Acid sulfate soil testing is often triggered when excavation, dewatering or other earthworks are proposed within an area mapped as having potential acid sulfate soils.
But testing acid sulfate soils is not as simple as drilling a few holes, measuring pH and deciding whether a site “passes” or “fails”.
A properly designed investigation needs to answer several different questions:
What soil will actually be disturbed? How deep will excavation extend? Will groundwater levels change? Does the soil contain reduced inorganic sulfur capable of generating acidity? Is existing acidity actually associated with acid sulfate soil processes? And, if acid sulfate soils are present, will the proposed disturbance create an environmental risk that needs to be managed?
These distinctions can materially change both the investigation scope and what needs to happen during construction.
Here are five things worth understanding before acid sulfate soil testing begins.
1. Acid Sulfate Soil Mapping Is a Starting Point — Not the Answer
Being located within an acid sulfate soil mapped area does not confirm that acid sulfate soils are present beneath a particular excavation.
The investigation should start by developing an understanding of the site's geology, geomorphology, elevation, soil landscape, topography and surrounding environment.
The 2018 National Acid Sulfate Soil Guidance adopts this type of ground-model approach. The training material used by Confluence identifies review of acid sulfate soil mapping, geological information, soil landscapes, topography and aerial photography as important preliminary steps, ideally supported by a site inspection before intrusive investigation begins.
This matters because acid sulfate soils are associated with particular depositional environments.
In coastal NSW, they are commonly associated with low-lying estuarine, floodplain and coastal sediments. However, the broader national understanding of acid sulfate soils now also recognises their occurrence in other environments.
The consultant therefore needs to ask more than:
“What ASS class is the property?”
The better questions are:
What geological materials are likely beneath the site?
At what elevation and depth might susceptible sediments occur?
What does the soil profile actually look like?
How does that relate to the proposed excavation?
A mapped risk area may warrant investigation, but the site-specific soil profile and proposed disturbance ultimately determine the project risk.
What the soil looks like matters too
Stratigraphy is an important part of acid sulfate soil interpretation.
Dark grey or black estuarine clays and sands may be consistent with sulfidic materials, whereas red, orange and brown residual soils may indicate a different geological origin.
That does not mean colour alone can identify acid sulfate soils. It means the field observations need to make sense when considered together with the geology, elevation, field screening and laboratory results.
One of the key lessons from the ASS training is that the results should be interpreted against a ground model, rather than treating each laboratory result in isolation.
2. The Investigation Needs to Follow the Excavation — and the Groundwater
One of the most important things to establish before acid sulfate soil testing is exactly what the development proposes to disturb.
That means understanding:
excavation depth;
excavation area;
approximate soil volume;
whether the works are localised or linear;
whether groundwater will be encountered; and
whether groundwater levels will be lowered.
The National Guidance approach presented in the training recommends that, where groundwater levels will not change, the investigation generally extend approximately 1 metre below the proposed depth of disturbance.
Where the works will lower groundwater, investigation should extend approximately 1 metre below the estimated final groundwater drawdown level.
That distinction is important.
Excavation is not the only way acid sulfate soils can be disturbed
Potential acid sulfate soils contain reduced inorganic sulfur that has not yet fully oxidised.
Excavation exposes that soil directly to oxygen.
But lowering the groundwater table can have a similar effect by exposing previously saturated sulfidic materials without physically excavating them.
A basement excavation might therefore affect a deeper soil profile than the excavation footprint alone suggests if substantial dewatering is required.
The ASS training specifically identifies groundwater drawdown, the zone of influence and hydrogeological pathways as important considerations when assessing dewatering in acid sulfate soil environments.
Sampling frequency also matters
Acid sulfate soils can occur in relatively discrete layers.
The training adopts a nominal vertical sampling frequency of one sample every 0.25 metres, with closer sampling where thin layers need to be characterised. Preliminary investigations may use wider intervals depending on the assessment purpose.
Spatial sampling also scales with the investigation.
The National Guidance comparison presented in the training uses increasing numbers of investigation locations as the disturbed area or material volume increases, and different rates for linear works such as pipelines and trenches.
