Acid Sulfate Soils and Development Site Constraints

Acid sulfate soils sit under a large share of Australia’s coastal development land, and on most sites they cause no trouble at all until something digs into them. The moment excavation or dewatering lets oxygen reach the iron sulfides, the reaction starts, and what would have been ordinary spoil becomes material that has to be tested, treated, validated and tracked off site. For a developer the question is rarely whether the soils are present. It is how deep the scheme has to go to reach them, and what that does to the excavation rate, the water treatment, the spoil disposal and the programme.

The awkward part of the timing is that the design decision which drives most of the cost, basement depth against the standing water level, tends to be locked months before anyone has drilled a hole. Mapped classes, guideline versions and the figures below were current at the date of writing and do change. The linked state source is the place to confirm the position for a specific site.

Three people hold the inputs. The environmental consultant’s investigation sets the extent of the soils and the net acidity by depth. The geotechnical engineer’s borehole logs and standing water level decide how much of the excavation sits below that level. The quantity surveyor prices treatment, dewatering, cartage and disposal off the back of both. Council sets the trigger through the planning instrument. None of them can say much that is useful until the site has been drilled, which is an argument for drilling early rather than a reason to wait.

What are acid sulfate soils, and what actually goes wrong?

Acid sulfate soils are natural sediments containing iron sulfides, mainly pyrite, and they are generally harmless while they stay waterlogged. The NSW Department of Climate Change, Energy, the Environment and Water describes them as posing no risk left undisturbed, and as producing sulfuric acid and toxic metals when exposed to air, with consequences for structures, waterways, and plant and animal life.

The reaction itself is fast. The Queensland Government’s WetlandInfo notes that framboidal pyrite has a large surface area and oxidises very rapidly, calling the result “the most acidic weathering reaction known at the earth’s surface”. Soil pH can fall from close to neutral to below 2. Iron, aluminium and sometimes heavy metals come out of the soil with the acid, which is why the problem travels in water rather than staying in the stockpile.

Two terms recur in the reports and they mean different things for the budget. Potential acid sulfate soils are unoxidised, typically dark grey, and generally sit lower in the profile below the water table. Actual acid sulfate soils have already oxidised, tend to be yellow to orange with red mottling, and sit nearer the surface. Waikato Regional Council sets out that distinction plainly. A site can carry both, and the treatment approach differs: existing acidity has to be neutralised, while potential acidity has to be prevented from forming at all.

A third category, monosulfidic black ooze, turns up in drains and channels and behaves differently again, releasing acidity and stripping oxygen from water very quickly when it is disturbed. It is one of the specific subjects covered in the Commonwealth’s National Acid Sulfate Soils Guidance, developed with the National Committee for Acid Sulfate Soils as non-prescriptive material that sits alongside state guidance rather than replacing it.

Where do acid sulfate soils occur in Australia?

Mostly in low-lying coastal land formed over roughly the last 10,000 years, which is also where a great deal of infill and fringe development happens. There are inland occurrences as well, so a coastal postcode is a strong indicator rather than a complete test.

In New South Wales the exposure is substantial. The state’s environment department puts the soils in every coastal estuary, usually below one metre elevation, and estimates they may affect more than 260,000 hectares, around 150,000 of which is in agricultural production. The largest areas are the northern coastal floodplains, including the Tweed, Richmond, Clarence, Macleay and Hastings rivers, with further areas on the Hunter and Shoalhaven. Inland occurrences include the Edward-Wakool river system.

In South Australia the Environment Protection Authority reports actual and potential coastal acid sulfate soils covering about 2,410 square kilometres, with an estimated acid reservoir of two million tonnes. The same guidance notes metal sulfides associated with historical copper, gold, silver and lead, and coal deposits, so the issue is not confined to the coastline.

Queensland frames it by elevation. Moreton Bay Regional Council’s guidance places the soils in low-lying coastal areas below 5 metres Australian Height Datum (AHD), and Brisbane City Plan 2014 splits its overlay into land at or below 5 metres Australian Height Datum (AHD) and land above 5 metres and below 20 metres.

