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NATIONAL WATER COMMISSION — WATERLINES 1
Sulfidic sediments in inland
waterways
DS Baldwin and SJ Capon
Waterlines Report Series No 55, September 2011
NATIONAL WATER COMMISSION — WATERLINES ii
Waterlines
This paper is part of a series of works commissioned by the National Water Commission on
key water issues. This work has been undertaken by the Murray Wetlands Working Group in
collaboration with the Murray–Darling Freshwater Research Centre on behalf of the National
Water Commission.
NATIONAL WATER COMMISSION — WATERLINES iii
© Commonwealth of Australia 2011
This work is copyright.
Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by
any process without prior written permission from the Commonwealth.
Requests and enquiries concerning reproduction and rights should be addressed to the
Commonwealth Copyright Administration, Attorney General’s Department, National Circuit,
Barton ACT 2600 or posted at www.ag.gov.au/cca.
Online ISBN: 978-1-921853-32-6
Sulfidic sediments in inland waterways, September 2011
Authors: DS Baldwin and SJ Capon
Published by the National Water Commission
95 Northbourne Avenue
Canberra ACT 2600
Tel: 02 6102 6000
Email: enquiries@nwc.gov.au
Date of publication: September 2011
Cover design by: Angelink
Front cover image courtesy of Darren Baldwin
An appropriate citation for this report is:
Baldwin DS and Capon SJ 2011, Sulfidic sediments in inland waterways, Waterlines report,
National Water Commission, Canberra
Disclaimer
This paper is presented by the National Water Commission for the purpose of informing
discussion and does not necessarily reflect the views or opinions of the
Commission.
NATIONAL WATER COMMISSION — WATERLINES iv
Contents
Executive summary ..................................................................................................................1
1. Introduction ...................................................................................................................5
1.1. Project context ................................................................................................................5
1.2. Project approach .............................................................................................................6
1.2.1. The Action Support Tool (AST)...............................................................................7
1.3. Scope and structure of this current report ......................................................................7
2. Background to the science ..........................................................................................8
2.1. What are sulfidic sediments? ..........................................................................................8
2.2. What risks do sulfidic sediments in inland waterways pose? .........................................8
2.3. How do sulfidic sediments form in inland waterways? ................................................ 10
2.4. How widespread are sulfidic sediments in Australia’s inland waterways? .................. 12
2.5. What approaches might be taken to manage sulfidic sediments in inland waterways? ..
..................................................................................................................................... 12
3. Key findings ............................................................................................................... 15
3.1. Sulfate addition affects fundamental ecosystem processes ........................................ 15
3.2. Saline groundwater is the principal source of sulfate to inland water bodies .............. 15
3.3. Inland wetlands are primed for sulfate reduction to occur ........................................... 16
3.4. Reduced sulfur can accumulate very rapidly in wetland sediments ............................ 17
3.5. Reactive monosulfides are the principal form of sulfides in inland waterways ............ 18
3.6. Understanding the acidification paradox ..................................................................... 19
3.7. Sulfate reduction is not the only pathway for wetland acidification ............................. 20
3.8. Management of sulfidic sediments once formed is both expensive and not straight
forward ......................................................................................................................... 21
3.9. Long-term solutions must address the underlying causes of salinisation and changes
to river hydrology ......................................................................................................... 23
4. Action Support Tool .................................................................................................. 25
4.1. Design of Action Support Tool ..................................................................................... 25
4.2. Overview of Action Support Tool ................................................................................. 26
5. Outcomes, recommendations and conclusions .................................................... 30
5.1. Key outcomes .............................................................................................................. 30
5.2. Recommendations ....................................................................................................... 31
5.2.1. Management ........................................................................................................ 31
5.2.2. Policy ................................................................................................................... 32
5.3. Future research ........................................................................................................... 32
5.4. Conclusions ................................................................................................................. 33
Bibliography ........................................................................................................................... 34
Appendix 1—List of the project’s scientific activities, conclusions and outcomes ...... 37
NATIONAL WATER COMMISSION — WATERLINES v
Figures
Figure 1: National guidance for the management of acid sulfate soils in inland
aquatic ecosystems, 2011 .......................................................................................... 6
Figure 2: Conceptual model showing the major risks associated with the
oxidation of sulfidic sediments that may occur upon exposure to oxygen
as a result of a water body drying (A) and following rewetting of the
water body or via disturbance of sulfidic sediments while inundated (B)................... 9
Figure 3: Conceptual model showing the conditions necessary for the formation
of sulfidic sediments in an inland water body ........................................................... 10
Figure 4: Graphic representation of the Action Support Tool .................................................. 27
NATIONAL WATER COMMISSION — WATERLINES vi
Definitions and acronyms
acid neutralising capacity in the context of sulfidic sediments, the ability of an
ecosystem to withstand the production of acid without an
overall lowering of pH in the water column or sediments
acidification the process by which water or sediments become more
acidic, i.e. have their pH lowered
anaerobic biogeochemical
processes physical, chemical and biological processes which take
place in an ecosystem in the absence of oxygen
anaerobic conditions conditions under which oxygen is absent
aquifer a permeable layer of rock or sediments through which
groundwater moves
AVS (acid volatile sulfide)
sulfidic material present in sediments that is highly
reactive in air
bioavailable carbon carbon in an ecosystem that is in a form able to be used
by organisms, e.g. bacteria, in their metabolism
chemical ameliorants substances added to sediments or water to improve
conditions
deoxygenation the removal of oxygen
half-life the time taken by a decaying substance to reduce its
mass by half
ionic composition the totality of ions in solution, including anions and
cations
MDBA Murray-Darling Basin Authority
mesocosm experiments experiments which use small, partially closed
experimental units, e.g. pots or artificial ponds, to
represent ecosystems on a larger scale
metalloids elements with properties intermediate between those of
metals and non-metals e.g. arsenic
NATIONAL WATER COMMISSION — WATERLINES vii
methanogenesis an important biogeochemical process, often the last step
of decomposition, in which the metabolism of certain
microbes, i.e. methanogens, produces methane
microbial respiration the metabolic processes by which the cells of
microorganisms, e.g. bacteria, create biochemical energy
neutralisation a chemical reaction in which an acid and a base combine
to form a salt and, usually, water
oxidation a chemical reaction in which an atom, molecule or ion
increases in oxidation number, usually by losing electrons
pyrite an iron sulfide mineral with the chemical formula FeS2
reduced inorganic sulfur sulfur of mineral origin in a reduced form, i.e. sulfide
reduction a chemical reaction in which an atom, molecule or ion
decreases in oxidation number, usually by gaining
electrons
residence time (of water) the duration for which water remains within a particular
water body
respiration the metabolic process by which cells, organisms or
ecosystems create biochemical energy, releasing carbon
dioxide (CO2) as a waste product
salinisation the process by which salt accumulates on soil surfaces or
in waterways as a result of rising groundwater levels
associated with land clearing and irrigation practices
SRB (sulfate reducing
bacteria) bacteria that use sulfate (SO2-4) rather than oxygen in
their respiration
sulfate reduction the biogeochemical process by which sulfate (SO2-4) is
converted to sulfide (S2-)
TAA (total actual acidity) the total amount of acidity occurring in sediments and/or
the water column at a particular time
NATIONAL WATER COMMISSION — WATERLINES viii
Acknowledgements
The project team would like to acknowledge and thank the National Water Commission for
funding this study under the Raising National Water Standards (RNWS) Program. We would
like to thank Anthea Brecknell and Ginni Glyde for their ongoing oversight of the project
We gratefully acknowledge the financial contribution, support and guidance to this project
from the NSW Murray Wetlands Working Group, and in particular, Dr Deb Nias. Anthea and
Deb are also thanked for reviewing and editing earlier drafts of this Waterlines report.
We also acknowledge the valuable contribution to this project made by members of the
National Advisory Committee:
Professor Leigh Sullivan (Southern Cross University)
Mr Bernie Powell (Qld Department of Natural Resources and Mines)
Mr Peter Waanders (SA Murray-Darling Basin Natural Resources Management Board) and
Professor Pierre Horwitz (Edith Cowan University).
Members of the Project Steering Committee:
Dr Deb Nias (NSW Murray Wetlands Working Group)
Ms Paula D’Santos (NSW Department of Environment, Climate Change and Water)
Ms Judy Frankenberg (NSW Murray Wetlands Working Group) and
Mr James Maguire (NSW Department of Environment, Climate Change and Water).
This work was only possible because of the hard work of the project team including:
Mrs Kerry Whitworth (La Trobe University and the Murray-Darling Freshwater Research
Centre)
Associate Professor Ewen Silvester (La Trobe University, Wodonga)
Dr Gavin Rees (CSIRO Land and Water and the Murray-Darling Freshwater Research
Centre)
Dr Mark Fraser (La Trobe University and the Murray-Darling Freshwater Research Centre)
Dr Peter Kappen (La Trobe University)
Associate Professor Jon Webb (La Trobe University, Bundoora)
Associate Professor Balwant Singh (University of Sydney)
Ms Fiona Glover (Honours student, La Trobe University)
Ms Anna Klein (Honours student, La Trobe University)
Ms Ishard Bibi (PhD student, University of Sydney) and
The 2nd Year Environmental Management Class of 2009, La Trobe University, Wodonga
campus.
NATIONAL WATER COMMISSION — WATERLINES 1
Executive summary
In 2007 the National Water Commission in collaboration with the NSW Murray Wetlands
Working Group Inc. funded the project Minimising Environmental Damage from Water
Recovery from Inland Wetlands: Determining water regimes to minimise the impact of sulfidic
sediments (potential acid sulfate soils). The principal objective of this project was to provide
management agencies with tools and guidelines, based on the best available science, on how
to appropriately manage wetlands in inland Australia to minimise the accumulation of sulfidic
sediments and remediate systems which are affected.
This Waterlines report presents a summary of key findings and management
recommendations arising from that project. The project has contributed significantly to the
development of the National guidance for the management of acid sulfate soils in inland
aquatic ecosystems and the development of an Action Support Tool (AST) which is freely
available at www.mdfrc.org.au.
Sulfidic sediments in Australia’s inland waterways have recently emerged as a potentially
serious environmental problem depending on the specific local conditions. They can cause
considerable ecological damage and have the potential to significantly detract from aesthetic
values and impact on human health. Sulfidic sediments are sediments that contain excessive
amounts of reduced inorganic sulfur, typically in the form of highly reactive sulfidic minerals.
Sulfidic sediments develop in the absence of oxygen when certain bacteria use sulfate rather
than oxygen for respiration, resulting in the production of sulfide. In turn, the sulfide reacts
with metal ions to form metal sulfides. Left undisturbed and covered with water, these sulfidic
sediments may cause relatively little harm. However, if disturbed or exposed to oxygen (e.g.
when a water body dries up), a range of biogeochemical processes can be triggered which
may lead to large-scale fish kills and the death of riparian vegetation. Acidification,
deoxygenation of the water column and the mobilisation of toxic heavy metals and metalloids
are the major processes of concern that can occur in response to the oxidation of sulfidic
sediments.
