The occurrence of inland acid sulphate soils in the floodplain wetlands of the Murray-Darling Basin,...

The occurrence of inland acid sulphate soils in thefloodplain wetlands of the Murray–Darling Basin,Australia, identified using a simplified incubation method

N. CREEPER1,2, R. FITZPATRICK

1,2 & P. SHAND1,2 ,3

1Acid Sulfate Soil Centre, EES, The University of Adelaide, Private Bag No 1, Glen Osmond, South Australia, 5064, Australia,2CSIRO Land and Water, Private Bag No 2, Glen Osmond, South Australia, 5064, Australia, and 3School of the Earth Sciences,

Flinders University, PO Box 2100, Adelaide, South Australia, 5001, Australia

Abstract

From 2006 to 2010, low water levels resulted in the drying of previously submerged inland acid

sulphate soils (IASS) in wetlands of the Murray–Darling Basin (MDB). The potential for

widespread severe acidification resulting from the oxidation of pyrite in these wetland soils triggered

a basin-wide study to assess the occurrence and risks posed by IASS material in the floodplain

wetlands of the MDB. The results of pH measurements before and following soil incubation from

more than 7200 samples (representing ca. 2500 profiles from 1055 georeferenced wetlands) were

used to assess the potential occurrence of sulphuric and sulphidic material in IASS across the

MDB. Their occurrence was investigated on a regional basis by dividing the MDB into 13

geographical regions whose boundaries roughly follow hydrological catchment boundaries. A total

of 238 floodplain wetlands, representing 23% of the total wetlands assessed, were found to contain

soils that became ultra-acidic (pH < 4) when oxidized and therefore present a severe acidification

hazard. These soils, the majority of which are likely to be IASS materials, were found in 11 of the

13 geographical regions. Among the 11 geographical regions likely containing IASS materials, the

proportion of wetlands that presented an acidification hazard varied between 2 and 52% of those

assessed. The geographical regions found to present the greatest acidification hazard were in the

southern MDB, downstream of the Murray–Darling confluence, and in catchments on the southern

side of the Murray River channel in Victoria. This study provided policy makers with a valuable

screening tool, which helped them to identify priority wetlands and regions that required more

detailed IASS investigations.

Keywords: Acid sulphate soils, inland regions, Murray–Darling Basin, distribution

Introduction

Soils or unconsolidated sediments that contain or are

affected by transformations of iron sulphide minerals are

termed acid sulphate soils (ASS) (Pons, 1973; Dent, 1986;

Dent & Pons, 1995; Isbell, 1996). ASS may either contain

sulphuric material or have the potential to form sulphuric

material in amounts that have an influence on the soil

characteristics. Where severe acidification occurs due to the

oxidation of sulphidic material, it can lead to a number of

detrimental impacts on the surrounding ecosystem. For

example, (i) flora can be impacted by soil acidification and

Fe and Al toxicity, (ii) water and/or soil quality can be

directly or indirectly impacted by increased concentration of

acidity and dissolved metals and metalloids and (iii)

amenities and structures can be damaged through the

corrosion of concrete and steel structures.

There are a number of soil classification systems used to

classify ASS materials. The Australian Soil Classification

(Isbell, 1996) and Soil Taxonomy (Soil Survey Staff, 2010)

are two such examples and use similar definitions for

sulphidic and sulphuric materials. In this study, the

Australian Soil Classification is used. Complete definitions

are available in Isbell (1996) but to summarize: (i) sulphuric

material has a field pH < 4 as a result of the oxidation of

sulphidic material and (ii) sulphidic material contains

oxidizable sulphur compounds and has a field pH of � 4 butCorrespondence: N. Creeper. E-mail: [email protected]

Received May 2012; accepted after revision October 2012

130 © 2012 The Authors. Journal compilation © 2012 British Society of Soil Science

Soil Use and Management, March 2013, 29, 130–139 doi: 10.1111/sum.12019

SoilUseandManagement

becomes ultra-acidic when oxidized and is identified by a

drop in pH of � 0.5 units to a pH < 4 following oxidation.

The incubation method is the most commonly used method

to identify sulphidic material and is preferred over other

methods because it is more likely to simulate the natural

acidification behaviour of the soil (Sullivan et al., 2009;

Creeper et al., 2012). The incubation method assesses a soil

acidification potential by exposing it to the atmosphere over

a period of time to allow acid generating oxidation reactions

to take place.