Tasmania’s Department of Natural Resources and Environment puts them under parts of the coastline on low-lying plains, wetlands and estuaries, with inland occurrences under peat bogs, salt lakes and wetlands. Victoria’s mapping, discussed below, works to a lower contour again.

The pattern worth taking from this is that each jurisdiction has picked a different elevation band as its screening device, so a rule of thumb carried across a state border tends to be wrong.

How do you find out whether a site is affected before settlement?

Start with the state risk mapping, then read the planning instrument that applies to the land, then drill. Mapping gives a probability and an indicative depth, not a finding, and the depth of the proposed works usually matters more than the mapped class.

In New South Wales the 1:25,000 risk maps are available through eSPADE and the SEED data portal, and they show the chance of occurrence, the estimated depth to the soil, the environmental risk from disturbance, and the landforms involved. That estimated depth is the figure to read against the proposed basement level, because it is what decides whether the scheme touches the material at all.

South Australia’s screening approach is set out in the state Environment Protection Authority’s guideline, which steps through checking the site’s elevation and the size of the excavation, then consulting the risk maps, with works in or within 500 metres of mapped high-risk classes triggering a field investigation. The maps sit on the state atlas and the national soil atlas.

Victoria’s statewide material is mapping and strategy rather than a class table. The state’s coastal acid sulfate soils mapping, now held in an archived Victorian Resources Online page whose maps and references date from 2003 to 2010, treats coastal land between the shoreline and the 2 metre Australian Height Datum (AHD) contour as prospective, reflecting a mid-Holocene sea level around 1.5 metres higher than today. Land above that contour is also mapped, because buried material can sit under younger sediments. Six regional maps run from Peterborough to Mallacoota, with maps 1 to 3 updated in May 2010. Whether a particular site carries an overlay or a permit condition is a question for the responsible authority, and the planning scheme is where to check it.

Timing is the practical point. A desktop check costs very little and can be done inside a due diligence period, while the borehole programme that actually answers the question takes longer and costs more. Where the mapping flags a risk, the question becomes whether the due diligence period is long enough to get a result, or whether the contract needs to carry the risk differently. Our guides on property due diligence and the due diligence clause cover how that period tends to be structured.

What triggers an acid sulfate soils assessment in New South Wales?

The class shown on the local environmental plan’s acid sulfate soils map, combined with how deep the works go. Most New South Wales local environmental plans carry a standard acid sulfate soils clause, numbered 7.1 in some instruments and 6.1 or 6.3 in others, which makes consent necessary for specified works on each class of land.

The class definitions describe how close to the surface the soils are expected to be. Eurobodalla Shire Council sets them out as Class 1 where the soils are found at or below the natural ground surface, Class 2 below the natural ground surface, Class 3 more than one metre below, Class 4 more than two metres below, and Class 5 where they are generally not found but the land sits within 500 metres of Class 1 to 4 land.

The works that attract consent scale with the class. City of Newcastle summarises the table this way:

Two details in that table tend to catch people out.

The first is the definition of works. Newcastle treats any disturbance of more than one tonne of soil as works, along with drain construction or maintenance, extractive industries, dredging, artificial water bodies, footings or foundations, flood works, and anything else likely to lower the water table. One tonne is a very small volume, somewhere around half a cubic metre of typical soil, so the threshold is reached by almost any footing excavation. Definitions are set council by council, so the local instrument is where to confirm it.

The second is Class 5. A Class 5 site is defined by its effect on neighbouring land, which means a scheme can be caught without ever excavating into acid sulfate soils on its own title. Dewatering is the mechanism. If the drawdown from a basement excavation pulls the water table down on adjacent Class 1 to 4 land below 5 metres Australian Height Datum (AHD), the trigger can be met. That makes the dewatering strategy a planning question, not only a construction one.

A preliminary assessment can close the matter out. Where testing in the area of potential disturbance shows the soils are not present or will not be affected, a management plan may not be required. Where it does find them, the plan is prepared before the works are approved, following the assessment and management guidelines in the state manual.

Moving the material off site brings in a separate regime. The state environment department notes that offsite transport and treatment are subject to the Environment Protection Authority’s waste classification guidelines, which is where the spoil disposal rate in the estimate comes from.

