What Are Acid Sulfate Soils and Why Are They a Problem?
Acid sulfate soils (ASS) are naturally occurring soils and sediments that contain iron sulfides. When they remain waterlogged and undisturbed, they generally present little environmental risk. The problem occurs when these soils are excavated, drained or otherwise exposed to oxygen, causing the sulfides to oxidise and generate sulfuric acid.
The resulting acidity can mobilise metals, affect groundwater and surface water, damage vegetation and aquatic ecosystems, and corrode concrete, steel and other infrastructure. For development and construction projects, the key issue is therefore not simply whether acid sulfate soils may be present — it is whether the proposed works will disturb them or alter the groundwater conditions that keep them stable.
This is why acid sulfate soil assessment is commonly required for excavation, earthworks, drainage, dewatering and infrastructure projects in low-lying coastal areas of NSW.
How Acid Sulfate Soils Form
Acid sulfate soils typically form in waterlogged, low-oxygen environments where iron, sulfate and organic matter are present.
These conditions are particularly common in coastal and estuarine environments such as:
tidal flats;
mangroves and saltmarshes;
coastal floodplains;
backswamps;
estuarine margins; and
low-lying land adjoining rivers, lakes and wetlands.
Under these saturated conditions, iron sulfides — particularly pyrite — can accumulate within the soil or sediment.
While the material remains saturated and oxygen is excluded, the sulfides are generally stable. Problems arise when drainage, excavation, groundwater lowering or other disturbance introduces oxygen into the soil profile.
The sulfides then oxidise and can generate sulfuric acid.
Acid sulfate soils occur throughout coastal NSW and have been identified in every coastal estuary in the state. Significant areas occur within the floodplains of the Hunter, Shoalhaven, Hastings, Macleay, Clarence, Richmond and Tweed river systems.
Potential Acid Sulfate Soils and Actual Acid Sulfate Soils
Acid sulfate soils are commonly discussed as either Potential Acid Sulfate Soils (PASS) or Actual Acid Sulfate Soils (AASS).
Potential Acid Sulfate Soils
Potential Acid Sulfate Soils contain sulfide minerals but have not yet undergone sufficient oxidation to generate significant acidity.
They are commonly found below the groundwater table or within permanently or periodically saturated soils.
PASS can therefore remain relatively benign while undisturbed.
However, excavation, drainage or groundwater lowering can expose the material to oxygen and initiate sulfide oxidation.
Actual Acid Sulfate Soils
Actual Acid Sulfate Soils have already undergone oxidation and contain existing acidity resulting from the oxidation of sulfide-bearing materials.
The NSW Acid Sulfate Soil Risk Map guidance describes actual acid sulfate soils as acidic horizons formed following aeration of sulfide-rich soil materials, while potential acid sulfate soils remain waterlogged and unoxidised.
A site can contain both actual and potential acid sulfate soil materials at different depths within the same soil profile.
Why Are Acid Sulfate Soils a Problem?
The primary environmental risk is the generation and movement of acidity.
When sulfide minerals oxidise, the resulting acidic conditions can change both the chemistry of the soil and the surrounding water environment.
Acidification of Soil and Water
Sulfuric acid generated within disturbed soils can significantly reduce soil pH.
Rainfall, groundwater movement or site drainage can then transport acidic water away from the disturbed area and into nearby drains, wetlands, creeks, rivers or estuaries.
This means the environmental impact may extend beyond the excavation itself.
Mobilisation of Metals
Acidic conditions can increase the solubility of naturally occurring metals such as iron and aluminium.
These metals can then be transported through groundwater or surface water and may reach concentrations that are harmful to aquatic ecosystems.
Impacts on Aquatic Ecosystems
Acidic drainage and mobilised metals can adversely affect fish, oysters and other aquatic organisms.
NSW Government guidance identifies impacts including reduced survival and growth, damage to waterways and aquatic ecosystems, and fish disease and mortality.