The levels of management responses required to address sulfidic sediments in inland
waterways will vary depending on the context but can involve strategies to prevent dangerous
amounts of sulfidic sediments from accumulating or, where this has already occurred, actions
to reduce environmental damage resulting from their disturbance. Where feasible, such
management actions may focus on minimising the potential for oxidation of sulfidic sediments
by keeping them wet. In cases where this is not possible, for example under drought
conditions, the priority may instead be to prevent acidification of a water body by applying
chemical ameliorants or enhancing a system’s capacity to neutralise any acid produced. In
extreme cases, where rehabilitation of water bodies affected by sulfidic sediments is not
possible, water bodies may need to be isolated to prevent flushing of this toxic water into
downstream waterways.
Three broad areas of knowledge gaps concerning the mitigation and management of sulfidic
sediments in inland waterways were identified: fundamental knowledge, assessment and
rehabilitation. A range of research activities, including a comprehensive literature review, field
surveys, numerous laboratory and field experiments as well as a major intervention at a
severely degraded field site, were then undertaken to address these knowledge gaps.
A synthesis of the outcomes of the research activities undertaken during this project has
provided nine key points that need to be considered in relation to the production and
management of acid sulfate materials in Australia’s inland waterways.
1. Sulfate addition affects fundamental ecosystem processes: Experimental results
indicate that addition of sulfate to wetland sediments can fundamentally transform the
way in which carbon moves through an ecosystem. In particular, the microbial process of
methanogenesis, which is usually the final step of decomposition in freshwater
ecosystems, is strongly inhibited by sulfate addition.
NATIONAL WATER COMMISSION — WATERLINES 2
2. Saline groundwater is the principal source of sulfate to inland water bodies: A
strong correlation between groundwater salinity and sulfate concentration was identified
from an extensive survey of bores between Mildura and the South Australian border.
These results also indicate that groundwater salinity levels are an appropriate indicator of
the potential for sulfidic sediments to be present or to form in inland waterways that
receive discharges of high salinity groundwater.
3. Inland wetlands are primed for sulfate reduction to occur: Inland wetland sediments
already contain sulphur-reducing bacterial communities that can rapidly respond to the
addition of higher than background levels of sulfate and changes in salinity, even in
wetlands not previously exposed to sulfur or salt.
4. Reduced sulfur can accumulate very rapidly in wetland sediments: Experiments
conducted in this project indicate that, given sufficient carbon availability, all of the
aqueous sulfate associated with saline water can be converted into reduced sulfidic
compounds in wetland sediments within a few months. Temperature has a strong
influence on sulfate reduction rate. The rate of sulfate reduction triples when temperature
increases from 20°C to 30°C and virtually no sulfate reduction occurs at 5°C.
The rates at which sulfidic sediments will form can be relatively easily predicted from the
sulfate load entering a water body, the residence time of water in the water body,
temperature and the amount of bioavailable carbon present. This knowledge can support
the design of managed flow regimes to prevent the accumulation of dangerous levels of
sulfidic sediments, e.g. establishing appropriate frequencies for the sequencing of dry and
wet (flushing) periods.
5. Reactive monosulfides are the principal sulfides in inland waterways: The iron
sulfide mackinawite is the major constituent of sulfidic sediments in Australia’s inland
waterways. Oxidation of mackinawite is considerably faster than that of pyritic sediments,
which are typically associated with coastal acid sulfate soils, and can lead to extremely
rapid deoxygenation and acidification of water bodies.
6. Not all wetlands with sulfidic sediments will acidify when their sediments are
exposed to oxygen: Some wetlands have a higher capacity than others to neutralise any
acid produced through the exposure of sulfidic sediments. A significant component of this
‘acid neutralising capacity’ actually results from the alkalinity that is trapped in a water
body during the process of sulfate reduction in which sulfidic sediments are formed in the
first place. The alkalinity created during sulfate reduction equals the acidity that is
produced upon subsequent re-oxidation of these sulfidic materials. In some systems,
however, the aqueous component of this alkalinity is transported away from the site
following sulfate reduction and this reduces the alkalinity available at that site to buffer
against later increases in acidity that may occur if sulfidic sediments are exposed to
oxygen.
The ratio of calcium to sulfate in source water entering a wetland is an important
determinant of a system’s acid neutralising capacity. The presence of calcium leads to the
precipitation of calcium carbonate during the formation of sulfidic sediments and,
therefore, a higher retention of alkalinity at the site available to buffer against future
increases in acidity.
7. Sulfate reduction is not the only pathway for wetland acidification: Salt-induced
acidification of inland waterways can occur via other biogeochemical pathways and in the
absence of sulfidic sediments. Potential for acidification, where sulfur concentrations are
low, may still be high in areas with significant levels of reduced iron present in sediments.
Salt can displace reduced iron from the sediments into the overlying water column.
Oxidation of this reduced iron can lead to acidification of the water column. Since clay-
bound reduced iron is known to be widespread in wetlands with a high clay fraction and
low sulfur loads, this finding is of considerable concern.
NATIONAL WATER COMMISSION — WATERLINES 3
8. Management of sulfidic sediments once formed is difficult and expensive: A major
field experiment investigating the effectiveness of 20 treatments aimed at neutralising
oxidised sulfidic sediments in a severely degraded wetland revealed very few that
successfully reduced acidity levels after six months, particularly at sediment depths
greater than 5 cm. It was concluded moreover that implementation at a wetland scale of
any effective treatments, such as the application of calcium carbonate (aglime), may be
very expensive and logistically challenging.
9. Long-term solutions must address the underlying causes of salinisation and
changes to river hydrology: Attempts to rehabilitate or restore water bodies affected by
sulfidic sediments will only be short-term remedies at best unless the underlying causes
are addressed by applying a holistic management framework. The content of the most
appropriate management strategy for a given location can be developed from the
information presented in this report and use of the associated Action Support Tool.
The findings from this project clearly demonstrate that sulfidic sediments are a symptom of
systems under stress, due primarily to salinisation resulting from rising groundwater tables
and altered river flows. Therefore manipulation of environmental flows may be used as an
effective tool for the sustainable management and mitigation of sulfidic sediments in inland
waterways. Reinstating periods of low or no flows into regulated waterways will minimise the
accumulation of potentially harmful amounts of sulfidic sediments, while returning high flows
at appropriate times will flush salts and acid, scour sediments and dilute affected waterways,
as well as protect wetland sediments from saline groundwater by providing a freshwater lens.
Recommendations
Policy for flow regime
• The effective management of potential or actual sulfidic sediments should be included as
a key objective when determining allocation and use of environmental water in Australia’s
inland river catchments.
• Key objectives for management of sulfidic sediments should include the reinstatement of
wet and dry cycles to prevent the build-up of dangerous concentrations of reduced sulfur.
Reinstating periods of low or no flows into regulated waterways will minimise the
accumulation of potentially harmful amounts of sulfidic sediments, while returning high
flows at appropriate times will flush salts and acid, scour sediments and dilute affected
waterways as well as protect wetland sediments from saline groundwater by providing a
freshwater lens.
• Appropriate flood durations and frequency of drying cycles may be calculated, with expert
assistance, given a knowledge of:
o total sulfate load entering water body
o residence time of water in water body
o temperature, and
o the amount of bioavailable carbon.
Management
• Rehabilitation of waterways affected by sulfidic sediments is not simple and there is no
single approach that will work in all cases. Each system should be considered on a case-
by-case approach, utilising the Action Support Tool developed by this project.
• Use of agricultural chemicals containing sulfate, e.g. gypsum, in areas that can affect
adjacent waterways (including groundwater), should be limited as far as practicable.
• Monitoring of shallow groundwater composition, especially the concentration of dissolved
salt, should be incorporated into wetland and river assessments as an indicator of the
potential for the presence or formation of sulfidic sediments.
NATIONAL WATER COMMISSION — WATERLINES 4
• The monitoring of groundwater should also include measuring the ratio of calcium to
sulfate as this can provide an indication of the potential for acidification to occur if sulfidic
sediments are exposed.
• Sulfidic sediments can form fairly rapidly. Just because they were not identified in a
wetland previously, does not mean they will not form in the water body into the future, and
ongoing assessment is recommended.
• The measurement of clay-bound reduced iron should be included in wetland assessments
to identify areas at risk of acidification via pathways other than the oxidation of sulfidic
sediments.
Future research
Two key knowledge gaps were identified:
• Understanding the dynamics of sulfidic sediments in flowing water bodies, and
• Determining the impact of sulfidic sediment formation and disturbance on the ecology of
inland waterways at different spatial scales.
Salt impacted Brilka Creek contains sulfidic sediments. Photo by D Baldwin
NATIONAL WATER COMMISSION — WATERLINES 5
Introduction
Sulfidic sediments (also called acid sulfate soils) in Australia’s inland waterways have recently
been recognised as posing a potentially serious environmental threat. Sediments that contain
excessive reduced inorganic sulfur are referred to as sulfidic sediments. This material forms
in waterlogged conditions as a result of various biogeochemical reactions involving the
conversion by bacteria of the sulfate present in saline water to sulfide and its subsequent
reactions with metals, especially iron.
Sulfidic sediments are relatively harmless if they remain undisturbed and covered by water.
However, when disturbed or exposed to oxygen (as a result of a water body drying up or by
resuspension by scouring during floods) sulfidic sediments can cause acidification and
deoxygenation of the water column as well as the release of toxic heavy metals (such as
cadmium and lead) and metalloids (such as arsenic) into the environment. These processes,
in turn, can lead to the death of aquatic organisms and may also adversely impact human
drinking water supplies.
While sulfidic sediments and acid sulfate soils are a familiar problem in coastal areas, it has
conventionally been considered that sulfate levels in Australia’s inland waterways were
sufficiently low that the processes of sulfate reduction that lead to the formation of excessive
amounts of sulfidic sediments were not significant in these environments. However, recent
surveys of wetlands in south-eastern Australia indicate that a not-insignificant number had
sulfidic sediments at levels high enough to result in environmental harm if inappropriately
managed (Hall et al., 2006, Lamontange et al., 2006, Murray-Darling Basin Authority, 2011).
The amount of sulfate occurring in Australia’s inland waterways has risen in recent times due
to processes such as salinisation and the application of some agricultural chemicals (e.g.
gypsum). Moreover, permanent inundation of historically ephemeral water bodies as a result
of river regulation, e.g. weir pools, has probably exacerbated the problem by allowing the
accumulation of excessive amounts of sulfidic sediments which would have been prevented
by frequent drying phases under more variable flow regimes prior to river regulation.
In 2007 the National Water Commission in collaboration with the NSW Murray Wetlands
Working Group Inc. funded the project Minimising Environmental Damage from Water
Recovery from Inland Wetlands: Determining water regimes to minimise the impact of sulfidic
sediments (potential acid sulfate soils). The principal objective of this project was to provide
management agencies with tools and guidelines, based on the best available science, on how
to appropriately manage wetlands in inland Australia to minimise the accumulation of sulfidic
sediments and remediate systems which are affected.
Project context
Due largely to the drought that affected much of the continent over the last decade, there has
been growing concern regarding the risk of disturbance and exposure of sulfidic sediments in
Australia’s inland waterways. Guidance regarding the assessment and management of
sulfidic sediments in inland waterways has also been lacking until quite recently with water
and land managers often forced to rely on information from coastal regions where the risks
and management approaches associated with sulfidic sediments may differ considerably.
In recognition of the significance of this issue, the Environment Protection and Heritage
Council (EPHC) and the Natural Resource Management Standing Committee endorsed the
development of national guidance for inland aquatic ecosystems. The National guidance for
the management of acid sulfate soils in inland aquatic ecosystems was published by EPHC
and the Natural Resource Management Ministerial Council in May 2011.