Acid sulphate soils are associated with a range of

geomorphologically varied environments that provide

adequate concentrations of available organic matter,

sulphate and iron along with reducing conditions

(Fitzpatrick & Shand, 2008a; Fitzpatrick et al., 2009a). ASS

are commonly found along Australia’s modern-day coastal

zones [defined as those areas landwards of the coastal waters

influenced by processes or activities that affect the coast and

its values (Natural Resource Management Ministerial

Council 2006)] in environments such as mangroves, back

swamps and estuarine systems. In these environments, ASS

are often termed Coastal ASS or Coastal Lowland ASS.

Acid sulphate soils can also be encountered in inland

environments (those areas landward of modern-day coastal

zones) such as river and stream channels, lakes, wetlands,

drains and floodplains. ASS in these environments are

termed Inland ASS (IASS). There are an estimated

215 000 km2 of ASS in Australia, the majority

(157 000 km2, 73%) of which occur inland of the modern-

day coastal zones and are thus classified as IASS

(Fitzpatrick et al., 2008b). Whilst the prevalence of ASS in

Australia’s modern-day coastal zone is widely known, until

recently the occurrence and properties of IASS in freshwater

inland systems have received significantly less attention. This

is in part due to the more recent appreciation of the extent

of IASS in inland systems, including the Murray–Darling

Basin (MDB). Field investigations into the occurrence of

IASS in the floodplain wetlands of the MDB only began to

increase in intensity circa 2006, correlating with the onset of

the worst drought conditions experienced by the MDB in

recent history.

In large parts of the MDB, the installation of locks, weirs,

blocking banks and barrages led to a shift from natural

wetting, flushing and partial drying cycles prior to European

occupation, to prolonged periods of inundation (circa last

50–80 years). Where changes to the hydrological conditions

in the regulated sections of the river and stream channels,

lakes and wetlands of the MDB resulted in prolonged

periods of inundation, the promotion of anoxic conditions

and subsequent accumulation of sulphidic material became

more likely. In addition, the naturally high organic carbon

loading of wetlands and increases to sulphate concentrations

due to practices such as leakage from agricultural or

industrial systems (Lamontagne et al., 2006) and rising

groundwater (Hicks et al., 2003) have enhanced the potential

of sulphide mineral formation throughout the floodplain

wetlands of the MDB. From 2006 to 2010, low water levels,

ensuing from the worst drought conditions experienced

throughout the MDB in recent history, resulted in the

widespread drying of previously submerged IASS in

floodplain wetlands of the MDB (Fitzpatrick et al., 2009a).

This provided ideal conditions for the oxidation of pyrite

that had accumulated in these soils over the preceding 50–

80 years.

This study represents the most extensive broad-scale

investigation into the occurrence of IASS in the floodplain

wetlands of the MDB. A number of existing reports have

investigated the occurrence of IASS in the MDB at a semi-

regional scale, for example, in South Australia (Lamontagne

et al., 2006; Wallace et al., 2008a,b; Fitzpatrick et al., 2009a)

and New South Wales (Hall et al., 2006). Hall et al. (2006)

sampled 81 wetlands in an attempt to establish how

extensive sulphidic sediments were in the MBD. It was

concluded that 17 wetlands (21%) had levels of reduced

sulphur that may be of concern. It was also stated that

although the study did not have the statistical power to

generalize its results over all wetlands in the MDB, the

number of wetlands that contained sulphidic material was

high enough to suggest that the occurrence of IASS in the

MDB is not uncommon. There have also been a growing

number of detailed studies on specific wetlands or small

areas in the MDB that are beginning to provide a reliable

picture of the spatial extent of IASS in the floodplain

wetlands of the MDB. For example, the occurrence of IASS

in the lower lakes (Fitzpatrick et al., 2010) and the Murray

River (Fitzpatrick et al., 2008b; Shand et al., 2008, 2009,

2010) has been covered in significant detail.

This study makes use of the soil samples collected as part of a

basin-wide study initiated in March 2008 by the Murray–Darling

Basin Authority (MDBA) to assess the spatial extent of, and

risks posed by, IASS material in the MDB (Murray–Darling

Basin Authority, 2011). The results of field pH

measurements and pH measurement following incubation,

comprising of more than 7200 individual pH measurements,

representing ca. 2500 profiles sampled within 1055

georeferenced wetlands, were used to gain an insight into the

potential occurrence of IASS in the floodplain wetlands of

the MDB.

