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Assessment of peroxide oxidation for acid sulfate soil analysis. 1. Reduced inorganic sulfur.

Introduction

It is important to have an accurate and reliable measure of the reduced inorganic sulfur (generally assumed to be pyrite) content of acid sulfate soil (ASS) materials to avoid inappropriate or unnecessary treatment leading to the possibility of environmental damage or unnecessary cost. The oxidation of reduced inorganic sulfur with hydrogen peroxide is a commonly used technique in the determination of the potential environmental risk of ASS, pyritic mine waste, and coal residue (Sobek et al. 1978; Konsten et al. 1988; O'Shay et al. 1990; Lin and Melville 1993; Dent and Bowman 1996; Ahern et al. 1998, 2000; Jennings et al. 2000). In Australia, the determination of reduced inorganic sulfur is used to identify the presence of ASS, trigger the need for ASS management plans, and calculate lime requirements (Sullivan et al. 2000a). In the peroxide oxidation techniques the potential environmental risk of materials is quantified by determining: (1) the sulfur released as sulfate by pyrite oxidation (i.e. peroxide oxidisable sulfur), (2) the net acidity produced by pyrite oxidation, or (3) a combination of both of these measures. The underlying assumption upon which these methods are based is that there is complete oxidation of any pyrite present, and that the reaction stoichiometry is as follows (O'Shay et al. 1990):

(1) Fe[S.sub.2] + 15/2 [H.sub.2][O.sub.2] [right arrow] Fe[(OH).sub.3] + 2[H.sub.2]S[O.sub.4] + 4[H.sub.2]O

From Eqn 1, 2 moles of [H.sub.2]S[O.sub.4] can be theoretically liberated from every mole of pyrite oxidised. While there is evidence of this stoichiometry with the oxidation of pure mineral pyrite (Jennings et al. 2000; Latham et al. 2000), for complex mixtures of sulfidic and silicate minerals (e.g. ASS materials) the oxidation process may be complicated by interference from clays, organic matter, and sulfate minerals (White and Melville 1993; Clark et al. 1996; Lin et al. 1996; Ahern et al. 1998; Sullivan et al. 1998, 1999). These interferences may be substantial; for example, they may result in errors that are an order of magnitude greater than some of the action criteria (e.g. 0.03 %S) that are currently used to identify ASS in Australia (Sullivan et al. 1999). Such interferences with hydrogen peroxide techniques, particularly from the presence of organic matter, has meant that the modified chromium reducible sulfur (CRS) method of Sullivan et al. (2000b) is currently the recommended procedure in Australia for low sulfide-content soils (<0.2% [S.sub.CR]), and coffee rock and organic rich soils (Sullivan et al. 1999; Ahern et al. 2000; Latham et al. 2000).

The objective of this study was to determine the reliability of some widely used peroxide oxidation techniques for the quantification of the reduced inorganic sulfur content in ASS. The reliability of peroxide oxidation techniques for the quantification of both net acidity and sulfidic acidity in ASS is examined in the second paper of this series (Ward et al. 2002).

Materials and methods

Four ASS were collected from 3 coastal floodplain sites in north-eastern New South Wales: McLeods Creek, a tributary of the Tweed River (28 [degrees] 18"7'S, 153 [degrees] 30"50'E), Tuckean Swamp (28 [degrees] 56"00'S, 153 [degrees] 23"30'E), and Bungawalbin Swamp (29 [degrees] 06"27'S, 153 [degrees] 13"30'E) on the Richmond River floodplain. Samples were collected using a Russian D-section corer and the outer layer of each core was removed and discarded to prevent any contamination from the sampling process. The sections were mixed and stored in a thick plastic bag to minimise oxidation. Within 48 h of sampling all samples were freeze-dried using liquid nitrogen and a Dynavac freeze-drier. All samples were hand crushed using a porcelain mortar and pestle prior to analysis. Profile sampling depths and some of the characteristics of the ASS materials are shown in Table 1. All samples were in the reduced unoxidised zone as evidenced by the lack of visual iron and jarosite precipitates, except McLeods Creek A, which had experienced some oxidation and contained iron oxide segregations around vacant root channels.

