial support from a

NERC/CASE studentship (04/99/FS/184) with

A.G. Gault et al. / Science of the Total Environment 345 (2005) 219–228 227

CETAC Technologies to AGG. EXAFS analysis was

supported by SRC Flexible Direct Access beamtime

award 38/286 to AGG and DAP. We would like to

thank Bob Bilsborrow and Fred Mosselmans (CLRC,

Daresbury) for their invaluable assistance with the

EXAFS data collection and Pete Morris for his help in

performing the BET analysis. Phil Robinson and Andy

Thompson at CODES are also thanked for their help in

preparing the sampling equipment and collecting the

samples in the field, respectively. Kaye Savage and

Dogan Paktunc are gratefully acknowledged for dis-

cussions regarding the incorporation of arsenic into

jarosite. We thank two anonymous reviewers for their

perceptive comments, which improved the manuscript.

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