Methods

Wetland selection

The MDB contains over 30 000 wetlands >1 ha in size, and

assessment of every wetland in the basin was not feasible. To

better target wetlands of interest, wetlands were subjected

to a multi-tiered selection process (Figure 1) designed to

preferentially select those wetlands that had a high

© 2012 The Authors. Journal compilation © 2012 British Society of Soil Science, Soil Use and Management, 29, 130–139

The occurrence of IASS in the floodplain wetlands of the Murray–Darling Basin 131

environmental significance and/or an increased likelihood of

containing IASS (Murray–Darling Basin Authority, 2009;

Murray–Darling Basin Authority, 2011).

The initial selection process (identification of wetlands for

assessment, Figure 1) utilized existing databases and data

from questionnaires sent to regional wetland managers. This

process resulted in the generation of a preliminary list

containing over 19 000 wetlands. Wetlands included in this

list satisfied at least one of the six following criteria: (i)

Ramsar wetlands or wetlands listed in the Directory of

Important Wetlands in Australia, (ii) wetlands affected by

regulated flows in the River Murray system, (iii) managed

wetlands, (iv) wetlands and creek systems receiving irrigation

return water, (v) wetlands within close proximity to domestic

water supply off-takes and (vi) wetlands identified as high

priority by jurisdictional representatives. Wetlands that did

not satisfy any of these criteria were not included in the

assessment process.

A shortlist was then resolved from the preliminary list

(desktop assessment, Figure 1). Further criteria were

developed to identify those wetlands that were at an

increased risk of containing IASS materials. The criteria

included whether (i) the wetland received irrigation return

water or wastewater, (ii) whether it intercepted saline

groundwater or (iii) if it had an EC of >1 500 dS/m. This

process identified almost 1500 wetlands with an increased

risk of containing IASS materials and required further

assessment. Of the almost 1500 wetlands identified, 1329

wetlands underwent a rapid assessment. Of the 1329

wetlands that underwent rapid assessment, pH results before

and after incubation were collected from 1055 of these

wetlands. The remaining wetlands do not form part of this

study as they either did not undergo a rapid assessment due

to access limitations or did not undergo pH incubation

measurements due to time constraints.

Wetland rapid assessment

The assessment and collection of soil and water samples

from a shortlisted wetland was referred to as a ‘rapid

assessment’. The rapid assessment was a field-based assessment

procedure that could be completed in ca. 30 min and was

designed to answer a series of questions aimed at

determining the likelihood that a wetland contained IASS

materials. The rapid assessments were conducted by state

and regional Natural Resource Management (NRM) agency

staff that had attended a specifically designed training

course. An easy to understand guide, which included

sampling protocols and a data recording sheet, was

developed by the MDBA to assist the often non-soil

specialist regional NRM agency staff when conducting rapid

assessments (Murray–Darling Basin Authority, 2009).

The rapid assessment of a wetland included the collection

of soil samples at three different sites within a toposequence

in each wetland (Table 1). At each site, soil samples were

collected for three different layers: surface (0–5 cm),

subsurface (5–30 cm) and subsoil (>30 cm). Collected soil

samples were placed in chip trays (Creeper et al., 2010, 2012)

and posted to the laboratory for analysis. Further details

including an accurate description of the rapid assessment

procedure are given by the Murray–Darling Basin Authority

(2009).