The National Water Commission project Minimising Environmental Damage from Water
Recovery from Inland Wetlands: Determining water regimes to minimise the impact of sulfidic
sediments (potential acid sulfate soils) contributed significantly to the development of this
NATIONAL WATER COMMISSION — WATERLINES 6
national guidance. The key findings of this project provided critical scientific understanding
that underpinned the development of management and mitigation recommendations provided
in the national guidance document. In particular, a review of rehabilitation options for inland
wetlands affected by acid sulfate soils undertaken for the project (Baldwin and Fraser, 2009),
provided a major foundation document for the national guidance document.
The National guidance for the management of acid sulfate soils in inland aquatic ecosystems
is available on the Australian Government Department of Sustainability, Environment, Water,
Population and Communities’ website:
http://www.environment.gov.au/water/publications/quality/guidance-for-management-of-acid-
sulfate-soils.html
Figure 1: National guidance for the management of acid sulfate soils in inland aquatic
ecosystems, 2011
Project approach
The purpose of the project was to examine options for the mitigation and management of
sulfidic sediments in Australia’s inland waterways. Although there is a substantial body of
work regarding the management of sulfidic sediments and acid sulfate soils in coastal areas,
this is not always applicable to inland waterways. Consequently, while this project built upon
existing literature, a key aim was to address knowledge gaps concerning the formation,
impacts and management of sulfidic sediments specifically with respect to inland waterways.
Research activities were undertaken to address knowledge gaps identified in each of three
broad areas:
1. Fundamental knowledge
2. Assessment
3. Rehabilitation.
Research activities included reviews and syntheses of existing knowledge, field surveys and
experiments ranging from controlled micro-scaled laboratory experiments through to larger,
mesocosm experiments and a number of field trials including a major intervention at a
severely degraded field site, Bottle Bend Lagoon, near Mildura, New South Wales. A
summary of each research activity, and its scientific and management outcomes and outputs,
e.g. published scientific papers, is provided in Appendix 1.
NATIONAL WATER COMMISSION — WATERLINES 7
The Action Support Tool (AST)
Given the applied nature of the project, an important output was the provision of the key
findings in a form suitable for assisting on-ground managers in making operational decisions
for waterways that have been or could be affected by sulfidic sediments. Thus, a major output
of this project is the Action Support Tool for managing sulfidic sediments in inland wetlands
(AST). This tool has been developed based on the best available science and is consistent
with the National guidance for the management of acid sulfate soils in inland aquatic
ecosystems (see Section 1.1). The AST is easy to use and does not require the end-user to
have knowledge of sediment biogeochemistry, although a background in natural resource
management is assumed. The AST is available for free download at:
www.mdfrc.org.au
A brief overview of the AST is provided in this document in Chapter 4. Any comments or
suggestions involving the improvement to the tool can be sent to darren.baldwin@csiro.au.
Scope and structure of this current report
This Waterlines report provides a summary of the scientific and management outcomes of the
National Water Commission project Minimising Environmental Damage from Water Recovery
from Inland Wetlands: Determining water regimes to minimise the impact of sulfidic sediments
(potential acid sulfate soils).
Chapter 2 presents a brief introduction to the science of sulfidic sediments in inland
waterways including their formation, potential impacts, options for their management and why
they differ as an environmental issue from those in coastal areas.
Chapter 3 provides a synthesis of the key findings of the project with respect to knowledge
gaps identified in the broad areas of fundamental knowledge, assessment and rehabilitation.
Chapter 4 provides a broad overview of the design and structure of the Action Support Tool.
Chapter 5 presents a summary of key outcomes along with recommendations for
management, policy and future research of Australian inland waterways based on the key
findings of the project.
A list of the project’s scientific activities, conclusions and outcomes is provided at Appendix 1.
References for further detail are also provided.
NATIONAL WATER COMMISSION — WATERLINES 8
Background to the science
What are sulfidic sediments?
The term ‘sulfidic sediments’, in the context of inland waterways, refers to waterlogged
sediments (i.e. mud), that comprise excessive amounts of material containing reduced
inorganic sulfur, or sulfide (S2-). Typically, this sulfide occurs in the form of sulfidic minerals,
i.e. metal ores that contain sulfide. Iron sulfides, in which sulfide is combined in various forms
with iron (Fe), are particularly common in waterlogged sediments (Rickard and Luther, 2007).
Mackinawite (FeS) and pyrite (FeS2) are two of the most common iron sulfides found in
sulfidic sediments. Elemental sulfur may also be occur (Burton et al., 2006).
How can sulfidic sediments be detected?
There are a number of visual cues which may indicate the presence of sulfidic sediments
in an inland water body, including:
What risks do sulfidic sediments in inland
waterways pose?
• discoloured and reddish bank and bed sediments from bank seepage
• banks and organic debris covered in copper coloured scum
• appearance of an oil slick and reddish deposits
• sediments beneath any scum resembling black ‘ooze’
• unhealthy looking, murky water with an orange to brown tinge, or
• a distinctive salty odour.
The key risks posed by sulfidic sediments in inland waterways are associated with the
oxidation of the reduced metal sulfides. Oxidation of sulfidic sediments occurs when they are
exposed to oxygen as a result of disturbance (e.g. a flood or bioturbation by animals) or when
a water body dries out. When sulfidic sediments are undisturbed and covered with water, they
have relatively little impact on the environment. This is because the water layer keeps the
reactive sulfidic minerals surrounded by an environment that is relatively low in oxygen,
preventing their oxidation.
However, when sulfidic sediments are exposed to oxygen, their oxidation can trigger
biogeochemical processes that may cause significant environmental harm. The main
processes of concern include acidification, deoxygenation and the release of heavy metals
and metalloids, all of which have the potential to result in significant impacts on inland
waterways such as large-scale mortality of fish and riparian vegetation. The aesthetic value of
inland waterways and human health (via contamination of drinking water) can also be
threatened by processes related to the disturbance of sulfidic sediments.
Acidification
Exposure of sulfidic sediments to oxygen results in a complex series of chemical reactions
involving the oxidation of reduced sulfur minerals, which eventually produces acid. If the
amount of acid produced by these reactions exceeds the capacity of the sediments or the
water body to absorb acid, referred to as a system’s ‘acid neutralising capacity’, then the pH
will drop. The Australian and New Zealand Environment and Conservation Council (ANZECC)
Guidelines (ANZECC and ARMCANZ, 2000) recommend a pH above 6.5 to protect the health
of aquatic ecosystems. Any decline below this level therefore has the potential to cause
ecological harm, e.g. fish kills.
NATIONAL WATER COMMISSION — WATERLINES 9
Acidification of the water column does not always follow the disturbance of sulfidic sediments.
The ability of a water body to buffer against increases in acidity or, in other words, neutralise
the acid produced (the acid neutralising capacity), may be increased by the presence of
shells, some types of organic matter, or clay sediments (see Baldwin and Fraser, 2009 and
references therein).
Figure 2: Conceptual model showing the major risks associated with the oxidation of sulfidic
sediments that may occur upon exposure to oxygen as a result of a water body drying (A) and
following rewetting of the water body or via disturbance of sulfidic sediments while
inundated (B)
Deoxygenation
When sulfidic sediments are resuspended in a water body as a result of some disturbance,
oxygen is used. In some situations, this process can actually deplete the water column of all
of its oxygen leading to the death of aquatic organisms (Sullivan et al., 2002).
Release of heavy metals and metalloids
The oxidation of some types of sulfidic minerals, e.g. galena (lead sulfide), covellite (copper
sulfide) or aresonopyrite (iron arsenic sulfide), can result directly in toxic heavy metals, e.g.
cadmium and lead, and metalloids, e.g. arsenic, being released into the environment where
they may cause direct toxicity as well as being assimilated into the tissues of aquatic
organisms, posing a chronic toxicity issue as well as being of potential concern to humans
who consume them.
Metals can also be mobilised when the acid produced by the oxidation of sulfidic sediments
dissolves other minerals that may be present. For example, aluminium, which is toxic to many
NATIONAL WATER COMMISSION — WATERLINES 10
aquatic organisms (ANZECC and ARMCANZ, 2000), is produced when acidic conditions
cause the breakdown of clay (Lottermoser, 2007).
Aesthetic and human health impacts
The disturbance of sulfidic sediments can also release volatile sulfur compounds (hydrogen
sulfide, dimethyl sulfide) that produce noxious odours (Kinsela et al., 2007). In addition to
being an aesthetic issue these compounds also have potential adverse implications for
human health (Hicks and Lamontange, 2006).
How do sulfidic sediments form in inland
waterways?
Sulfidic sediments are formed when certain types of bacteria, i.e. sulfate reducing bacteria
(SRB), use sulfate (SO2-4) for respiration. As a result, sulfate is converted or reduced to
sulfide (S2-) which then combines with metal ions, iron in particular, to form various types of
metal sulfide minerals (Figure 3). The reduction by bacteria of sulfate to sulfide occurs in the
absence of oxygen, i.e. under anaerobic conditions. In addition to the presence of sulfate it
requires a source of bioavailable carbon. Such anaerobic conditions typically occur in the
bottom sediments of slow-flowing or stagnant water bodies below the first few millimetres of
the sediment surface, especially those with fine sediments. Under appropriate conditions,
sandy sediments in flowing streams can also be anoxic, especially at depth (e.g. Atkinson et
al., 2008).
Figure 3: Conceptual model showing the conditions necessary for the formation of sulfidic
sediments in an inland water body
Despite the presence of anoxic sediments and often ample supplies of bioavailable carbon,
sulfate reduction in inland waterways has typically been assumed to be a relatively
insignificant process, mainly due to a perceived lack of sufficient quantities of sulfate (Homler
and Storkholm, 2001). However, the concentration of sulfate in inland Australia has increased
in recent decades as a result of human activities including the application of agricultural
chemicals, e.g. gypsum (calcium sulfate dihydrate) and mining of sulfide-containing mineral
deposits. However, it is the salinisation of inland aquatic systems, as a result of changes to
hydrology associated with land clearing and irrigated agriculture, that has been the chief
contributor to increases in the amount of sulfate in these systems. This is because the salt in
the inland Australian landscape is mainly of ancient marine origin, and sulfate occurs in
NATIONAL WATER COMMISSION — WATERLINES 11
seawater. Consequently, high levels of salt in inland Australia are usually associated with
significant concentrations of sulfate and therefore increase the potential for formation of
sulfidic sediments (Sullivan et al., 2002).
The prolonged periods of inundation that have resulted from river regulation in many inland
waterways will also have facilitated the accumulation of sulfidic sediments. Prior to river
regulation more frequent drying events would have occurred, during which the oxidation of
much smaller, less damaging amounts of sulfidic sediments would have occurred with the
oxidation products being flushed out by subsequent flood events.
Example – Bottle Bend Lagoon
Bottle Bend Lagoon provides a classic example of an inland wetland affected by sulfidic
sediments. Located on the Murray River in New South Wales near the South Australian
border, Bottle Bend Lagoon had been permanently full for at least three decades. In
2002, however, the wetland underwent a partial drying event as a result of the lowering of
weir pool in the adjacent river and the pH subsequently fell from near neutral to below 3 in
under four months leading to a significant fish kill. The pH remained at that level for the
next eight years (McCarthy et al., 2006). Even when the wetland filled to above its normal
high water mark when it was reconnected to the river during the September 2010 floods,
the pH of the water column remained below 4. To put this into context, the natural range
of pH in freshwaters is between 6.5 – 9.0.