Peroxide oxidisable sulfur was determined using 2 methods: (1) that outlined in the `peroxide oxidation combined acidity and sulfate (POCAS) method' (Ahern et al. 1998), and (2) the modified peroxide method (POCASm) (Ahern et al. 2000). In the POCAS method, peroxide oxidisable sulfur ([S.sub.POS]%) is determined by measuring the difference between sulfate after peroxide oxidation ([S.sub.P]%) and KCl extractable sulfate ([S.sub.KCI]%). As the oxidation step is not standardised in this method (in practice, different analytical laboratories employ different oxidation procedures, varying for example, the duration and temperature of the peroxide oxidation), a variety of different procedures were examined here. The POCAS method was undertaken at 3 different temperatures on triplicate samples: cold (20 [degrees] C), warm (<60 [degrees] C), and hot (<80 [degrees] C). As it is often practically difficult to precisely determine when oxidation is complete, a variety of peroxide oxidation durations were examined; these were 1 day, 2 days, and 4 days. In addition, all samples in the cold oxidation treatment had an additional peroxide oxidation duration of 10 days. Ahern et al. (1998) considered that visually the reaction had gone to completion when there was no further reaction (as evidenced by the evolution of bubbles from the samples) on standing, the mineral soil had become grey to light brown, and the supernatant was clear and transparent. Using these criteria the reaction was visually complete after 2 days for the McLeods Creek samples and 4 days for the other 2 samples, except for the samples analysed using the cold procedure (20 [degrees] C) (all of these samples were still bubbling after even 10 days of the cold treatment). All data, unless otherwise stated, were obtained at the optimum peroxide oxidation duration (i.e. following Ahern et al. 1998). The peroxide oxidisable sulfur using a modified peroxide method (POCASm) (Ahern et al. 2000) was also determined. This modified procedure uses a standard peroxide oxidation procedure in which the oxidation temperature is aimed to be 80 [degrees] C with an oxidation duration of 1 day. Two grams of sample (in quadruplet) was used for this procedure as recommended by Latham et al. (2000). All methods used analytical grade 30% hydrogen peroxide and blanks were run throughout.

The pH in a 1: 5 soil/water extract ([pH.sub.[H.sub.2]O]) and after peroxide oxidation ([pH.sub.OX]) were measured using an Orion 720A glass electrode. Electrical conductivity (EC) was also measured in a 1:5 soil/water extract using a TPS 900C conductivity meter. KCl extractable sulfate ([S.sub.KCl]%) and sulfate after peroxide oxidation ([S.sub.p]%) were analysed turbidimetrically using flow injection analysis colorimetry (Lachat QuikChem 8000); standards were made up in the same matrix (i.e. a KCl extract of the same molarity as the sample analysed) and all samples were filtered using a 0.45-[micro]m filter prior to analysis. The reduced inorganic sulfur fraction (which includes pyrite and other iron disulfides, elemental sulfur, and acid volatile sulfides) was measured in duplicate on all samples before and after peroxide digestion using the modified CRS technique of Sullivan et al. (2000b). After peroxide oxidation, all residues were washed 3 times with distilled water, centrifuged, and oven-dried at 60 [degrees] C prior to CRS analysis. The KCl extractable sulfur fraction ([S.sub.KCl]%) (Ahern et al. 1998) was removed from these residues, and the residue was then washed once with distilled water. The HC1 (4 M) acid extractable sulfur ([S.sub.HC1]%) (a measure of jarosite concentration) was determined on some of the residues after peroxide oxidation by extracting a 1:40 soil suspension with 4 M HC1 (Ahern et al. 1998); 0.5 g samples in duplicate were used. The sulfur extracted by the HC1 was analysed using ICP-AES. Total sulfur (%S) and carbon (%C) were measured on a LECO 220 Sulfur/Carbon analyser (using duplicates) for the samples before peroxide oxidation, and also for the residues after both hydrogen peroxide oxidation and subsequent HC1 extraction. Dry weights of all residues were recorded to allow sulfur budgeting; sulfur budgets were calculated on basis of the original oven-dry weight. The mineralogy of some of the ASS materials, before and after peroxide oxidation, was determined using powdered samples using a Philips PW 1820 diffractometer (XRD).