Sample analysis

Field soil pH was measured immediately following collection

of soil samples. Field soil pH was measured with pH

indicator strips [Merck item number: 1.09541.0001 (pH 2.5–

4.5); 1.09541.0002 (pH 4.0–7.0)] when conducting rapid

assessments due to their ease of use by nonsoil specialist

regional NRM agency staff. The pH of a soil sample

following incubation was determined using the simplified

incubation method. Full details and discussion of the

simplified incubation method can be found in earlier

papers (Creeper et al., 2010, 2012). In summary,

representative soil samples were placed into individual chip

tray compartments forming a layer of soil ca. 10 mm

thick. Chip trays were used as incubation vessels for the

soil samples as they produce similar incubation conditions

to soil slabs (Soil Survey Staff, 1999, 2010; Sullivan et al.,

2010) but offer some advantages over soil slabs such as

ease of use during sampling, storage and analysis. Soil

samples were incubated under aerobic conditions, and

during incubation, soil samples were maintained in a moist

state (i.e. near field capacity). After � 9 weeks of

incubation, soil pH was measured for all samples. For

soil samples that had a pH between 4 and 6.5

(4 � pH � 6.5), following the initial � 9-week incubation

period, soil pH was remeasured after an additional

� 10 weeks of incubation (total incubation period

� 19 weeks). Measurement of soil pH took place within

the chip tray compartments. Prior to pH measurement, the

soil sample was homogenized by mixing with a glass rod,

whilst the minimum amount of deionized water was added

until a stable junction between the pH electrode and soil

water was achieved (approximate soil-to-solution ratio of

1:1 or less).

Identification of wetlands for assessment

Desktop assessment

Rapid assessment

1329 wetlands

19 000 wetlands

Figure 1 Flow chart of the multi-tiered wetland selection process.


132 N. Creeper et al.

Geographical regions

The potential occurrence of sulphuric and sulphidic materials

in IASS throughout the floodplain wetlands of the MDB

was investigated on a regional basis. This was achieved by

dividing the MDB into 13 geographical regions (Figure 2).

Geographical region boundaries were adapted from CSIRO

Water for a Healthy Country sustainable yields project

(CSIRO, 2008) and roughly follow catchment boundaries. A

number of smaller catchments or catchments with a small

number of rapid assessed wetlands were combined into single

geographical regions (Table 2). The large Murray and

Darling catchments were split into smaller geographical

regions. Table 2 provides details of the boundary limits for

these geographical regions.

Results

Acidification of soils in the floodplain wetlands of the MDB

Throughout this study, a wetland was considered to present a

severe acidification hazard if one or more soil samples within

the wetland were found to be ultra-acidic (Schoeneberger

et al., 2002) at the time of collection (field pH < 4) or

became ultra-acidic upon oxidation (pH < 4 following

Table 1 Sampling locations for the rapid assessment of a ‘wet’ and ‘dry’ wetland

Sampling site Wet wetland Dry wetland

Site 1 Below the waterline at 5–10 cm water depth Visible lowest point of the wetland

Site 2 Above the water line at the water’s edge Approximately midway between site 1

and the high water mark of the wetland

Site 3 Approximately midway between site 2 and

the high water mark of the wetland

High water mark of the wetland

Extracted from the guide provided to regional NRM agency staff to assist them during the rapid assessment of a wetland (Murray–Darling

Basin Authority, 2009).

Murray-DarlingBasin

km

4002000

Upper Darling-Paroo-Warrego

Brisbane

Sydney

Canberra

Melbourne

Adelaide

SouthernOcean

PacificOcean

Lower Darling

LowerMurray

Sunraysia

Murray Sunset

WimmeraLoddon-Avoca-Campaspe

Headwaters-Ovens

Lachlan-Murrumbidgee

Condamine-Balonne

BorderRivers-Moonie

Riverina-Goulburn-Broken

Riverland

NMurray-Darling Basin boundaryMBD catchment boundariesGeographical region boundaries

Assessed wetlands

Figure 2 Geographical regions used to

investigate the occurrence of inland acid

sulphate soils in the Murray–Darling Basin

and locality of the wetlands assessed.



incubation). For ease of referral, wetlands found to present a

severe acidification hazard were denoted as ‘Acid Wetlands’

(AW). Across the MDB, 238 (23%) of the 1055 wetlands

assessed were found to present a severe acidification hazard.

However, there were differences in the proportions of AW

among the geographical regions. The proportion of AW

within a region was found to vary between 2 and 52% of

assessed wetlands among the geographical regions found to

present a severe acidification hazard (Figure 3). Two

geographical regions registered no severe acidification hazard

(i.e. ‘Lower Darling’ and Upper Darling-Paroo-Warrego).

The largest proportion of AW was found in the ‘Lower

Murray’ and ‘Headwater-Ovens’ geographical regions, where

more than half of the wetlands that underwent rapid

assessment were found to be AW. The geographical region

encompassing the Loddon, Avoca and Campaspe catchments

also exhibited a severe acidification hazard with 29% of the

rapid assessed wetlands identified as AW.