A healthy wetland, Kings Billabong (l), and a wetland impacted by sulfidic sediments, Bottle Bend Lagoon (r).
Photos by D Baldwin
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How widespread are sulfidic sediments in
Australia’s inland waterways?
While sulfidic sediments have traditionally been considered as mainly a coastal problem,
recent surveys reveal that sulfidic sediments are present in many inland water bodies (Hall et
al., 2006; Lamontagne et al., 2006; Murray-Darling Basin Authority, 2011). For example, a
survey of 80 wetlands in inland New South Wales conducted on behalf of the NSW
Environmental Trust and the NSW Murray Wetlands Working Group Inc., indicated that
sulfidic sediments were present in around 20 per cent of wetlands at levels high enough to
cause significant environmental damage should they be disturbed (Hall et al., 2006). Sulfidic
sediments have also been found in inland waterways in South Australia, Western Australia
and Victoria, particularly in areas affected by salinisation (EPHC and NRMMC, 2010 and
references therein).
Which water bodies are most at risk?
In inland Australia, the water bodies most likely to have accumulated harmful levels of
sulfidic sediments include those that:
• have been inundated for prolonged durations
• occur in areas with elevated groundwater levels
• contain water with an electrical conductivity of > 1750 µS cm, or
• contain sediments with an electrical conductivity of > 400 µS cm.
What approaches might be taken to manage
sulfidic sediments in inland waterways?
A review of potential approaches to the management of sulfidic sediments in inland
waterways, drawing from existing literature on the management of coastal acid sulfate soils,
acid mine drainage and waterways affected by acid rain, was conducted during the project
and published in the Journal of Environmental Management (Baldwin and Fraser, 2009).
The principal management approaches reviewed included:
i) minimising the formation of sulfidic sediments
ii) the rehabilitation of affected water bodies by a) preventing oxidation of reduced
inorganic sulfur, b) the managed oxidation of reduced sediments, c) neutralising
acidified water bodies using chemical ameliorants, or (d) promoting alkalinity in a
system via aquatic plants, clays or microbial processes, and
iii) protecting waterways that are connected to impacted sites.
A brief summary of these approaches is presented here and further discussion is provided
with respect to the project’s key findings in Section 3.8.
NATIONAL WATER COMMISSION — WATERLINES 13
Minimising the formation of sulfidic sediments
It is not feasible to eliminate two of the conditions which enable sulfate reduction to occur in
inland waterways, i.e. sediment anoxia and the presence of bioavailable carbon, since these
are present in all aquatic systems. Consequently, minimising the potential for formation of
potentially harmful amounts of sulfidic sediments in inland waterways is likely to be best
achieved by either:
a. reducing the amount of sulfate in a system, or
b. preventing excessive concentrations of sulfidic materials from accumulating in bottom
sediments.
If feasible, reducing the amount of sulfate entering a system may be assisted by limiting the
use of agricultural chemicals containing sulfur. However, overall strategies must address the
mobilisation of salt in the landscape.
Preventing excessive concentrations of sulfidic materials from accumulating over time in the
first place will involve incorporating drying periods into flow regimes so that reduced sulfidic
materials are regularly oxidised in small doses thus preventing the build-up of dangerous
deposits of reduced sulfur. However, such reinstatement of drying phases to managed water
bodies for this purpose must take into account the existing concentration and rate of formation
of sulfidic sediments. In practice this may require the installation of flow regulators (Baldwin
and Fraser, 2009).
Rehabilitating affected water bodies
In water bodies in which excessive levels of sulfidic sediments have already accumulated,
management approaches could focus on preventing or limiting the occurrence of damaging
biogeochemical processes associated with the oxidation of reduced sulfidic minerals. In cases
where acidification has already occurred, neutralising the produced acid would be the likely
focus. While such intervention may be the only immediate recovery option for those wetlands
that have already been affected, it should be recognised that the accumulation of excessive
amounts of sulfidic minerals in an inland water body is indicative of a system under stress as
a result of salinisation and altered hydrology. Thus treatment of affected water bodies by the
above approaches will really just be treating the symptoms rather than addressing the
fundamental causes of the problem (Baldwin and Fraser, 2009).
Preventing the oxidation of sulfidic sediments
As described above (Section 2.2), the oxidation of reduced sulfidic minerals can be prevented
by keeping sulfidic sediments inundated and thus maintaining anoxic conditions around the
reactive surfaces of the reduced sulfur materials. Given the likelihood of a drying climate in
much of inland Australia, keeping sulfidic sediments in inland waterways flooded may not
always be feasible and, even if it were, this strategy may aggravate the situation by
maintaining the accumulation of reduced sulfur (see Section 2.3). The use of groundwater,
which is typically saline and therefore a source of sulfate, to flood sulfidic sediments is likely
to similarly exacerbate the problem.
Other approaches may include the addition of a carbon source, e.g. in the form of mulch,
which promotes microbial activity and maintains moisture and anoxic conditions within and
over the surface of bottom sediments. A major risk associated with this approach, however, is
that oxygen in the water column is also stripped as a result of this microbial activity and the
subsequent anoxia may result in fish kills (Baldwin et al., 2001).
NATIONAL WATER COMMISSION — WATERLINES 14
Oxidation of reduced sulfidic sediments
Where the underlying causes of the accumulation of sulfidic sediments can be treated, there
may be some situations in which deliberate oxidation of existing sulfidic sediments, e.g.
through the partial or total drying of a water body, is warranted. Such a management strategy
should be undertaken with extreme caution and with due consideration of a given wetland’s
biogeochemical state since there are clearly many risks associated with this approach. These
risks include the potential mobilisation of toxic heavy metals, deoxygenation and, in the case
of completely drying an affected water body, the death of aquatic organisms. In particular, for
such an approach to be effective, a system should have a high capacity to neutralise any acid
produced as a result of the oxidation of the sulfidic material that is present (Baldwin and
Fraser, 2009).
Neutralising acidified water bodies
Acid produced as a result of the oxidation of sulfidic sediments can be neutralised by the
addition, either to water or sediments, of a wide range of chemical ameliorants including
calcium carbonate (aglime), calcium oxide (quicklime), magnesium oxide (calcined
magnesia), calcium hydroxide (slacked lime), sodium carbonate (soda ash), treated red mud,
fly ash and biochar (Baldwin and Fraser, 2009). However, there are considerable difficulties
and expense associated with the efficient application of these chemical ameliorants, and a
risk that heavy metals mobilised as a result of the acid dissolving surrounding minerals (see
Section 2.2) will be precipitated, potentially affecting benthic organisms.
Water bodies may also have potential intrinsic sources of alkalinity stored in aquatic plants or
clay or calcareous sediments which may contribute to a system’s capacity to neutralise any
acid produced by the oxidation of sulfidic sediments. The alkaline effect of plant material,
either in existing vegetation or that introduced as part of rehabilitation efforts, occurs as a
result of decomposition but may also be promoted by burning. Clay sediments can also
contribute neutralising capacity through ion exchange and dissolution processes.
There are risks associated with both pathways to neutralisation however, including possible
increases in acidity in the case of plant material, and the mobilisation of toxic metals (e.g.
aluminium) from the dissolution of some clays (see Section 2.2; Baldwin and Fraser, 2009).
Stimulation of microbial activity, i.e. through the addition of bioavailable carbon (e.g. mulch),
may be similarly used to create alkalinity in an affected system but this is really only a stop-
gap measure as the approach works by recreating the conditions which enabled the formation
of the reduced inorganic sulfur compounds in the first place (Baldwin and Fraser, 2009). This
strategy would actually result in more sulfidic material being produced, thereby increasing the
risk of future acidification in the event the wetland completely dried out.
Protecting receiving waterways
When rehabilitation of a water body affected by oxidation of sulfidic sediments is not possible,
it may be necessary to consider measures to protect connected downstream waterways.
These might include the physical isolation of the affected water body, via block banks or
regulators, dilution of poor quality waters impacted by the disturbance of sulfidic sediments, or
the physical or biological treatment of waters discharging from affected areas (Baldwin and
Fraser, 2009). As with other potential management approaches, each of these have potential
risks including the precipitation of toxic heavy metals and further acidification.
NATIONAL WATER COMMISSION — WATERLINES 15
Key findings
The nine major issues highlighted in the executive summary are discussed in more detail in
this chapter which synthesises the project’s results and explains their relevance to the
ongoing management of sulfidic sediments in Australia’s inland waterways. Summaries of the
conclusions for each of the individual scientific activities are provided in Appendix 1.
Sulfate addition affects fundamental ecosystem
processes
The occurrence of elevated levels, compared with the natural condition, of sulfate is
increasingly becoming a major issue in the management of inland aquatic ecosystems, mainly
because of the risks associated with the oxidation of reduced sulfidic minerals that have
accumulated in the sediments of some waterways and wetlands (see Section 2.2). Excess
sulfate also has the potential to influence a range of other biogeochemical processes which
may in turn alter the function and character of affected aquatic ecosystems.
To explore the potential effects of sulfate pollution on other aspects of wetland
biogeochemistry, mesocosm experiments were conducted in which sulfate, with or without
carbon, was added to wetland sediments that had not previously been exposed to high levels
of sulfate. A range of anaerobic biogeochemical cycles (i.e. processes occurring in the
absence of oxygen), including the cycling of sulfur, carbon, nitrogen and the metals iron and
manganese, were then examined.
The addition of sulfate was found to fundamentally affect the way in which carbon, and
therefore energy, moved through the system. In particular, sulfate addition, even at very low
levels, inhibited methanogenesis. Methanogenesis is an important anaerobic biogeochemical
process whereby the metabolism of certain microbes results in the production of methane.
This is usually the final step in the respiration of carbon in freshwater ecosystems.
The results of this study indicate that the way inland wetlands function, and therefore their
ecological character, can be severely altered when the inputs of sulfate into them are
increased. This in turn can radically affect the ecosystem services these wetlands provide
(Woodward and Wui, 2001 and references therein).
Implications for management
• Addition of sulfate to inland waterways should be minimised as far as possible,
e.g. by limiting the use of agricultural chemicals containing sulfur such as
gypsum.
Saline groundwater is the principal source of
sulfate to inland water bodies
As part of this project, a survey was undertaken to examine the quality of groundwater in 100
bores along the Murray River between Mildura and the South Australian border. Although all
of the samples collected in this survey were from the same major aquifer, groundwater quality
varied significantly with salinity values ranging from a few 100 µS cm-1 to nearly
100 000 µS cm-1 (for reference, seawater has a salinity of about 55 000 µS cm-1). In many of
the bores saline groundwater that was quite close to the surface (i.e. < 4 m) contained
concentrations of sulfate high enough, in the event that water entered a wetland, to allow the
development of sulfidic sediments at levels likely to cause environmental degradation should
they be disturbed. Significantly, the concentration of sulfate in groundwater samples was
NATIONAL WATER COMMISSION — WATERLINES 16
found to correlate strongly with salinity levels. Consequently, groundwater salinity can be
used as an indicator of the potential for the formation of sulfidic sediments in inland water
bodies.