Moist filter paper was placed over 1-L peroxide oxidation beakers in triplicate to ascertain whether there were atmospheric losses during the POCASm method for selected samples. The filter papers were shaken for 1 h in 25 mL of distilled water and the sulfate content analysed turbidimetrically.

Results and discussion

Sulfur fractions in ASS materials

The total sulfur content of the 4 ASS soil materials prior to peroxide oxidation ranged between 2.36 and 6.80% (Table 2). The reduced inorganic sulfur fraction (i.e. the CRS fraction) was the dominant sulfur fraction in the 4 soils analysed, and ranged between 60 and 75% of the total sulfur present. The `insoluble + organic sulfur' fraction was approximately 25% of the sulfur present, whereas the `soluble and exchangeable sulfur' fraction was a relatively minor component for all of the ASS materials.

Effect of procedure on peroxide oxidisable sulfur determination

There was considerable variation in [S.sub.POS] depending on the duration of the peroxide oxidation for each oxidation method (Fig. 1). A decrease in [S.sub.POS] was generally observed with time (e.g. Fig. 1c), although some ASS materials showed an increase in [S.sub.POS] after 2 days (e.g. Fig. 1b). The variation in [S.sub.POS] with the peroxide procedure used at the completion of oxidation (or after 10 days oxidation duration for the cold treatment) is shown in Table 3. Larger [S.sub.POS] values were generally measured using the POCASm procedure compared with those using the POCAS procedure as was observed by Latham et al. (2000).

[FIGURE 1 OMITTED]

Chemical and microscopic evidence has previously shown that not all the pyrite present is oxidised after treatment with hydrogen peroxide (Konsten et al. 1988). While also observed in this study, the majority of the CRS fraction (>91% of the initial CRS concentration) had been oxidised after 1 day for all treatments. The POCASm was very efficient in oxidising the pyrite present, with <3% of the initial CRS concentration remaining after 24 h of oxidation. Clearly the considerable variations in the reduced inorganic sulfur contents determined using the various peroxide oxidation techniques employed in this study were not due to variations in the amount of incomplete oxidation of the pyrite in the ASS materials during peroxide oxidation.

As most of the reduced inorganic sulfur present had been oxidised by the hydrogen peroxide there should have been a good agreement between [S.sub.POS] and CRS, provided the assumptions underlying the peroxide oxidation procedure hold. However, for the non-peaty ASS materials [S.sub.POS] was observed to underestimate the reduced inorganic sulfur oxidised, with recoveries as low as 42% (see Table 3). Although the results gained using the POCASm method for these samples generally showed the best agreement between [S.sub.POS] and CRS, the [S.sub.POS] in the POCASm method still greatly underestimated reduced inorganic sulfur content with recoveries of only 69-76%.

One possible explanation of this underestimate in [S.sub.POS] maybe due to sulfate retention, through which the sulfate produced upon oxidation may be partially retained as adsorbed sulfates and basic A1 and Fe sulfate minerals (Lin et al. 1996). Analysis of the sulfur fractions in the residues after peroxide oxidation demonstrates that there is often an increase in the `insoluble and organic sulfur' fraction (up to 72%) from what was initially present in the ASS materials before peroxide oxidation, although this was not observed using the POCASm method (Table 4). The concentration of this fraction was also often observed to increase with the duration of the peroxide oxidation digestion. It is expected that organic sulfur should be greatly diminished by peroxide oxidation, as shown by the reduced total carbon contents of all of the ASS material residues (Table 5). Thus, an increase in the `insoluble and organic sulfur' fraction after peroxide oxidation clearly indicates an increase in the insoluble sulfur fraction. Further analysis of the remaining `insoluble and organic sulfur' fraction demonstrates that a large proportion (>84%) of this fraction was not extractable by 1 M KCl but was extractable by 4 m HC1 ([S.sub.HCI]%) (Table 4). X-ray diffraction of powdered samples identified considerable quantities of jarosite in the peroxide oxidation residues of all 4 samples (Fig. 2). None of the samples contained detectable (by XRD) quantities of jarosite prior to peroxide oxidation, [S.sub.HCI]% is largely a measure of jarosite formed during peroxide oxidation as HC1 will not extract organic sulfur (Ahern et al. 1998), and any water-soluble sulfate, exchangeable sulfate, and gypsum sulfate were already removed from the ASS residues.