The ‘Riverland’, ‘Riverina-Goulburn-Broken’, ‘Sunraysia’

and ‘Wimmera’ geographical regions present a similar

severe acidification hazard. Approximately 13–20% of the

wetlands assessed in these regions were identified as AW. The

severe acidification hazard of the above regions is not as great as

the hazard posed by the ‘Lower Murray’, ‘Headwaters-Ovens’

and ‘Loddon-Avoca-Campaspe’ geographical regions but is still

considered to be of significance. The ‘Lachlan-Murrumbidgee’

and ‘Murray Sunset’ geographical regions with ca. 10% AW

and the ‘Border Rivers-Moonie’ and ‘Condamine-Balonne’

geographical regions with <5% AW pose only a minor severe

acidification hazard relative to the other geographical regions in

this study. The remaining ‘Upper Darling-Paroo-Warrego’ and

‘Lower Darling’ geographical regions were found to pose no

acidification hazard in this study.

Proportion of soil samples within an AW that were ultra-

acidic or became ultra-acidic following incubation

The number of soil samples in an individual AW that were

ultra-acidic or became ultra-acidic following incubation

varied between a single sample and 100% of samples

(Figure 4). Wetlands denoted as AW typically contained

soils that were ultra-acidic or became ultra-acidic following

Table 2 Upstream and downstream boundaries for geographical regions

Geographical

regions

Split, single or

combined

catchment(s)

Downstream region

boundary Upstream region boundary Wetlands

Border Rivers-Moonie Combined – – 90

Condamine-

Balonne

Combined – – 54

Headwaters-Ovens Split Lock 16 (Yarrawonga Weir,

Victoria)

Headwaters and Ovens catchment 58

Lachlan-

Murrumbidgee

Combined – – 21

Loddon-Avoca-C

ampaspe

Combined – – 31

Lower Darling Split Lock 9 (Murray–Darling

Junction,

New South Wales)

Lake Menindee 14

Lower Murray Split Mouth of River Murray Lock 1 (Blanchetown, South Australia) 182

Murray Sunset Split Grouped samples near

Murray

Sunset National Park

29

Riverina-Goulburn-

Broken

Split Lock 15 (Euston, Victoria) Lock 16 (Yarrawonga Weir, Victoria)

and

Goulburn and Broken catchments

95

Riverland Split Lock 1 (Blanchetown,

South

Australia)

Lock 9 (Murray–Darling junction,

New South Wales)

344

Sunraysia Split Lock 9 (Murray–Darling

junction, New South

Wales)

Lock 15 (Euston, Victoria) 64

Upper Darling-

Paroo-Warrego

Split Lake Menindee Paroo and Warrego catchments 43

Wimmera Single catchment – – 30



incubation in 20–40% of all samples collected during the

rapid assessment. The proportion of soils that were ultra-

acidic or became ultra-acidic following incubation within an

individual AW was found to vary among geographical

regions. Individual AW were often found to contain a higher

proportion of soils that were ultra-acidic or became ultra-

acidic following when located in geographical regions that

also had a high proportion of AW.

Location of ultra-acidic soils or soils that became

ultra-acidic following incubation within an AW

Within a wetland, the occurrence of soils that were ultra-

acidic or became ultra-acidic following incubation varied

with sampling location. The sampling locations of sites 1, 2

and 3 are listed in Table 1. Generally, soils that were ultra-

acidic or became ultra-acidic following incubation were most

common in the subsoil layer at site 1 (Figure 5). Overall, a

greater prevalence of soils that were ultra-acidic or became

ultra-acidic following incubation was observed at site 1 and

site 2 than for site 3. Within each individual site, the

prevalence of soils that were ultra-acidic or became ultra-

acidic following incubation generally increased with

increasing depth (subsoil > subsurface > surface. This trend

was consistent across all sites.

Discussion

General discussion

The degree of acidification hazard posed by a geographical

region will depend largely on the proportion of AW within

that geographical region. A geographical region that

contains a high proportion of AW will pose a higher degree

of acidification hazard. Generally, the geographical regions

presenting the greatest severe acidification hazard were

located in the southern regions of the MDB, especially those

downstream from the Murray–Darling confluence and those

located on the southern side of the Murray River channel in

Victoria.