The practical point is simple:
A pool excavation, a 4-metre basement and a 500-metre service trench should not all receive the same investigation design.
The sampling should reflect the actual disturbance.
3. Field pH and Peroxide Testing Are Useful — But They Can Mislead You
Field screening is an important component of acid sulfate soil investigation.
Two commonly recorded parameters are:
pHF — the field pH of the soil; and
pHFOX — the pH following peroxide oxidation of the sample.
Peroxide testing can help identify materials that may contain reduced sulfur capable of oxidising and generating acidity.
However, these tests are screening tools.
They do not conclusively determine whether a soil is an acid sulfate soil, and they should not be interpreted in isolation.
The National Guidance cited in the training stresses that field screening should be undertaken as soon as practical after sample collection because soil pH can change materially during storage.
More importantly, both false positives and false negatives can occur.
A low pHFOX does not automatically mean PASS
A pHFOX below approximately 3.5 may appear to indicate reduced inorganic sulfur.
But the training provides examples where peroxide testing produced a strong pH response despite laboratory analysis identifying no measurable reduced inorganic sulfur.
One potential mechanism is the reaction of iron oxides with hydrogen peroxide, producing iron hydrolysis and a reduction in pH.
A vigorous reaction or elevated temperature during peroxide testing can therefore look convincing while still being unrelated to pyrite oxidation.
A high pHFOX does not necessarily rule PASS out either
The opposite can also occur.
A soil containing substantial carbonate material may buffer the acidity produced during peroxide oxidation.
The pHFOX can therefore remain relatively high even though laboratory analysis identifies reduced inorganic sulfur above the applicable action criterion.
The training identifies carbonate buffering and, in some circumstances, elevated salinity as potential causes of these false-negative field screening results.
This is why ASS investigations should not be reduced to:
pHFOX below 3.5 = PASS
pHFOX above 3.5 = no PASS
The consultant needs to consider the soil description, geology, elevation, field response and laboratory data together.
4. Laboratory Testing Is About Much More Than Soil pH
One of the most important differences between acid sulfate soil assessment and routine soil testing is that the laboratory investigation is not simply measuring whether the soil is currently acidic.
Potential acid sulfate soil can have a relatively neutral or even alkaline pH while still containing substantial reduced inorganic sulfur (RIS).
If that material is subsequently excavated or drained, oxidation can generate acidity.
The laboratory assessment therefore needs to distinguish between several different components of the soil's acidity hazard.
Chromium Reducible Sulfur — Scr
Chromium Reducible Sulfur (Scr) measures reduced inorganic sulfur within the sample.
The method captures sulfur associated with materials such as pyrite and other iron disulfides, elemental sulfur and acid volatile sulfides.
It therefore provides a measure of the soil's potential sulfidic acidity — the acidity that could be generated if reduced sulfur is oxidised.
This is one reason Scr is such an important parameter.
A soil may currently have a neutral pH but still contain sufficient reduced sulfur to create an acid-generating risk after disturbance.
Titratable Actual Acidity — TAA
Titratable Actual Acidity (TAA) measures readily available soluble and exchangeable acidity.
It is therefore relevant to acidity that is already present within the soil.
However, TAA needs to be interpreted carefully because the measured acidity can include contributions from sources other than oxidation of reduced inorganic sulfur.
That leads to an important point.
Acidic Soil Is Not Automatically Acid Sulfate Soil
A low soil pH does not, by itself, demonstrate that acid sulfate soil processes are responsible.
The training identifies several other potential causes of acidic soil, including:
organic acids;
organic sulfur;
iron and aluminium hydrolysis; and
fertiliser inputs.
For a soil to be interpreted as acid sulfate soil, the consultant should look for evidence connecting the acidity to reduced inorganic sulfur or its oxidation products, together with supporting geological and field evidence.
This is why simply identifying a high TAA result or low pH and calling the material “acid sulfate soil” can be misleading.
Retained acidity — SNAS
Some acidity may be stored within poorly soluble iron and aluminium hydroxy-sulfate minerals such as jarosite.