[FIGURE 2 OMITTED]

The use of peroxide oxidation digestion procedures often creates the ideal conditions for the formation of jarosite which requires acid oxidising conditions (pH 2-4) and the presence of sufficient iron, potassium and sulfate (van Breemen 1973; Carson et al. 1982). Acidic solutions ([pH.sub.OX] ranged from 1.3 to 2.5 in this study) with high concentrations of iron and sulfate (from the oxidation of pyrite) and the use of 1 M KCl provides such conditions. Lower quantities of jarosite formed during the POCASm method probably because KCl is only added after the heating stage and the procedure is complete within one day. Nonetheless, the quantities of jarosite forming during peroxide oxidation in all peroxide methods examined were considerable with the proportion of sulfur being trapped as jarosite during peroxide oxidation being between 13% and 30% of the initial reduced inorganic sulfur content. The conversion of reduced inorganic sulfur to non-soluble sulfate during peroxide oxidation resulted in considerable underestimation of the reduced inorganic sulfur content of the ASS materials with low organic matter content.

While jarosite formation could account for the lower than expected peroxide oxidisable sulfur in the ASS materials with low organic matter, an overestimate was observed for the Bungawalbin ASS material using the POCASm method even despite the formation of appreciable jarosite in this sample during the peroxide oxidation (Fig. 2c and Table 4). The Bungawalbin soil is a peaty soil with a high organic matter content; a substantial portion of the sulfur in the organic matter would be oxidised by peroxide and included as peroxide oxidisable sulfur (Ahern et al. 1998, 2000; Sullivan et al. 1999). The total carbon content of the Bungawalbin ASS material decreased from 11.8% to 0.6% after oxidation with peroxide (Table 5), and the higher yields of reduced inorganic sulfur found using peroxide oxidation techniques for this sample (in one case an overestimation of reduced inorganic sulfur) despite considerable jarosite formation during peroxide oxidation, can be attributed to the oxidation of organic sulfur in this sample during peroxide oxidation to sulfate.

Overall sulfur budget

The determination of a sulfur budget for all of the ASS materials (Table 6) show a deficit in sulfur during the peroxide oxidation ranging between 22% and 28%, indicating that the observed underestimates in peroxide oxidisable sulfur for reduced inorganic sulfur found for ASS materials with low organic matter contents are not entirely due to the formation of jarosite during peroxide oxidation.

A possible reason for the observed sulfur deficit could include losses from washing the residues from peroxide oxidation with distilled water prior to analysis. However, solutions collected from the washing process were analysed for sulfate and found to contain negligible amounts of sulfate. While additive errors in the various determinations is a potential cause of this deficit, there was a good agreement between the techniques used (i.e. LECO and ICP-MS); potential errors were minimised from the analysis of standards at regular intervals, and it is unlikely that these errors would have resulted in a consistent loss. A more likely reason for the observed sulfur deficit could be the volatilisation of sulfur dioxide during the peroxide oxidation. Konsten (1984, cited in Konsten et al. 1988) found the sulfur budgets of some of the Dutch samples showed a decrease by 20-40% in total sulfur content after oxidation, and Konsten et al. (1988) showed that S[O.sub.2] is lost as a gas during peroxide oxidation of ASS materials. A decrease in the sulfur budget of a similar magnitude has also been observed by van Breemen (1976). Indeed, sulfur was trapped in moist filter papers that were placed (for a duration of 24 h) over the beakers during peroxide oxidation, although the amount of sulfur trapped by the moist filter paper only accounted for approximately 2% of observed sulfur deficit (i.e. 0.03 %S). Given that atmospheric losses of sulfur have been observed previously during peroxide oxidation (Konsten et al. 1988), it is most likely that this process has contributed to the sulfur deficit observed during the peroxide oxidation of these samples.

Interfering processes with peroxide techniques

All the peroxide methods used in this study assume that Eqn 1 holds. Essentially, the underlying assumptions are that there are not any sources of sulfur additional to reduced inorganic sulfur that produce sulfate during the peroxide oxidation, although the organic sulfur fraction is a known interference (Ahern et al. 1998, 2000; Sullivan et al. 1999), and that the sulfur liberated is converted to soluble or readily exchangeable sulfate. However, it is clear that when ASS materials are oxidised by hydrogen peroxide, additional processes interfere with the determination of the reduced inorganic sulfur fraction. These processes may result in either an underestimate or overestimate of the reduced inorganic sulfur fraction. The processes interfering with the determination of the reduced inorganic sulfur fraction identified from this study include:

* The formation of sulfur compounds during the peroxide oxidation not extractable by 1 M KCl (i.e. jarosite).