The proportion of soil samples that were ultra-acidic or

became ultra-acidic following incubation was found to vary

among wetlands denoted as AW (Figure 4). AW that

contained soils that were ultra-acidic or became ultra-acidic

following incubation in a large number of samples were

typically located in geographical regions that also contained

a large proportion of AW. Hence, the severe acidification

hazard posed by individual AW was commonly greater when

located in a geographical region found to pose a high overall

severe acidification hazard (e.g. ‘Lower Murray’, ‘Headwater-

Ovens’ and ‘Loddon-Avoca-Campaspe’ geographical regions).

Minimum pH in wetland following incubation

Cum

ulat

ive

freq

uenc

y

0.0

0.2

0.4

0.6

0.8

1.0Upper Darling-Paroo-WarregoLower Darling

0.0

0.2

0.4

0.6

0.8

1.0Lachlan-MurrumbidgeeMurray Sunset

0.0

0.2

0.4

0.6

0.8

1.0SunraysiaRiverland

Border Rivers-MoonieCondamine-Balonne

WimmeraRiverina-Goulburn-Broken

0 2 4 6 8 10

0 2 4 6 8 10

0 2 4 6 8 10 0 2 4 6 8 10

2 4 6 8 100

2 4 6 8 100

Loddon-Avoca-CampaspeLower MurrayHeadwaters-Ovens

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Figure 3 Cumulative frequency plots of the minimum pH in a wetland following incubation.



AW in these regions would likely require management of the

wetland in its entirety rather than the management of isolated

occurrences of soils that were ultra-acidic or became ultra-acidic

following incubation within the wetland.

The results were also used to gain a generalized assessment

on what may be a typical distribution of soils that were

ultra-acidic or became ultra-acidic following incubation

within a wetland. The prevalence of soils that were ultra-

acidic or became ultra-acidic following incubation increased

for sites nearest to the lowest point of the wetland

depression (site 1 and site 2) (Figure 5). Due to their lower

elevation, sites 1 and 2 are likely to experience subaqueous

conditions more frequently and for longer periods of time

than sites higher in elevation. An increase in the frequency

and duration of subaqueous conditions will usually

correspond to an increase in the opportunity for sulphide

formation. Hence, where the source of acidification is related

to the oxidation of sulphides, the greater prevalence of soils

that were ultra-acidic or became ultra-acidic following

incubation at these sites may be explained by their lower

elevation. Although the likelihood for sulphide formation is

higher for sites of lower elevation, the likelihood of forming

sulphuric material under a natural low water scenario is

lowest at these sites.

The prevalence of soils that were ultra-acidic or became

ultra-acidic following incubation was also found to increase

with depth at all sites. Ultra-acidic soils and soils that

become ultra-acidic following incubation that are located at

depth are less likely to be disturbed and hence will usually

pose a reduced acidification hazard relative to those that are

located at or near the surface. The above observations

detailing the varying likelihood for the occurrence of soils

that were ultra-acidic or became ultra-acidic following

incubation between different sites and different depths within

a wetland remained consistent among geographical regions.

Source(s) of ultra-acidic soils in the floodplain wetlands of

the MDB

In this study, the incubation method was used to assess the

acidification potential of soils in the floodplain wetlands of

the MDB, with the intention of identifying sulphidic materials.

The large acidification potential that is characteristic of soils

containing sulphides makes these materials easily identifiable

by the incubation method. There are other nonsulphide-

related processes that can result in acidification during the

incubation method, but the majority of these processes do

not cause the soil to acidify strongly enough to produce a

positive result in the incubation method. However, the

presence of acidic organic materials in peaty soils and the

hydrolysis of reduced iron (Fe2+) (Klein et al., 2010) are

two nonsulphide-related processes that have been suggested

to result in severe acidification in low sulphate and/or poorly

buffered systems.

Results from various studies show that sulphide minerals

are commonly found in floodplain wetland soils located

throughout the MDB, including many of the wetlands

studied here (Hall et al., 2006; Lamontagne et al., 2006;

Fitzpatrick et al., 2009a,b, 2010; Thomas et al., 2009a,b,c;

Grealish et al., 2010). Hence, it is expected that many of the

soils identified in this study that were ultra-acidic at the time

of collection or became ultra-acidic following incubation are

IASS materials. For example, based on the verified published

occurrences of IASS, the source of acidity in the soils

encountered in the ‘Lower Murray’ geographical region that

were ultra-acidic or became ultra-acidic following incubation

can be attributed to IASS processes. This is also the case for

the ‘Riverland’, ‘Sunraysia’, ‘Wimmera’ and ‘Loddon-Avoca-

Campaspe’ geographical regions.