Further analysis of the groundwater quality survey data shows that acidification in local
wetlands of this region is unlikely to occur as a result of acidic groundwater. Acidic
groundwater has previously been implicated in wetland acidification in other parts of Victoria
(Macumber,1992) and in the Western Australian wheatbelt (Peiffer et al., 2009). However, all
but four groundwater samples in the current survey were alkaline.
Implications for management
• Monitoring of shallow groundwater composition, especially salinity
concentrations, should be incorporated into wetland and river assessments as a
leading edge indicator of the potential for the presence or formation of sulfidic
sediments.
Inland wetlands are primed for sulfate reduction
to occur
The conditions necessary for sulfate reduction, i.e. the process by which sulfidic sediments
develop, to occur include:
i) presence of sulfate
ii) anoxic conditions
iii) a source of bioavailable carbon, and
iv) the occurrence of sulfate reducing bacteria (SRB), which use the sulfate in their
metabolism to produce sulfide (see Section 2.3).
As discussed in Section 2.3, sulfate reduction has traditionally been perceived as relatively
unimportant in inland waterways compared with other anaerobic biogeochemical processes
such as methanogenesis. Since anoxic conditions and abundant bioavailable carbon supplies
are common in the bottom sediments of many inland waterways and wetlands, this
assumption has been based primarily on the perception that sulfate concentrations were quite
low in these environments. Whether or not viable communities of SRB occur in Australia’s
inland waterways was identified as a major knowledge gap prior to the start of this project.
Mesocosm experiments were conducted to investigate how sulfate-reducing bacteria present
in inland wetland sediments might respond to being exposed to increasing levels of sulfate.
Since saline groundwater is the main source of sulfate in Australian inland waterways (see
Section 3.2), concentrations of sulfate were adjusted by changing the surface water salinity.
The structure of the SRB community and levels of inorganic mineral sulfides produced were
then monitored over six months.
Changes in the overall structure of SRB communities were found to be clearly and
significantly related to the level of salt (added as sea salt and therefore containing sulfate)
added to wetland sediments. One group of SRB was identified, however, that occurred under
both the freshwater and saline treatments. These results demonstrate that sediments in
freshwater inland wetlands not only contain significant communities of SRB, but that these
SRB communities have varying capacities to respond to salinity and can also respond rapidly
to increases in salinity.
NATIONAL WATER COMMISSION — WATERLINES 17
This capacity of SRBs to rapidly respond to increasing salinity (and hence increasing sulfate
levels) explains the speed with which harmful levels of reduced sulfur can form, even in
wetland sediments with no previous exposure to either salt or sulfate. The results showed that
sediments subjected to salt levels of 15 g/L -1 (added as sea salt) accumulated significantly
higher levels of acid volatile sulfides than treatments in which 0, 1 or 5 g/L -1 of salt were
added. Even in the reasonably short duration of this experiment, the amounts of metal
sulfides produced were greater than the values at which potential environmental harm could
occur should the sediment be disturbed.
Implications for management
• Limiting the activity of sulfate reducing bacteria will only be possible by preventing
sulfate (usually from salt) entering a wetland. Once a wetland is salinised, sulfate
reduction will occur, with the rate and extent controlled by the amount of
bioavailable carbon that is present.
• Since the rate of accumulation of sulfidic sediments is affected by salt levels,
knowledge of salt loads entering wetlands may assist with the design of managed
flow regimes to prevent the accumulation of dangerous levels of sulfidic
sediments, e.g. prediction of maximum intervals of inundation.
Reduced sulfur can accumulate very rapidly in
wetland sediments
Management of waterways in which excessive amounts of sulfidic sediments have
accumulated is extremely challenging (see Section 2.5; Baldwin and Fraser, 2009).
Consequently, strategies that minimise the formation of sulfidic sediments in potentially
harmful quantities are preferable. Understanding the main factors determining the rate at
which reduced sulfur materials accumulate is crucial for informing management actions.
An experiment was conducted to investigate the combined effects of salinity (and therefore
sulfate concentrations, see Section 3.2), organic carbon and temperature on the rate at which
reduced sulfur materials accumulate in sediments of a freshwater inland wetland with no prior
history of salinisation. The results indicate that, given sufficient carbon availability, all of the
sulfate present in the saline input water can be converted to reduced sulfur compounds in the
sediment within a few months. Rates of sulfate reduction were also found to be strongly
influenced by temperature. Virtually no sulfate reduction occurred at 5°C while there was a
tripling of the rate of sulfate reduction between 20°C and 30°C.
The concentration of acid volatile sulfide (AVS) in salinised sediments can, depending on
antecedent conditions, increase by orders of magnitude in a short time, within months.
However, there may be a lag time between the loss of sulfate in a system and the formation
of AVS. During this lag phase intermediate forms of sulfur, e.g. organic sulfur, are formed
(Whitworth and Baldwin, 2011). Consequently, snapshot assessment protocols that only
measure sulfate and AVS to determine the presence or potential formation of sulfidic
sediments will not detect wetlands in which the first stages of the process leading to the
formation of sulfidic sediments are underway.
In summary, the potential for reduced sulfur to form in a particular water body can be
predicted relatively simply given a knowledge of:
• the sulfate load (i.e. total amount, not simply concentration) entering the water body
• the residence time of water in the water body
• the temperature, and
• the amount of bioavailable carbon in the system.
NATIONAL WATER COMMISSION — WATERLINES 18
Such an ability to predict rates of accumulation of sulfidic materials is critical for planning
water delivery to minimise the formation of potentially harmful levels of sulfidic sediments in
inland waterways.
Implications for management
• Rates of accumulation of sulfidic sediments can be predicted, given a knowledge
of: - total sulfate load entering water body
- residence time of water in water body
- temperature
- amount of bioavailable carbon.
• Estimates of the rates of accumulation of sulfidic sediments can assist
development of managed flow regimes (wet and dry cycling) required to prevent
the build-up of harmful levels of reduced sulfur.
• Wetland and river assessments that only monitor the presence of sulfate and acid
volatile sulfide (AVS) might not detect wetlands in which sulfidic sediments are at
intermediate stages of formation (see comment above).
Reactive monosulfides are the principal form of
sulfides in inland waterways
Sulfate reduction can occur extremely quickly when wetland sediments are exposed to sulfate
in the presence of carbon (see Section 3.4). The type of sulfides present may vary both
spatially across wetlands and over time within wetlands. The type of sulfide present has
fundamental implications for estimating the risks associated with their exposure (see
Section 2.2) and for identifying the most effective strategy for their monitoring and
management (see Section 3.4). In the early stages of sulfate reduction forms of sulfur that are
intermediate between sulfate and AVS, such as reduced organic sulfur and elemental sulfur,
may be present in significant quantities. Within several months, however, the pool of reduced
sulfur is likely to be dominated by AVS which mostly comprises reactive monosulfide minerals
(Rickard and Morse, 2005).
In this project, laboratory experiments were designed to track the composition of sulfidic
sediments that formed in the sediments of previously non-saline inland wetlands in response
to the addition of sea salt (and therefore sulfate), and carbon. The iron monosulfide mineral
mackinawite was found to be the dominant sulfide mineral present. Sulfidic sediments were
also collected from salinised inland wetlands and subjected to mineralogical examination.
This survey revealed that highly reactive forms of reduced sulfur, similar to those observed in
the laboratory, were the dominant forms of sulfides in the sediments of these wetlands. AVS
mainly comprising mackinawite, was the major form of reduced sulfur in the sediments of two
salinised wetlands (Bottle Bend and Psyche Bend lagoons) along the Murray River near
Mildura.
Mackinawite is a highly reactive monosulfide mineral and reacts with oxygen much more
rapidly than the pyritic (iron disulfide) minerals that tend to be the primary forms of sulfide in
coastal acid sulfate soils. Under aerobic conditions, mackinawite undergoes rapid oxidation
(with a half-life of only minutes to hours) to form elemental sulfur (Burton et al., 2006). This
oxidation process can rapidly remove oxygen from the water column and result in hypoxia
which may, in turn, cause fish kills and the death of other aquatic organisms (Sullivan et al.,
2002). The oxidation of elemental sulfur to sulfate that follows is slightly slower than the initial
oxidation of mackinawite to elemental sulfur, with a half-life in the order of days. This process
can, however, produce substantial acidity which may cause the pH of a water body to
decrease rapidly if its capacity to neutralise this acid is exceeded (Burton et al., 2006).
NATIONAL WATER COMMISSION — WATERLINES 19
Implications for management
• Rapid oxidation of monosulfides in inland waterways may initially result in rapid
(minutes to hours) deoxygenation of water bodies, causing fish kills and the death
of other aquatic organisms.
• Acidification is then likely to occur somewhat more slowly (i.e. days) when the
acid neutralising capacity of the water body is exceeded.
• Careful consideration should be given on the impacts of disturbing sediments
containing monosulfides prior to any management intervention (e.g. increasing
flows down a reach of river known to contain sulfidic sediments).
Understanding the acidification paradox
Understanding the processes that are triggered when wetlands containing sulfidic sediments
undergo drying is essential for informing management actions aimed at minimising the
potential of sulfidic sediments to cause ecological harm after they are formed. Management
strategies to prevent acidification will be particularly vital (see Sections 2.2 and 2.5).
Acidification does not always occur following the drying of wetlands containing sulfidic
sediments; therefore, knowledge of the factors influencing the outcome is crucial for
identifying the most appropriate management approach. The outcome depends on the
balance between the acid generating capacity and the acid neutralising capacity in each
affected water body. Previous studies have established that some water bodies have the
capacity to neutralise the acid produced when sulfidic materials are oxidised, thereby
preventing harmful acidification of the water body (see Baldwin and Fraser, 2009).
This ‘acid neutralising capacity’ of a water body containing sulfidic sediments arises from two
main sources:
1. The pre-existing neutralising capacity of the wetland, i.e. stored alkalinity in the form
of organic matter (e.g. aquatic plants), clay sediments, old shells or carbonate
minerals, and
2. The alkalinity generated during the process of sulfate reduction.
In a closed system, the reduction of sulfate (i.e. the production of reduced sulfidic materials)
and subsequent re-oxidation should be acid neutral. This means that the alkalinity generated
during sulfate reduction, which may be aqueous or trapped in a solid phase (e.g. as
precipitated carbonate minerals), is equivalent to the amount of acid produced as a result of
the subsequent oxidation phase. A fundamental question, therefore, is why some wetlands
containing sulfidic sediments acidify upon drying while others do not.
Since inland wetlands are rarely closed systems, one hypothesis is that some systems
undergo acidification because the alkalinity that was initially generated during the sulfate
reduction process is transported away from the site of production via surface or groundwater
flow pathways, while the potential acidity remains behind in the form of reduced sulfidic
materials. Flushing of wetlands during periods of high flow, leakage of surface water to
groundwater and groundwater flow paths across wetlands can all result in the transport of
dissolved alkaline materials away from a wetland (Jolly et al., 2008).
Alkalinity that is not trapped in the solid state (typically as calcite) during sulfate reduction has
the potential to migrate away from the site of reduction, before oxidation of the formed
sulfides occurs at some later time. If this happens in a wetland which does not have sufficient
residual capacity to neutralise acid produced when the reduced sulfur is re-oxidised (e.g. from
alkalinity stored in organic matter, existing carbonate minerals or carbonate in the water
column) then acidification is likely to occur.