This retained acidity is assessed using Net Acid Soluble Sulfur, or SNAS.
The training notes that retained acidity should be considered where pHKCl is below 4.5 or where jarosite has been identified.
Acid Neutralising Capacity — ANC
Soils may also contain naturally occurring materials capable of buffering acidity.
This is measured as Acid Neutralising Capacity (ANC).
However, ANC needs particularly careful interpretation.
The 2018 National Guidance presented during the training recommends that measured ANC should not automatically be deducted from net acidity.
Its effectiveness needs to be corroborated — for example through incubation testing — before relying on it to demonstrate that the soil will provide effective long-term buffering.
That matters because a laboratory result can otherwise make a highly sulfidic soil appear to have little net acidity simply because a large calculated ANC value has been subtracted.
What about SPOCAS?
The training also identifies limitations with the sPOCAS analytical method, particularly in soils containing organic matter.
The 2018 National Guidance presented in the workshop recommends limiting its use to soils with less than approximately 0.6% organic matter, because organic material can lead to overestimation of oxidisable sulfur.
For modern ASS interpretation, the Chromium Reducible Sulfur method is therefore particularly important.
What Should You Look for in an ASS Laboratory Report?
The training identifies six useful parameters when interpreting an acid sulfate soil laboratory report:
Total Actual Acidity;
Chromium Reducible Sulfur;
retained acidity;
Acid Neutralising Capacity;
net acidity calculated with ANC; and
net acidity calculated without ANC.
These values form part of the acid-base account.
Conceptually:
Net acidity = actual acidity + potential acidity + retained acidity
with ANC only deducted where there is adequate evidence that it represents effective buffering.
The results then need to be considered against applicable action criteria, which vary according to factors including soil texture and the quantity of material being disturbed.
This is another reason why there is not one universal sulfur concentration that determines the outcome of every ASS investigation.
Fine-grained clayey soil, coarse sandy soil and different excavation volumes may have different applicable criteria.
5. Finding Acid Sulfate Soils Does Not Automatically Mean the Soil Must Be Disposed Of
A laboratory result above an ASS action criterion does not necessarily mean the excavation cannot proceed.
And it does not automatically mean the soil needs to be transported to landfill.
Instead, the results help determine how the material should be managed during disturbance.
That may include:
minimising the volume disturbed;
separating ASS from unaffected materials;
controlled excavation;
short-term stockpile management;
preventing uncontrolled oxidation;
treatment using an appropriate neutralising agent;
managing runoff and leachate;
groundwater management;
verification testing following treatment; and
contingency measures if unexpected conditions are encountered.
Where management is required, these measures are generally documented in an Acid Sulfate Soil Management Plan (ASSMP).
The training identifies several core components of an ASSMP, including the environmental setting, vertical and horizontal extent of ASS, proposed excavation depth, field and laboratory results, treatment strategy, construction sequencing, monitoring, validation, groundwater and surface-water considerations, contingency measures and lime-rate calculations.
Laboratory results can determine the treatment rate
Where neutralisation is proposed, the acid-base account can be used to calculate a theoretical liming rate.
The training approach calculates lime demand from net acidity while applying a factor of safety and accounting for the neutralising value of the treatment material.
Importantly, the recommended calculation is based on net acidity without ANC unless the effectiveness of the ANC has been independently corroborated.
So an ASS laboratory report is not merely confirming whether acid sulfate soils exist.
It can directly inform:
how much material requires management, where it occurs, how it should be treated and how treatment will ultimately be verified.
What Does an Acid Sulfate Soil Investigation Actually Look Like?
A good investigation generally starts before the drilling rig or hand auger arrives.
The consultant first needs to understand:
the development;
disturbance depth;
excavation footprint;
estimated material volume;
groundwater conditions;
potential dewatering;
ASS risk mapping;
geology and geomorphology; and
the likely soil profile.
The field investigation is then developed around that conceptual ground model.
Soil is logged with attention to stratigraphy, colour, texture, moisture conditions, shells, organic material, jarosite and other relevant features.
Field pH and peroxide screening are undertaken throughout the profile, with laboratory samples selected to characterise the materials of interest.