* The oxidation of sulfur fractions other than reduced inorganic sulfur (i.e. especially organic sulfur).

* The loss of sulfur during the peroxide oxidation digestion.

The magnitudes of these interferences are considerable (e.g. Fig. 3). While hydrogen peroxide methods may be represented by Eqn 1 when pure ground pyrite is used, the oxidation of pyrite by hydrogen peroxide in mineralogically diverse ASS materials is, rather than based on Eqn 1, better represented by Eqn 2 (where the equation is balanced only with respect to sulfur):

[FIGURE 3 OMITTED]

(2) aFe[S.sub.2] + b[S.sub.OM] + [H.sub.2][O.sub.2] [right arrow] (2a+b-0.5c-d)S[O.sub.4] + Ck[Fe.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6] + d[S.sub.LOSS]

where [S.sub.OM] is organic S; [S.sub.LOSS] is losses of sulfur during peroxide oxidation; and d [approximately equal to] s0.25(a+b)

Thins, in addition to the conversion of pyritic sulfur to soluble sulfate the results of this study indicate that organic sulfur is converted to soluble sulfate, that a considerable portion of this soluble sulfate is converted to insoluble jarosite during peroxide oxidation, and that substantial losses of sulfur (most likely atmospheric losses) occur during peroxide oxidation. Unless all of the processes implicit in Eqn 2 can be quantified then the use of peroxide oxidisable sulfur as reliable quantitative determination of reduced inorganic sulfur content in ASS materials is not warranted. Although the results in this paper were gained using ASS materials, it is expected that the findings of this paper may also be applicable for the analysis of other sulfidic materials such as pyritic mine waste and coal residue.

Conclusions

The determinations of peroxide oxidisable sulfur using all of these methods examined were subject to considerable interferences due to the precipitation of jarosite during the peroxide oxidation, the oxidation of organic sulfur, and the loss of sulfur (most likely atmospheric losses) during the peroxide oxidation. This study shows that the peroxide oxidisable sulfur procedures examined in this study are not able to provide reliable determinations of the reduced inorganic sulfur content of ASS materials.
Table 1. Characteristics of the ASS materials examined

Location Depth (m) Texture [pH.sub.[H.sub.2]O]

McLeods Creek A 1.10-1.25 Light clay 6.4
McLeods Creek B 1.90-2.40 Light clay 8.2
Bungawalbin 1.00-2.40 None (i.e. peat) 3.8
Tuckean Swamp 1.00-1.40 Light clay 3.2

Location EC (dS/m) Total C (%)

McLeods Creek A 1.57 1.70
McLeods Creek B 2.37 1.69
Bungawalbin 1.96 11.78
Tuckean Swamp 3.18 2.20
Table 2. Sulfur fractions (%S) present in the ASS materials prior to
peroxide oxidaiion

`Soluble + exchangeable sulfur' was determined by KCl extraction
([S.sub.KCl]%); `Insoluble + organic sulfur' = total S - CRS -
[S.sub.KCl]

Site Total S CRS Soluble + Insoluble +
 exchangeable S organic S

McLeods Creek A 2.36 1.71 0.11 0.54
McLeods Creek B 3.59 2.70 0.08 0.80
Bungawalbin 5.16 3.07 0.16 1.92
Tuckean Swamp 6.80 4.38 0.78 1.63
Table 3. Variation in [S.sub.POS] (%) with peroxide oxidation method

Numbers in parentheses show [S.sub.POS] as a percentage of the CRS
value for that sample

Site Cold Warm
 (20 [degrees] C) (<60 [degrees] C)

McLeods Creek A 0.85 (50%) 0.71 (42%)
McLeods Creek B 1.59 (59%) 1.43 (53%)
Bungawalbin 3.03 (100%) 2.50 (82%)
Tuckean Swamp 2.23 (53%) 2.99 (70%)