NATIONAL WATER COMMISSION — WATERLINES 20
The intrinsic acid neutralising capacity of wetlands is of particular interest in a management
context since this represents a potential variable that may be manipulated, e.g. via variation of
the flow regime or source water composition, or addition of organic matter to prevent
acidification of systems containing sulfidic sediments.
A combination of laboratory experiments and computer modelling was undertaken to explore
the factors affecting the trapping of acid neutralising capacity during sulfate reduction. The
results showed that the ionic composition of a wetland’s source water is a major factor in
determining whether the alkalinity captured during sulfate reduction is likely to be sufficient to
neutralise acid produced as a result of re-oxidation. In particular, the results demonstrate that
the proportion of alkalinity that can be trapped via formation of calcite in the sediment is
closely linked to the ratio of calcium to sulfate in source water.
The results of this study suggest there is some potential for wetland managers to reduce the
risk of future acidification by manipulating hydrology so that the loss of aqueous phase
alkalinity is prevented. Although this study specifically used the composition of groundwater to
model the potential for trapping alkalinity in wetlands in which sulfate reduction was occurring,
the findings are applicable to other saline source waters such as irrigation return waters and
salt interception schemes.
Sulfate reduction is not the only pathway for
wetland acidification
During a recent survey of sulfidic sediments in wetlands of the Murray-Darling Basin (Murray-
Darling Basin Authority, 2011) sediments from a number of wetlands appeared to be neutral
when their pH was measured directly using pH strips, but appeared to be acidic when the pH
was measured using the standard soil pH method (where the pH of a 1:5 soil:potassium
chloride extract is measured). No evidence of the presence of sulfides was found in these
wetlands. These observations suggest the possibility that there are pathways for salt-induced
acidification in inland waterways other than those involving the re-oxidation of reduced
sulfate. Since an increasing number of inland waterways are affected by rising salinity,
understanding any alternative pathways for salt-induced wetland acidification is clearly critical
for managing such risks.
In this project, a preliminary assessment of clay sediments from a wetland near the Murray
River in south-eastern New South Wales revealed the presence of significant levels of salt-
exchangeable reduced iron (ferrous iron, Fe2+). When released into solution ferrous iron
oxidises and produces acid. Despite these wetlands containing only very low amounts of
sulfides they have considerable potential for acidification. A study was subsequently
conducted demonstrating that the acidification occurring upon the addition of salt to these
sediments can be explained by the displacement of clay-bound reduced iron by salt and the
subsequent oxidation and hydrolysis of this reduced iron.
Reduced iron is known to be widespread in clays in the sediments of inland wetlands. The
potential for acidification of these wetlands once they are salinised needs to be incorporated
into their management plans.
NATIONAL WATER COMMISSION — WATERLINES 21
Implications for management
• Salinisation of wetlands containing excessive levels of clay-bound reduced iron is
likely to result in acidification. Assessments of wetlands and rivers should
therefore measure the concentrations of clay-bound reduced iron as well as
sulfidic sediments when assessing wetlands for acidification potential.
Management of sulfidic sediments once formed
is both expensive and not straight forward
A comprehensive review of potential rehabilitation strategies for inland wetlands affected by
sulfidic sediments was undertaken during this project (Baldwin and Fraser, 2009) and the
overall conclusions briefly summarised in Section 2.5. Due to the paucity of literature on
rehabilitation of inland wetlands containing sulfidic sediments, this synthesis relied on case
studies of coastal acid sulfate soils, acid rock and acid mine drainage, acid mine lakes and
waterways affected by acid rain. This review identified a variety of management strategies.
By far the best management strategy for reducing risk associated with sulfidic sediments in
inland waterways is to prevent their accumulation in the first place. This requires removing the
source of sulfate from the wetland, e.g. by lowering saline water tables, or by introducing
frequent wetting and drying cycles in order to limit the amount of sulfidic material that can
build up in sediments during wet phases and thereby reducing the possible environmental
damage that can occur as a consequence of drying.
Once sulfidic sediments have formed in a wetland, prevention of oxidation, usually by keeping
sediments inundated to a sufficient depth, is a potential strategy. However, this may not
always be feasible in a drying climate. If oxidation of sulfidic sediments does occur and leads
to the acidification of sediments and/or the water column, then management actions to
neutralise this acid will be necessary.
Approaches to neutralising sediments and/or the water column (Baldwin and Fraser, 2009)
include:
• the addition of chemical ameliorants such as lime
• the use of clay and carbonates lying beneath the sulfidic sediments as an acid
neutralising agent
• introducing alkalinity into the system by burning plant material, and
• the promotion of microbial respiration via the addition of a carbon source (e.g. mulch)
since some microbial processes, such as iron and sulfate reduction, produce
alkalinity.
The effectiveness of these strategies for neutralising oxidised sulfidic sediments was
assessed in the current project via a large-scale field-based experiment in a highly degraded
wetland, Bottle Bend Lagoon. Each of the above management approaches were tested in
replicated plots in the field. These treatments were:
1. addition of three types of chemical ameliorants (calcium hydroxide, calcium carbonate
and biochar)
2. deep ripping and mixing of underlying clay layer into the surface (i.e. sulfidic) layer of
sediments
3. burning of two types of biomass applied to the surface of the wetland: wood (hot long
burn) and straw (cool quick burn)
4. the introduction of imported organic carbon (barley straw), and
NATIONAL WATER COMMISSION — WATERLINES 22
5. the re-establishment of three different macrophyte species in the wetland to provide
an in situ carbon source:
i) the common reed (Phragmites australis)
ii) cumbungi (Typha domingensis), and
iii) old-man saltbush (Atriplex nummularia).
Plants were either directly planted into the experimental plots or planted into plots that
had also been treated with calcium carbonate (Treatment 1), treated with biochar (P.
australis only; Treatment 1), deep ripped (Treatment 2), or isolated from the
underlying sulfidic sediments by layers of paper and garden mulch.
Two weeks after the commencement of the experiment, the only treatments (out of 20 trialled)
that decreased total actual acidity (TAA) in the top 5 cm of the sediment were the addition of
calcium hydroxide, the addition of calcium carbonate, burning of wood, and the planting of
common reed, cumbungi and old-man saltbush into beds of topsoil and mulch. After six
months, the only treatments that sustained decreased TAA were the addition of calcium
hydroxide and the planting of common reed, cumbungi and old-man saltbush into beds
prepared with calcium carbonate. The only treatment which reduced TAA in the layer of
sediments between 5 and 30 cm was the planting of common reed into beds prepared with
calcium carbonate. This result was solely due to the incorporation of carbonate to a greater
depth.
Bottle Bend Lagoon was also surveyed to estimate the total amount of actual and potential
acidity stored in the sediments and overlying water. This survey showed that up to
1200 tonnes of calcium carbonate would be required to neutralise all of the actual and
potential acidity in this 10 ha wetland. This calcium carbonate would need to be dug into the
wetland to a depth of 0.5 m to ensure contact with the full depth of potentially acid-generation
material. This would be extremely difficult to achieve in practice given the very soft mud.
Furthermore, neutralisation of the acidic volume of water in the wetland (which was about
12.5 ML at the time of the survey) would result in the production of approximately 2750 m3 of
metal-rich sludge that would require separate disposal.
Such a rehabilitation technique would prove costly and complex to implement, restricting its
application to sites of high economic or environmental value.
Implications for management
• Long-term reductions in acidity of shallow sediments are only likely to be
achieved by application of a strong ameliorant like calcium hydroxide.
• Application of such rehabilitation techniques at a wetland scale is likely to be
prohibitively expensive and logistically difficult.
NATIONAL WATER COMMISSION — WATERLINES 23
Long-term solutions must address the
underlying causes of salinisation and changes
to river hydrology
The presence of sulfidic sediments in Australia’s inland waterways can be directly linked to
changes in land use and water flow and is, therefore, a symptom of ecosystems under stress.
Consequently, unless the underlying causes that result in formation of sulfidic sediments are
addressed, any attempts to rehabilitate or restore water bodies currently affected by sulfidic
sediments will at best be postponing a reoccurrence of the issue.
The two principal and strongly inter-related areas that need to be addressed are:
1. salinisation (with sulfate-containing water) of the landscape, and
2. changes to riverine hydrology.
Salinisation
For the most part, the occurrence of sulfidic sediments in inland Australia is directly linked to
salinisation. The largest pool of sulfate in inland Australia occurs in saline groundwater.
Sulfidic sediments in inland waterways are therefore just another consequence of rising
groundwater levels bringing subterranean salt to the surface. Consequently, sulfidic
sediments will continue to form and be a management issue until the causes of rising saline
water tables are addressed.
Changes to riverine hydrology
Prior to river regulation, most floodplain wetlands would have experienced drying events on a
fairly regular basis. Even if sulfidic sediments did form during wet phases, these frequent
drying events would mean that sulfidic material would probably not accumulate over time to
the degree that it posed an environmental risk. Periodic flooding under unregulated flow
regimes, even by small to intermediate floods, would have ‘reset’ water bodies by flushing out
Project field trials at Bottle Bend Lagoon, NSW. Photo by K Whitworth
NATIONAL WATER COMMISSION — WATERLINES 24
salt and acid that may have formed by oxidation of dried sediment, and higher flows will have
scoured out affected sediments. Any potentially adverse water quality effects such as
acidification, deoxygenation or the mobilisation of toxic metals would have been mitigated by
dilution. Overbank floods would also have increased the likelihood of a freshwater lens
forming on top of saline groundwater, insulating wetland sediments from the presence of salt.
While the presence of sulfidic sediments is a symptom of salinisation, changes in river
management, particularly the loss of the cycling low flow and flood flow regime that has
resulted in many places from river regulation, has substantively contributed to the problem.
Changes in river operation have meant that many wetlands are now inundated for much
longer periods, or in different seasons, than they would have been prior to regulation.
Examples include:
• saline irrigation flows filling adjacent wetlands in high summer at a time when they
normally would be drying out
• weir pools and other water storages raising the local water table or maintaining
surface connectivity via channels, keeping nearby wetlands inundated for extended
periods of time.
Because such water bodies have been inundated with sulfate-rich water for much longer than
they would have been given unregulated flow regimes then sulfidic sediments can accumulate
at levels that, if disturbed, can result in adverse risk to the environment. Filling of wetlands
during times of higher temperatures will also dramatically increase the rate of sulfate
reduction and hence the rate that sulfidic sediments accumulate (see Section 3.4).
Consequently, changes to both high and low flow elements of flow regimes under river
regulation are likely to have contributed significantly to the accumulation of sulfidic sediments
in inland waterways.
This project has demonstrated that inputs of excessive levels of sulfate can fundamentally
change the biogeochemical processes underlying the functioning of aquatic ecosystems in
ways which make it impossible to retro-engineer resilience into affected systems. Options for
the rehabilitation of inland waterways affected by sulfidic sediments, for example the
application of lime, really only treat the symptoms of the problem and do not address the
underlying causes. However, wise management of riverine flow regimes may prevent the
formation of sulfidic sediments in inland waterways or, if they are already present, mitigate
against their worst effects.
The findings of this project indicate that environmental flows may be used as an effective tool
in the management and mitigation of sulfidic sediments in inland waterways. Preventing the
formation of sulfidic sediments in inland waterways and the rehabilitation of waterways
impacted by sulfidic sediments should be specifically addressed when options are being
assessed for the use of environmental water.