How does Queensland treat acid sulfate soils?

Through the State Planning Policy state interest and the planning scheme overlay code, with avoidance stated as the preference rather than one option among several.

The Queensland Government’s page on legislation, policies and risk assessments lists the Environmental Protection Act 1994 (Qld), the Planning Act 2016 (Qld), the Vegetation Management Act 1999 (Qld), the Fisheries Act 1994 (Qld) and the State Development and Public Works Organisation Act 1971 (Qld), with the Environment Protection and Biodiversity Conservation Act 1999 (Cth) at the Commonwealth level. It names the Queensland State Planning Policy 2017 state interest for emissions and hazardous activities as the instrument applying to acid sulfate soils under the Planning Act 2016 (Qld), and sets the objective for affected areas as identifying high-probability areas, preferring land uses that avoid disturbance, and requiring management of any disturbance that does occur. The department states that avoiding disturbance is always the preferred strategy.

In practice the trigger arrives through the local planning scheme’s acid sulfate soils overlay, mapped against the elevation bands noted earlier. The assessment benchmarks in those overlay codes generally run along two paths: avoid disturbing the soils at all, or investigate, treat and manage the disturbance and the drainage water over time. Filling is treated as a disturbance in its own right, because placing fill on low-lying land can push existing acidic material below the water table or aerate previously saturated material.

Investigation and analysis follow the Queensland sampling and laboratory methods guidelines, or AS 4969, the Standards Australia series on the analysis of acid sulfate soil. The Queensland soil management guidelines are the reference for treatment, and their influence runs beyond the state: the New South Wales reuse framework described below points to the same Queensland treatment guideline.

What applies in the other states and territories?

The destination is similar everywhere, which is investigate, avoid where you can, neutralise what you disturb and validate the result. The route differs.

South Australia frames the issue as site contamination. The state Environment Protection Authority’s guideline, Site contamination: acid sulfate soil materials (EPA 638/07), was issued in November 2007, so its currency is worth checking before relying on it. It steps from elevation and excavation-size screening, through a field inspection by a suitably qualified environmental consultant using visual and water indicators, to laboratory assessment following the National Environment Protection Measure (NEPM) Schedule B2 and AS 4482.1. Classification criteria depend on soil texture, with clay content shifting the applicable trigger, which is a detail worth raising with a consultant rather than assuming a single action level. For coastal development the guideline anticipates a thorough investigation, a long-term management plan, an on-site remediation trial and contingency plans. It states that on-site treatment is preferred to off-site disposal and that the authority is unlikely to approve disposal to marine spoil grounds, and it requires discharges to meet the Environment Protection (Water Quality) Policy 2003 (SA) or to hold an exemption. Sitting over all of it is the general environmental duty under section 25 of the Environment Protection Act 1993 (SA).

Western Australia works from the Department of Water and Environmental Regulation’s guideline Identification and investigation of acid sulfate soils and acidic landscapes, whose current version is dated June 2015. It is addressed to decision-making authorities, proponents and consultants involved in the planning, development and use of areas that may contain the soils, and the department invites contact on policy issues the guideline does not reach. State risk mapping sits alongside it.

Tasmania applies the Tasmanian Acid Sulfate Soil Management Guidelines, structured around background and legislation, assessment, management principles, and management strategies of avoidance, minimisation and neutralisation. The guidelines focus on development below 20 metres Australian Height Datum (AHD), a notably higher band than the other states, while stating that the requirement for a management plan should apply wherever significant disturbance may occur anywhere in the state.

Victoria is covered in the mapping section above. The material is a strategy and a prospective-land map rather than a class table, and the planning scheme is where a site-specific control would appear.

The Australian Capital Territory is inland, and the issue arises rarely if at all. For the Northern Territory, the national guidance and the relevant territory authority are the places to confirm the position for a coastal site.

What does an investigation actually involve?

A desktop review, a field inspection, then sampling and laboratory analysis, with the laboratory turnaround generally setting the pace.