Laboratory data are then incorporated back into the soil profile rather than being considered as isolated numbers.
The approach presented during the ASS training can be summarised as:
build the ground model → understand the stratigraphy → add field screening and laboratory data → assess the acidity hazard → develop management requirements around the proposed development.
That is a much more reliable approach than simply comparing a handful of laboratory numbers against a single threshold.
When Should Acid Sulfate Soil Testing Be Undertaken?
Ideally, the assessment should occur early enough that the findings can influence excavation planning.
This can be particularly important for:
basements;
swimming pools;
sewer and stormwater trenches;
road and infrastructure projects;
subdivisions;
deep footings;
drainage works;
bulk earthworks;
foreshore developments; and
projects involving dewatering.
Early investigation allows the project team to determine whether ASS management needs to be incorporated into the earthworks methodology before excavation begins.
If treatment is required, the project may need space for a treatment area, stockpile controls, lime supply, runoff management and verification testing.
Those requirements are much easier to accommodate during planning than after sulfidic material has already been excavated.
What Information Should You Give Your Consultant?
Before requesting acid sulfate soil testing, provide as much information as possible about the proposed works.
Useful information includes:
site address;
development plans;
proposed excavation depth;
approximate excavation footprint;
estimated excavation volume;
proposed basement, pool or trench depths;
whether groundwater is expected;
whether temporary or permanent dewatering is proposed;
any existing geotechnical information;
previous ASS investigations;
proposed reuse or disposal of excavated soil; and
the construction program where known.
The better the consultant understands the proposed disturbance, the more appropriately the investigation can be designed.
Frequently Asked Questions (FAQ):
Q: Does being mapped for acid sulfate soils mean ASS is definitely present?
No. Mapping indicates the potential for acid sulfate soils based on factors such as geology, geomorphology, elevation and landscape setting. Site-specific investigation may still be required to determine whether relevant ASS materials occur within the proposed disturbance zone.
Q: How deep should acid sulfate soil testing go?
The investigation should be designed around the proposed disturbance. The approach presented in the National Guidance training is generally to investigate approximately 1 metre below the proposed excavation depth where groundwater levels will remain unchanged, or approximately 1 metre below the predicted groundwater drawdown level where dewatering will occur.
Q: Can pHFOX confirm whether soil is PASS?
No. Peroxide testing is a useful screening technique, but both false-positive and false-negative results can occur. Laboratory testing and interpretation of the broader soil profile are required to determine whether reduced inorganic sulfur is present.
Q: What is Chromium Reducible Sulfur?
Chromium Reducible Sulfur, or Scr, measures reduced inorganic sulfur within the sample and provides an indication of potential sulfidic acidity that may be generated following oxidation.
Q: Does low soil pH mean the soil is acid sulfate soil?
Not necessarily. Acidic soil can result from processes unrelated to oxidation of reduced inorganic sulfur. ASS interpretation requires consideration of RIS, oxidation products, laboratory results, geology, stratigraphy and other supporting evidence.
Q: Does acid sulfate soil always require lime treatment?
No. The management response depends on the soil characteristics, acidity hazard, quantity disturbed, development methodology and receiving environment. Some projects may avoid disturbance, while others require controlled treatment and verification through an ASSMP.
Q: Does acid sulfate soil have to be disposed of off site?
Not necessarily. Depending on the circumstances, ASS may be capable of being appropriately treated and managed on site. The management approach needs to be determined from the investigation results and project requirements.
Acid Sulfate Soil Testing and Assessment in NSW
Confluence Environmental undertakes acid sulfate soil testing, investigation and management planning across NSW for residential, commercial, civil and infrastructure projects.
Our investigations are designed around the proposed development rather than applying a generic sampling program. Assessment may include desktop review, soil profiling, field screening, laboratory analysis, interpretation of acid-base accounting, groundwater considerations and preparation of an Acid Sulfate Soil Management Plan where required.
Where excavation or dewatering is proposed in an acid sulfate soil mapped area, provide the site address, proposed excavation depth and development plans and we can determine an appropriate investigation scope.