Site Hot Modified hot
 (<80 [degrees] C) (POCASm)

McLeods Creek A 0.91 (53%) 1.29 (76%)
McLeods Creek B 1.90 (70%) 1.96 (73%)
Bungawalbin 2.38 (78%) 3.40 (111%)
Tuckean Swamp 2.08 (48%) 2.93 (69%)
Table 4. Insoluble and organic sulfur (%S) fraction before and
remaining after peroxide oxidation

Sulfur measured in residue excludes CRS remaining after oxidation;
numbers in parentheses indicate the HCl (4 M) extractable sulfur (i.e.
mainly jarositic sulfur)

Site Initial Peroxide oxidation method used

 Cold Warm
 (20 [degrees] C) (<60 [degrees] C)

McLeods Creek A 0.54 0.80 0.93 (0.89)
McLeods Creek B 0.80 1.05 1.07 (1.02)
Bungawalbin 1.92 0.78 1.25
Tuckean Swamp 1.63 1.96 2.21

Site Peroxide oxidation method used

 Hot Modified hot
 (<80 [degrees] C) (POCASm)

McLeods Creek A 0.95 0.31 (0.26)
McLeods Creek B 1.06 0.67 (0.63)
Bungawalbin 1.19 (1.16) 0.46 (0.39)
Tuckean Swamp 2.05 (2.02) 1.39 (1.29)
Table 5. Total carbon fraction (%C) before and remaining after
peroxide oxidation

Site Initial Peroxide oxidation method used

 Cold Warm
 (20 [degrees] C) (<60 [degrees] C)

McLeods Creek A 1.70 0.19 0.19
McLeods Creek B 1.69 0.19 0.21
Bungawalbin 11.78 1.11 1.00
Tuckean Swamp 2.20 0.26 0.29

Site Peroxide oxidation method used

 Hot Modified hot
 (<80 [degrees] C) (POCASm)

McLeods Creek A 0.15 0.12
McLeods Creek B 0.17 0.15
Bungawalbin 0.60 1.15
Tuckean Swamp 0.21 0.20
Table 6. Sulfur budget (%S)

Numbers in parentheses indicate the sulfur deficit as a proportion of
the initial total sulfur

Site Peroxide Initial [S.sub.POS] [S.sub.KCL]
 method used total (%) (%)
 (%)

McLeods A Warm 2.36 0.71 0.11
 POCASm 2.36 1.29 0.11
McLeods B Warm 3.59 1.43 0.08
 POCASm 3.59 1.96 0.08
Bungawalbin Hot 5.16 2.38 0.16
 POCASm 5.16 3.40 0.16
Tuckean Hot 6.80 2.08 0.78
 Swamp POCASm 6.80 2.93 0.78

Site Peroxide Residue S fractions
 method used
 [S.sub.HCl]% [S.sub.KCl]% Insoluble +
 organic S%

McLeods A Warm 0.89 0.03 0.01
 POCASm 0.26 0.05 0.01
McLeods B Warm 1.02 0.04 0.01
 POCASm 0.63 0.03 0.01
Bungawalbin Hot 1.16 0.02 0.02
 POCASm 0.39 0.03 0.04
Tuckean Hot 2.02 0.03 0.03
 Swamp POCASm 1.29 0.02 0.21

Site S deficit
 (%)

McLeods A 0.61 (26)
 0.65 (28)
McLeods B 1.01 (28)
 0.88 (25)
Bungawalbin 1.42 (28)
 1.14 (22)
Tuckean 1.87 (28)
 Swamp 1.58 (23)


Acknowledgments

This research was undertaken as part of Project 1.4 `Coastal soil processes and their management for sustainable tourism development' funded by the Cooperative Research Centre (CRC) for Sustainable Tourism.

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Manuscript received 18 February 2001, accepted 3 October 2001

Nicholas J. Ward (A), Leigh A. Sullivan, Richard T. Bush, and Chuxia Lin

Centre for Acid Sulfate Soil Research, Southern Cross University, Lismore, NSW 2480, Australia.

(A) Corresponding author; email: nward@scu.edu.au
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Author:Ward, Nicholas J.; Sullivan, Leigh A.; Bush, Richard T.; Lin, Chuxia
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Geographic Code:8AUST
Date:May 1, 2002
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