The field stage looks for indicators rather than measuring them. Jarosite, a pale yellow mineral deposit, is a classic sign of soil that has already oxidised, typically where land has been excavated or drained. Unoxidised material tends to be dark grey and may smell of sulfur. These observations guide the sampling, and they are not a substitute for it, because the number that matters is net acidity by depth and that only comes from a laboratory.

The analysis follows AS 4969 in Queensland practice and the methods set out in the national and state guidance elsewhere. Several routes exist, including incubation testing, which is the method the New South Wales reuse framework contemplates alongside chemical criteria as one way of demonstrating that material can be reused without treatment. Where laboratory work sits on the critical path for a development application, the turnaround is a programme input rather than an administrative detail, and it is worth asking for before the borehole programme is scoped.

Two efficiencies are usually available. The first is combining the acid sulfate soils sampling with the geotechnical borehole programme, since the holes are the same holes. The second is combining it with contaminated land sampling where the site history calls for that work anyway. Our guides on geotechnical reports and contaminated land assessment cover how those scopes are usually put together.

What does an acid sulfate soils management plan commit you to?

A hierarchy, and then a set of controls that each take space, time and money on site. Tasmania’s guidelines express the hierarchy as avoidance, minimisation and neutralisation, which is the same order the Queensland and South Australian material uses.

Waikato Regional Council sets out the control measures in common use, and the list travels well: shallower and wider drainage channels, piling to contain excavations, containing spoil on a treatment pad for testing and treatment, detaining stormwater and treating it before release, neutralising soil with calcium carbonate, bunding to protect waterways, neutralising material before off-site disposal to an authorised site, and specifying sulfate-resistant and acid-resistant concrete.

Agricultural lime, calcium carbonate, is the standard neutralising agent. In the material the New South Wales Environment Protection Authority published when consulting on its acid sulfate soil order and exemption, agricultural lime is the only permitted neutralising agent, with a maximum pH in potassium chloride of 10, and treatment is required to follow the Queensland guideline dated May 2024. Blending treated material with anything other than agricultural lime is not permitted under that framework.

The site planning consequence is the one that catches tight infill sites. A treatment pad is a bunded, impermeable area where spoil is spread, limed, mixed, left to react and then sampled before it goes anywhere. It needs real area, it needs to stay available across the earthworks period, and it competes with the crane pad, the site shed, the laydown area and the hoarding line. On a constrained footprint there may be nowhere to put it, which pushes the scheme towards either treating off site at an authorised facility, which changes the cartage and disposal rates, or towards a design that avoids the material in the first place.

Where does acid sulfate soil risk hit the feasibility?

Through excavation depth, dewatering, spoil, programme and contingency, roughly in that order of significance.

Excavation depth against the standing water level is the biggest single lever, because it determines how much material is affected at all. Every additional metre of basement below the water table typically adds a full footprint’s worth of saturated material to the treatment quantity, on top of the ordinary cost of excavating and retaining below water.

Dewatering and water treatment come next. Water pumped out of a basement excavation in these soils generally cannot go straight to the stormwater system. It has to be detained, tested, usually limed, and discharged under whatever approval applies, and the cost runs for the duration of the excavation rather than as a one-off. Where drawdown extends beyond the boundary, it can also create the planning trigger described in the New South Wales section.

Spoil classification and cartage is where the estimate can move sharply. Untreated, treated and contaminated material attract different classifications and different disposal destinations, and the rate per tonne steps up rather than sliding. A classification that moves one band after the earthworks have started is a variation, not a refinement.

Programme is affected at both ends. The investigation sits before lodgement, and the treatment, validation sampling and sign-off sit inside the bulk earthworks period. Validation is iterative by nature: material that fails is re-limed, re-mixed and re-tested, and wet weather interrupts the whole cycle.

Contract risk allocation decides who pays for the difference between the assumption and the reality. Whether acid sulfate soils fall within a latent conditions regime or have been passed to the contractor through the site information is a drafting question with a direct price. Our guide to extensions of time, variations and latent conditions sets out how that allocation usually works, and the construction contingency guide covers where an allowance of this kind tends to sit.