However, for some less well-documented geographical

regions, such extrapolation is not possible. In the ‘Riverina-

Goulburn-Broken’, ‘Murray sunset’, ‘Lachlan-Murrumbidgee’

and ‘Headwater-Ovens’ geographical regions, it is more

difficult to assign the source of severe acidification entirely to

IASS processes without supporting sulphide data. Although

sulphides and evidence of sulphide oxidation have been

observed in each of these geographical regions, soils that

strongly acidify following incubation but do not contain

sulphides in detectible quantities have also been identified

(Thomas et al., 2009a,b; Ward et al., 2010). The source(s) of

severe acidification in these ‘non-sulphide containing severely

acidifying soils’ is yet to be confirmed, and further work is

required to ascertain the cause(s) of such severe acidification.

Again, it is unlikely that nonsulphide acidification processes

are dominant in the majority of study regions found to pose

a severe acidification hazard, and importantly, the

Percent of samples with pH < 4 following incubation

Num

ber

of w

etla

nds

0–20% 20–40% 40–60% 60–80% 80–100%

100

80

60

40

20

0

Figure 4 Percent of samples within a wetland that contained soils

that were ultra-acidic or became ultra-acidic following incubation for

all geographic regions.



acidification hazard remains the same no matter the source

of the acidity.

Conclusions

Of the ca. 30 000 wetlands in the MDB, almost 1500 were

identified as having an increased likelihood of containing IASS

material. Of the 1055 wetlands that pH incubation data were

collected for 238 (23%) were found to present a severe

acidification hazard. The number of wetlands that presented a

severe acidification hazard within a geographical region ranged

from <5 to >50% (Figure 6). Geographical regions located in

the southern MDB, downstream from the Murray–Darling

confluence, and catchments located on the southern side of the

Murray River channel in Victoria were found to present the

greatest acidificationhazard.

The major cause of severe acidification over much of the

MDB is undoubtedly the result of sulphide oxidation.

Hence, this study indicates that IASS containing sulphuric

and sulphidic materials are common in the floodplain

wetlands of the MDB. Indeed, the observation of soils that

do not contain sulphides but become ultra-acidic following

incubation has shown that nonsulphide containing materials

can be wrongly identified as sulphidic materials when using

the incubation method. Care must be taken if implying the

presence of sulphides when a soil material is observed to

become ultra-acidic following incubation. Additionally, this

study has focussed on soils where the acidification potential

is much greater than the acid neutralizing capacity. Well-

buffered soils that do not strongly acidify when oxidized, but

contain high contents of sulphide, present other hazards such

as metal and metalloid release that may also be significant

and warrant further work.

The information in this paper improves the understanding

of the potential occurrence of IASS in the MDB and

represents the most extensive estimate of the basin-wide

occurrence of sulphidic and sulphuric materials provided thus

far. This study has demonstrated that the potential existence

of IASS in any floodplain wetland located in the MDB should

be a key consideration for any natural resource manager’s

wetland management plan. The non-uniform occurrence of

severe acidification hazard throughout the MDB also has

implications on the successful management of IASS in the

MDB whereby IASS should receive much greater attention in

the geographical regions identified as presenting a significant

acidification hazard. The study should help direct resources to

manage IASS effectively, especially in future occasions of low

water levels in the MDB.

Percent of samples with pH < 4 followingincubation



Site 2Site 1

Site 3

Surface

Sub-surface

Sub-soil

Surface

Sub-surface

Sub-soil

Surface

Sub-surface

Sub-soil

420 6 8 10 12

420 6 8 10 12

420 6 8 10 12

Figure 5 Percent of soils that were ultra-acidic or became ultra-acidic following incubation at each site and sampling depth for all geographical

regions.



Acknowledgements

We thank Rob Kingham, Richard Merry, Leigh Sullivan

and the two anonymous reviewers for their helpful

comments and Greg Rinder for preparing Figure 2. The

Murray–Darling Basin Authority is thanked for funding the

ASS risk assessment project, much of which allowed us to

construct this paper.

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