Implications for management
• Preventing the accumulation of sulfidic sediments should be included as a key
ecological objective for the management of environmental water in Australian
inland waterways.
NATIONAL WATER COMMISSION — WATERLINES 25
Action Support Tool
Rehabilitation of waterways affected by sulfidic sediments is not simple and there is no single
approach that will work in all cases. Each system should be considered on a case-by-case
approach, utilising the Action Support Tool (AST) developed by this project.
The AST is designed to assist on-ground managers to make operational decisions for
waterways that have been, or could be, affected by sulfidic sediments. In developing the tool
the following points were considered:
• It is assumed that the end-user has a background in natural resource management,
but there are no assumptions about their knowledge of sediment biogeochemistry
• The tool had to be based on the best available science
• The tool needed to be consistent with the National guidance for the management of
acid sulfate soils in inland aquatic ecosystems
• The tool needed to be freely available, and
• Above all else, the tool needed to be easy to use by a wide variety of users.
The AST can be found at www.mdfrc.org.au.
Any comments or suggestions involving the improvement to the tool can be sent to
darren.baldwin@csiro.au.
Design of Action Support Tool
The AST is a web-based interactive instrument. It consists of a series of either/or questions
that the user is asked to answer. These questions are based around four key issues:
1. whether or not sulfidic sediments are already present
2. whether or not acidification has occurred
3. whether it is a standing water body (like a wetland) or a channel (like a creek), and
4. whether management intervention is being considered or not.
While it is hoped that each of the questions is self explanatory, at a number of key junctures
further information is supplied to assist the user in making their decisions For example, the
first question page asks ‘whether or not sulfidic sediments are present’. The user is presented
with three options: ‘Yes’, ‘No’ and ‘Don’t Know’. Hitting the ‘Don’t Know’ button will take the
user to a page that outlines simple ways to recognise if sulfidic sediments are present,
including a previously published protocol for recognising sulfidic sediment in inland waterways
(Baldwin et al., 2007) as well as hyperlinks to more detailed assessment protocols.
At the end of the series of questions the tool leads to a page that lists the potential
environmental or social issues that may occur in that particular case. While acidification is
probably the most dramatic effect of disturbing sulfidic sediments, other issues are also
important depending on the context. For example, noxious odours may prove to be an
important consideration when assessing management interventions in waterways adjacent to
population centres.
The results page also lists suggested actions that may help to mitigate the risks posed by
sulfidic sediments. However, as noted above, the results are, by their nature, general.
Therefore, where it is appropriate, the user is directed to other useful resources through a
series of hyperlinks.
NATIONAL WATER COMMISSION — WATERLINES 26
In developing the AST it has been assumed that the user will at some stage need to engage
with experts with a more detailed knowledge of the assessment and treatment of sulfidic
sediments or acid sulfate soils. To facilitate that interaction the tool also informs the user
about the relevant terminology and methodologies. Key terms are hyperlinked to their
definition and links to other useful web pages and documents are included where appropriate.
Overview of Action Support Tool
A graphic outline of the broad structure and content of the AST is provided on the following
pages. The snapshot of material provided here is for illustration only and is not intended to
replace the depth and breadth of information that is provided online. Users are referred to
www.mdfrc.org.au to access the AST and the associated more detailed descriptions of the
risks and suggested actions for each step of the decision tree.
NATIONAL WATER COMMISSION — WATERLINES 27
Figure 4: Graphic representation of the Action Support Tool
NATIONAL WATER COMMISSION — WATERLINES 28
Figure 4: Graphic representation of the Action Support Tool (cont.)
NATIONAL WATER COMMISSION — WATERLINES 29
Figure 4: Graphic representation of the Action Support Tool (cont.)
NATIONAL WATER COMMISSION — WATERLINES 30
Outcomes, recommendations and
conclusions
The project Minimising Environmental Damage from Water Recovery from Inland Wetlands:
Determining water regimes to minimise the impact of sulfidic sediments (potential acid sulfate
soils) has generated considerable knowledge regarding the fundamental science, assessment
and rehabilitation of inland waterways affected by, or with the potential to become affected by,
the formation of sulfidic sediments.
In addition to contributing significantly to preparation of the National guidance for the
management of acid sulfate soils in inland aquatic ecosystems, the project has led to the
development of a freely available Action Support Tool which synthesises the current state of
knowledge to recommend appropriate management options to decision-makers.
Key outcomes
Fundamental knowledge
The research conducted in this project clearly demonstrates that sulfate pollution in inland
waterways can fundamentally transform key ecological processes, such as decomposition,
and therefore has substantial implications for the ecosystem services provided. Saline
groundwater has been identified as the primary source of excessive sulfate entering inland
water bodies with a high degree of correlation between measured salinity levels and sulfate
concentrations. This project has demonstrated that inland waterways are primed for sulfate
reduction and the formation of sulfidic sediments, even in wetlands not previously exposed to
above background levels of sulfate. Furthermore, reduced sulfur compounds can accumulate
extremely rapidly if sufficient amounts of bioavailable carbon are present in a wetland, with
the rate of accumulation being strongly influenced by temperature.
The main form of reactive sulfidic material present in sediments in inland waterways is highly
reactive iron monosulfide. Compared with the pyritic minerals (iron disulfides) that are
typically present in many coastal acid sulfate soils, monosulfides react with oxygen much
faster. The consequence can be rapid deoxygenation of water bodies if the material is
resuspended in the water column, and a high potential for subsequent acidification if a
system’s capacity to neutralise acid is exceeded. The acid neutralising capacity of inland
water bodies investigated by this study appears to be closely linked to the ratio of calcium to
sulfate in the original source water containing the input sulfate. This is the case because
higher calcium concentrations promote the precipitation of calcium carbonate in the sediment,
which increases the store of alkalinity in the system.
This project has shown that acidic groundwater is unlikely to be responsible for causing
wetland acidification along the Murray River despite being implied in acidification elsewhere.
The current work has, however, identified an alternative pathway for wetland acidification,
involving reduced iron that can occur in the absence of sulfur but is still strongly linked to
salinisation (Klein et al., 2010).
Assessment
The results of this project indicate that the concentration of salt in groundwater is a strong
indicator of the potential for sulfidic sediments to be present, or to form, in an inland water
body. The ionic composition of groundwater, especially the ratio of calcium to sulfate, may
also be a good indicator of the potential for acidification to occur if sulfidic sediments are
exposed. The potential rates of accumulation of sulfidic sediments can be predicted relatively
simply given a knowledge of the sulfate load entering a water body, the residence time of
water in the water body, the amount of bioavailable carbon and temperature. However, it is
NATIONAL WATER COMMISSION — WATERLINES 31
noted that since there may be a lag time between the initial reduction of sulfate and the
formation of AVS, during which intermediate forms of sulfur are present, assessment
procedures that only measure sulfate and AVS may fail to detect some wetlands in which
sulfidic sediments are in the very early stages of developing.
Overall, the findings show the critical need for wetland and river assessment protocols to
include measurements of the depth, composition and flow path of shallow groundwater in the
vicinity of a wetland since this is likely to be the main potential input source for sulfate.
Rehabilitation
Management options for rehabilitation of inland water bodies affected by sulfidic sediments
are likely to be complex, expensive and logistically difficult. Experiments conducted during
this project show that the addition of chemical ameliorants (e.g. calcium hydroxide and
calcium carbonate) and burning or planting macrophytes to introduce alkalinity can reduce the
acidity of an impacted wetland. However, the expense and effort required to execute such
rehabilitation options are likely to make such strategies unfeasible in many cases.
The presence of sulfidic sediments in inland waterways is a symptom of stress, primarily as a
result of salinisation and altered flow regimes. Management actions that aim to prevent the
oxidation of existing sulfidic sediments or neutralise acidified water bodies are only treating
the symptoms, not the underlying causes. Environmental flows in particular can be used as
an effective tool in the management of inland waterways including the prevention and
mitigation of sulfidic sediments. Reinstating periods of low or no flows into regulated
waterways will minimise the accumulation of potentially harmful amounts of sulfidic
sediments, while returning limited periods of high flow. High flows can flush salts and acid,
scour sediments and dilute affected waterways as well as protect wetland sediments from
saline groundwater by providing an overlying freshwater lens.
The most important recommendation from this project is that the management of sulfidic
sediments is included as a specific ecological objective for the use of environmental water.
Without management that addresses the consequences of hydrological alteration and
salinisation, sulfidic sediments will continue to be produced with potential for adverse
environmental consequences.
Recommendations
Management
Assessment
• Salt concentration in shallow groundwater should be routinely measured.
• Assessment of shallow groundwater should also include consideration of the ratio of
calcium to sulfate as this can provide an indication of the potential for acidification to
occur if sulfidic sediments are exposed.
• Assessment procedures should measure intermediate forms of sulfate and AVS as these
may indicate the formation of sulfidic sediments even where sulfate and AVS are not
detected.
• Determining the concentration of clay-bound reduced iron should be included in wetland
assessments to identify areas at risk of acidification via pathways other than the oxidation
of sulfidic sediments.
Rehabilitation
• Rehabilitation of waterways affected by sulfidic sediments may be approached by:
NATIONAL WATER COMMISSION — WATERLINES 32
o preventing the oxidation of sulfidic sediments (i.e. by maintaining water levels
over sediments)
o managed oxidation of reduced sulfidic sediments
o the use of chemical or other ameliorants (including stored alkalinity) to neutralise
acidified water bodies
o addition of organic matter (mulch), or
o the isolation of affected water bodies from adjacent waterways.
Implementation of any of the above management approaches requires careful
consideration of site-specific factors due to the contingent risks to aquatic ecosystems
including deoxygenation and the mobilisation of toxic heavy metals, in addition to
promoting further accumulation of sulfidic sediments.
• Managers contemplating the above options are strongly advised to consult the Action
Support Tool developed in this project (see Chapter 4).
Prevention
• Use of agricultural chemicals containing sulfate, e.g. gypsum, should be limited as far as
possible in the catchments of wetlands.
• Managed flow regimes should include the reinstatement of wet and dry cycles to prevent
the build-up of dangerous levels of reduced sulfur. Appropriate flood durations and
frequency of drying cycles may be calculated, with expert assistance, given a knowledge
of:
o total sulfate load entering water body
o residence time of water in water body
o temperature, and
o the amount of bioavailable carbon.
• Environmental water should be used to flush salts and acid from waterways (i.e. small to
medium ‘dilution’ flows) and scour affected sediments from water bodies (i.e. high flows).
Policy
• Management of sulfidic sediments should be included as a key ecological objective for
the use of environmental water in Australia’s inland river catchments.
• Long-term solutions to the problem of sulfidic sediments in inland waterways must
address the underlying causes of salinisation and altered river hydrology.
Future research
This project has increased our knowledge of sulfidic sediment in inland wetlands in Australia.
However, several knowledge gaps still remain which, if addressed, could substantially
improve the long-term management of these materials. There are two principal areas that
need addressing:
1. This project mostly dealt with the formation and transformations of sulfidic sediments in
wetlands (standing water bodies). We also know that sulfidic sediments can occur in river
channels. While the fundamental chemistry of the two will be the same, the difference in
hydrology will create a different environment for transformation and mobilisation of the
material.