Durability is the long tail. Austroads’ study of piles taken from a decommissioned bridge at Banora Point, published in 2011, found that concrete durability in acid sulfate environments is significantly influenced by the type of binder, and observed that AS 2159 and AS 3600 do not recognise binder effects and give limited support for a 100-year design life in those conditions. For a developer the relevance is that the structural engineer’s concrete specification for footings, piles and basement tanking may step up on an affected site, and that step belongs in the cost plan rather than in a late design development variation.

A worked example: what basement depth does to the quantity

Take a coastal infill site with a 1,800 square metre building footprint. The geotechnical report records a standing water level 2.0 metres below natural ground surface and an in-situ bulk density of 1.8 tonnes per cubic metre. The mapping puts potential acid sulfate soils below the water table.

Scheme A, one basement level excavated to 3.5 metres. Total excavation is 1,800 × 3.5 = 6,300 cubic metres. The portion below the standing water level is 1,800 × 1.5 = 2,700 cubic metres, which at 1.8 tonnes per cubic metre is 4,860 tonnes of potentially affected material.

Scheme B, same basement but the ground floor lifted 0.7 metres, so the excavation goes to 2.8 metres. Total excavation is 1,800 × 2.8 = 5,040 cubic metres. The portion below the standing water level is 1,800 × 0.8 = 1,440 cubic metres, or 2,592 tonnes.

The difference is 1,260 cubic metres less excavation overall, and 2,268 tonnes less material requiring treatment, testing and validation. That is the quantity the quantity surveyor prices treatment and disposal against, and our guide to quantity surveyor cost estimation covers how those rates are usually built up.

What the second scheme costs is paid elsewhere. Lifting the ground floor raises the finished floor level against the flood planning level and the street, pushes the overall building height against whatever height control applies, and lengthens or steepens the driveway ramp. On a tight site any of those can cost a level or a row of car spaces, and losing spaces can cap the unit yield. The arithmetic above does not answer which scheme is better. It shows where the trade is, and that the trade sits in the design, not in the earthworks contract.

Both figures move with the standing water level, which is seasonal. A water level recorded in a dry autumn and a water level recorded after a wet summer can differ by enough to change the answer, which is an argument for asking what season the boreholes were drilled in.

How does this interact with contaminated land and remediation work?

Closely enough that they are often the same investigation, and distinctly enough that they are separate findings.

Acid sulfate soils are a natural condition rather than a contamination event, but the regulatory treatment overlaps. South Australia handles the issue explicitly within the site contamination framework, with laboratory assessment under the National Environment Protection Measure (NEPM). Elsewhere the overlap shows up at the point the material leaves the site, where waste classification determines the destination and the rate.

Where a site has both a fill history and a low elevation, running the two scopes together is usually more efficient and gives a cleaner picture, because the same boreholes can be logged for both and the same consultant can set the disposal strategy once rather than twice. Where remediation is already in prospect, the sequencing matters: acid sulfate material that is excavated, stockpiled and left to oxidise while a remediation approach is settled becomes a harder problem than it was in the ground. Our guides on contaminated land assessment and remediation action plans and site audits cover the parallel process.

Can treated acid sulfate soil be reused, or does it all leave site?

Reuse is increasingly contemplated, and in New South Wales it now has a dedicated instrument. The Environment Protection Authority published an acid sulfate soil order and an acid sulfate soil exemption in 2025, which it describes as supporting the safe and beneficial reuse of acid sulfate soil across the state, with the aims of streamlining approvals and meeting demand for reuse.

From the authority’s consultation material, the general order is directed at natural material with minimal contamination, which would meet the virgin excavated natural material (VENM) or excavated natural material (ENM) requirements but for its sulfidic content, and other material may still need a site-specific order and exemption. An acid sulfate soil management plan prepared by an environmental practitioner, following the Queensland guideline, is expected, with independent review by a practitioner certified as a Registered Soil Practitioner in acid sulfate soils. Treatment follows the Queensland guideline, agricultural lime is the only permitted neutralising agent, and proof of performance trials precede any reduction in sampling frequency. Existing site-specific approvals continue unless the authority revokes them in writing. Those details come from the consultation material, so the published order and exemption are the documents that govern, and they are where to confirm the current conditions and sampling rates.

Elsewhere the direction is similar without a comparable instrument. South Australia’s guidance prefers on-site treatment to off-site disposal and indicates that disposal to marine spoil grounds is unlikely to be approved. Queensland’s policy preference is avoidance first, then treatment and management.

For a feasibility this is worth attention, because reuse on site removes cartage and tipping from the budget and replaces them with treatment, testing and the area to do it in. Whether that trade is favourable depends on the tonnage, the available footprint and the local disposal rates, and it is a question for the quantity surveyor once the investigation has produced a quantity. On larger land subdivisions the balance tends to tilt towards on-site treatment and reuse in earthworks fill, simply because the area exists. Our guide to land subdivision covers how bulk earthworks are usually staged.

How are acid sulfate soils handled in New Zealand?

As a regional council matter, through the consent for land disturbance, with risk mapping doing the screening.

Waikato Regional Council records that the presence of these soils in northern New Zealand has been known since the 1970s, with wider investigation more recent and driven largely by major infrastructure projects in Northland, Auckland and Waikato. A preliminary risk assessment prepared for the council classifies occurrence probability as high, medium, low or negligible, and the mapping can be viewed at a scale of at least 1:36,000.

The trigger is earthworks or drainage in medium to high risk areas, and the council directs applicants to the acid sulfate soil requirements within its land disturbance activities consent application. It offers a useful depth indication as well: disturbance of surface soil to no more than 1 metre is described as unlikely to pose a significant risk, while deeper disturbance is much more likely to reach unoxidised material. Larger volumes, proximity to waterways and interception of groundwater all raise the risk.

The control measures the council expects are the ones listed earlier, and it points to Australian guidance documents for technical detail, which is a reasonable signal that the Australian manuals are doing the heavy lifting on both sides of the Tasman.

What to ask your consultants

The questions below are the ones that decide the outcome on a specific site, and they are the ones this guide deliberately does not answer.

Ask your environmental consultant

• What class or probability does the mapping give this site, and at what depth does it put the soils?
• From the boreholes, what is the net acidity by depth, and which layers are actual rather than potential acid sulfate soils?
• Which laboratory method set are you using, and do the action criteria shift with the clay content of this profile?
• What is the realistic laboratory turnaround, and does any of it sit on the critical path for lodgement?
• Could a preliminary assessment close this out without a management plan, and what result would it take?
• If a management plan is needed, what area does the treatment pad require, and where would it go on this footprint?
• What validation regime does this council usually accept, and how many rounds of sampling does that imply?
• Would this material qualify for reuse under the applicable state order or exemption, and what would disqualify it?

Ask your geotechnical engineer

• What standing water level did you record, in what season, and how much does it move through the year?
• How much of the excavation sits below that level in each scheme option we are testing?
• What in-situ bulk density should we use to convert volumes to tonnes for the treatment estimate?
• What dewatering method does this profile call for, and what drawdown would it cause beyond the boundary?
• Does the sulfate content change the concrete specification for footings, piles or the basement tanking, and by how much?

Ask your quantity surveyor

• What tonnage and what treatment rate are you carrying, and where did the rate come from?
• What have you allowed for dewatering, water treatment and discharge, and over how many weeks?
• What spoil classification have you assumed, and what does the rate become if it moves one band?
• How much of this is in the measured works and how much is sitting in contingency?

Ask your lawyer or contract administrator

• Does the building contract treat acid sulfate soils as a latent condition, or has the risk been passed to the contractor through the site information?
• What do the draft consent conditions require by way of validation reporting, and who signs it off?
• If the vendor’s reports disclose the soils, what does that do to our position under the contract of sale?

The short version

The mapped class tells you the site may have a problem. The standing water level and the basement depth tell you whether the scheme does. Everything downstream of that, the treatment tonnage, the dewatering duration, the spoil classification and the programme float, follows from the depth decision, and that decision is made at concept design. Getting a borehole in early and asking the water level question before the basement level is fixed is the cheapest intervention available on a site like this.