2. The formation of sulfidic sediments can potentially have multiple impacts on the ecology
of inland waterways, at both the local and regional scale. At a fundamental level, the
formation of sulfidic sediments in wetlands in inland waterways occurs because energy (in
the form of carbon) follows an alternative pathway (sulfate reduction) than would occur in
NATIONAL WATER COMMISSION — WATERLINES 33
non-impacted wetlands (formation of methane). The implications of this switch in energy
pathways on the ecology of these systems are unknown. Furthermore, we do not know
what the impact of sulfide formation in sediments in wetlands has on the community
structure and food web dynamics in affected wetlands (whether acidified or not). We also
need to know how the distribution of wetlands and other water bodies containing sulfidic
sediments affects the functioning of ecosystems at the landscape scale.
Conclusions
Conditions in Australia’s inland waterways are such that the formation of sulfidic sediments
occurs rapidly in response to the addition of sulfate delivered in high concentration to many
water bodies via saline groundwater. The underlying causes of the problem are salinisation
and changes to river hydrology that have resulted from a range of past land and water
management practices.
Addition of excess sulfate alters fundamental ecological processes in water bodies. The
exposure of sulfidic sediments to oxygen can result in deoxygenation, acidification and the
mobilisation of toxic heavy metals and metalloids, all of which are potentially harmful to
aquatic organisms and human health. Once formed, sulfidic sediments are both difficult and
costly to effectively manage. Environmental flows have the greatest potential to be used as a
proactive tool for preventing the accumulation of harmful levels of sulfidic sediments in inland
waterways.
Dr D Baldwin (MDFRC) measuring water quality in a small wetland impacted by sulfidic
sediments, NSW. Photo by A Brecknell
NATIONAL WATER COMMISSION — WATERLINES 34
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Appendix 1—List of the project’s scientific activities, conclusions
and outcomes
Activity Description Conclusions Outcomes Outputs
FUNDAMENTAL KNOWLEDGE
1. Sulfate addition to
wetland sediment in
mesocosms
This project subjected anaerobic
mesocosms to additions of sulfate and
measured the time course responses
for a range of metabolites for anaerobic
respiration.
Sulfate reduction rates under ideal
conditions are quite rapid. Excessive
levels of sulfides can accumulate in
wetland sediments in less than one
month. Sulfate reduction also affects
nutrient cycling in wetland sediments.
DS Baldwin and AM Mitchell, Impact of
sulfate pollution on anaerobic
biogeochemical cycles in wetland
sediment: a mesocosm study
(submitted - unpublished).
2. Sulfate reducing
bacteria (SRB) in
mesocosms
Activity to determine the microbial
consortia responsible for sulfate
reduction in sediments from inland
wetlands.
SRB communities already exist in
wetland soil and are poised to become
active irrespective of conditions. Three
types of SRB are present – high salt
tolerant, not salt tolerant and ubiquitous.
Furthermore, reduced sulfur
accumulation (to quite high levels) can
occur within six months, dependent on
salt concentrations.
Management of SRB is only possible
by preventing sulfate (from saline
ground or surface water) entering a
wetland. Once a wetland is salinised,
sulfate reduction will occur. Sulfate
reduction to produce excessive levels
of reduced sulfur in sediments can
occur rapidly. Potential to model SRB
kinetics would allow prediction of
maximum intervals of inundation that
can occur without harmful levels of
sulfides being produced.
GN Rees, G Watson, DS Baldwin and
K Hall 2010, ‘Sulfide formation in
freshwater sediments by sulfate
reducing microorganisms with diverse
tolerance to salt’, Science of the Total
Environment 409, 134–139.
NATIONAL WATER COMMISSION — WATERLINES 38
Activity Description Conclusions Outcomes Outputs
ASSESSMENT
4. Determining the
effects of sulfate and
carbon loading and
temperature on the rate
and products of sulfate
reduction
Microcosm experiment to determine
the factors affecting the rate of sulfate
reduction in inland waterways.
Changes in sulfur speciation over time
were investigated. Reduced sulfur forms
rapidly when carbon is not limiting, even
in sediments that were not initially
anoxic and have not previously been
exposed to elevated levels of sulfate.
Temperature substantially influences
the rate of sulfate reduction. Elemental
sulfur and organic sulfur are major
components of the total reduced sulfur
during the early stages of sulfate
reduction. After several months, reactive
mineral sulfides dominate the reduced
sulfur pool.
This data will allow wetland managers
to develop an appropriate drying cycle
to limit the formation of sulfidic
sediments. Knowledge has been
incorporated into the Action Support
Tool (Chapter 3).
KL Whitworth and DS Baldwin 2011,
‘Reduced sulfur accumulation in
salinised sediments’, Environmental
Chemistry 8, 198–206.
5. Groundwater
composition and
acidification risk 1:
assessment and
modelling
A fundamental question regarding
sulfidic sediments is why wetlands
acidify. Sulfate reduction and re-
oxidation should be acid neutral in a
closed system. This activity surveyed
the composition of groundwater from
100 shallow bores along the Murray
River between Mildura and the SA
border and used the results to populate
a geochemical model of alkalinity
capture during sulfate reduction in
waterways that may intercept these
groundwaters.
Modelling predicts that the risk of
acidification increases as the calcium:
sulfate ratio decreases (with a critical
threshold of 1.0). If calcium
concentrations are insufficient to trap
alkalinity in solid phase carbonate
minerals, buffering capacity can be lost
from the system – especially through
groundwater gradients.
Reinforces the critical role shallow
groundwater can play in these
systems. New advice will suggest that
any wetland or river investigation
should include an assessment of the
depth, composition and (potentially)
flow path of groundwater. Knowledge
has been incorporated into the Action
Support Tool (Chapter 3).
See Activity 6
6. Groundwater
composition and
acidification risk 2:
experimental verification
Controlled laboratory experiment to
investigate impacts of various aqueous
ionic compositions on acid neutralising
capacity in sediments where sulfate
reduction has occurred.
Carbonate precipitation is kinetically
slow and can lead to super-saturation in
the water column, meaning that net
export of alkalinity is a real possibility.
The sediment matrix can contribute to
alkalinity retention and acid buffering
capacity.
See above KL Whitworth, EJ Silvester and
DS Baldwin, Alkalinity capture during
microbial sulfate reduction: the role of
aqueous ionic composition (to be
submitted - unpublished).
NATIONAL WATER COMMISSION — WATERLINES 39
7. Alternative pathways
for salt-induced
acidification
Preliminary experiments showed that
addition of moderate levels of salt to
wetlands resulted in almost immediate
acidification through release of
occluded iron and subsequent
oxidation. This project examined the
mechanisms underlying the process.
Experimental work shows that the
occluded iron is present on ion-
exchange sites in clay minerals.
Displacement of iron occurs even at low
salinity (< 500 EC). Oxidation of the
released iron decreases the pH and
initiates an autocatalytic acidification
process.
Evaporation and drying in salinised
wetlands containing occluded iron will
likely result in acidification by the
process of ferrolysis. Similarly,
salinisation of a freshwater wetland by
entry of saline water could induce
acidification. Simple sediment pH
measurements would be a good
predictor of the likelihood of this
occurring.
A Klein, DS Baldwin, B Singh and
E Silvester 2010, ‘Salinity-induced
acidification in a wetland sediment
through the displacement of clay-
bound iron(II)’, Environmental
Chemistry 7, 413–421.
8. Using remote sensing
to locate acid sulfate
sediments in the
environment
Pilot study to determine viability of
using remote sensing techniques to
either observe or predict occurrence of
sulfidic sediments in inland waterways.
Visual assessment from the air can be
used as a preliminary screening tool to
prioritise wetlands for more detailed
(and expensive) on-ground assessment.
Overflights have now been used to
assess occurrence of acid sulfate
soils by NSW Office of Environment
and Heritage and the Murray-Darling
Basin Authority along the Darling
Corridor. Other NSW agencies have
used this technique for the Wakool
River.
See for example Ralph, TJ and
Baldwin DS 2009. Aerial assessment
of acid sulfate soils hazards along the
Darling River corridor, western New
South Wales. Summary report of aerial
assessment conducted 19th – 21st
September 2008. Rivers and Wetlands
Unit, NSW Department of Environment,
Climate Change and Water, Sydney,
14 pp. A report to the Murray-Darling
Basin Authority.
REHABILITATION
9. Survey of
rehabilitation options for
wetlands affected by
sulfidic sediments
Literature review and subsequent
synthesis of appropriate methods for
rehabilitation of inland waterways
affected by acid sulfate soils.
A wide range of rehabilitation options
may be possible; however, each has
significant draw-backs, including cost.
The best option, where possible, is to
prevent initial accumulation of sulfidic
materials.
Foundation document for the National
guidance for the management of acid
sulfate soils in inland aquatic
ecosystems. Knowledge has been
incorporated into the Action Support
Tool (Chapter 3).
DS Baldwin and M Fraser 2009,
‘Rehabilitation options for inland
waterways impacted by sulfidic
sediments – a synthesis’, Journal of
Environmental Management 91, 311–
319.
10. Buffering capacity of
clays Preliminary work suggested that clay
minerals could play an important role in
buffering acidity in low pH
environments. Experimental work was
undertaken to determine the
weathering behaviour of clay minerals
Underlying clay layer certainly has the
possibility of being used to mitigate
acidification in wetlands but it may result
in high levels of dissolved aluminium in
the overlying water column (see also
Activity 12).
This data will allow assessment of the
potential for natural clay minerals to
buffer wetland pH.
I Bibi, B Singh and E Silvester 2011,
‘Dissolution study of illite in saline–
acidic solutions at 25°C ’, Geochimica
et Cosmochimica Acta 75, 3237–3249.
NATIONAL WATER COMMISSION — WATERLINES 40
within acidic wetlands.
11. Bottle Bend survey A survey of Bottle Bend sediments was
undertaken prior to the intervention
experiment. The profile of the wetland
was surveyed and about 60 sediment
cores were taken across the wetland.
Water samples were also collected.
It would require up to 1200 tonnes of
lime buried to a depth of 0.5 m to
neutralise the actual and potential
acidity in Bottle Bend Lagoon.
Neutralisation of the overlying water
column would produce about 2750 m3 of
metal-rich sludge.
This will allow us to calculate exactly
how much reduced sulfur is in the
sediments of Bottle Bend so to allow a
realistic assessment of the cost of
treatment options identified in the
intervention experiment. This protocol
can be applied to other wetlands.
Knowledge has been incorporated
into the Action Support Tool.
DS Baldwin, M Fraser, GN Rees,
E Silvester and K Whitworth,
Rehabilitation options for inland
waterways impacted by sulfidic
sediments – Field trials (submitted).
12. Bottle Bend
intervention A major field experiment of remediation
treatments has been established on
Bottle Bend, a severely degraded
wetland lagoon near Mildura.
Principally, these treatments will
determine effective methods to
neutralise acidity produced by the
oxidation of sulfidic sediments.
Of the 20 treatments trialled, only
addition of chemical ameliorants
(calcium carbonate or calcium
hydroxide) had a significant and
enduring impact on sediment pH.
The experiment allows determination
of an appropriate management
strategy of acid sulfate sediments for
implementation at the wetland scale,
which would be transferrable to other
wetland systems. Knowledge has
been incorporated into the Action
Support Tool (Chapter 3).
See activity 11
NATIONAL WATER COMMISSION — WATERLINES 41