Damage to Vegetation
Strongly acidic soil conditions can damage or kill vegetation and alter soil chemistry in ways that make revegetation difficult.
Corrosion of Infrastructure
Acidic soil and groundwater can also affect the built environment.
Sulfuric acid can corrode concrete, iron, steel and some aluminium alloys, making ASS relevant not only to environmental management but also to buried services, foundations and civil infrastructure.
What Activities Can Disturb Acid Sulfate Soils?
Acid sulfate soil risk is closely related to how a project interacts with the soil and groundwater environment.
Common disturbance mechanisms include:
bulk excavation;
basement construction;
trenching and service installation;
road and rail construction;
drainage works;
dredging;
piling and drilling;
excavation for pools, footings and foundations;
land forming and cut-to-fill earthworks;
dewatering; and
other activities that lower the groundwater table.
NSW Government guidance specifically identifies excavation, drainage, infrastructure works and construction as common activities capable of exposing acid sulfate soils to oxygen.
Importantly, direct excavation is not the only risk.
Dewatering can lower groundwater levels beyond the excavation footprint and expose sulfide-bearing soils that would otherwise remain saturated. The resulting oxidation can therefore occur outside the area physically excavated.
Does Acid Sulfate Soil Mapping Mean ASS Is Present?
Not necessarily.
Acid sulfate soil risk maps are predictive tools. They indicate matters such as:
the likelihood of acid sulfate soils occurring;
the approximate depth at which they may occur;
the environmental risk associated with disturbance; and
the landforms commonly associated with ASS.
They do not replace site-specific investigation.
A property may be mapped as having ASS risk while the proposed excavation remains above the relevant soil horizon. Conversely, site-specific conditions may warrant investigation even where the mapped probability is relatively low.
The important question for a project is therefore:
Will the proposed works encounter, expose or otherwise affect acid sulfate soil materials?
That is what an acid sulfate soil assessment is intended to determine.
How Are Acid Sulfate Soils Assessed?
An acid sulfate soil assessment combines an understanding of the proposed development with site-specific soil and environmental information.
The first step is generally to establish:
the proposed excavation depth;
the area and volume of disturbance;
whether groundwater will be lowered;
the site's elevation and geomorphic setting;
mapped ASS risk;
likely soil conditions; and
applicable planning requirements.
Where intrusive investigation is required, boreholes or other sampling locations are advanced through the soil profile to an appropriate depth relative to the proposed disturbance.
Soils may then be assessed using a combination of:
soil-profile observations;
field pH screening;
peroxide reaction testing; and
laboratory analysis using recognised acid sulfate soil methods.
The purpose is not simply to obtain a positive or negative ASS result.
The assessment should determine where acid-generating materials occur, whether the proposed works will disturb them and what management response is required.
National guidance currently covers soil sampling and identification, laboratory methods, groundwater dewatering and other aspects of ASS management and supplements the NSW Acid Sulfate Soil Manual.
What Happens if Acid Sulfate Soils Are Identified?
Finding acid sulfate soils does not necessarily prevent development.
The appropriate response depends on:
the location and depth of the ASS;
the amount of material being disturbed;
the acidity and sulfide characteristics of the soil;
groundwater conditions;
the construction methodology; and
whether the material will be reused, treated or removed from site.
Where possible, the preferred approach is often to avoid or minimise disturbance.
Where disturbance cannot be avoided, an Acid Sulfate Soil Management Plan (ASSMP) may be required.
An ASSMP can establish requirements for:
excavation and soil segregation;
temporary stockpiling;
containment of disturbed material;
neutralisation using suitable materials such as agricultural lime;
treatment rates;
verification sampling;
groundwater and dewatering management;
runoff and leachate control;
reuse or off-site management of excavated material; and
unexpected finds and contingency procedures.
The management approach should be developed around the actual construction methodology and the characteristics of the ASS identified during the investigation.
Acid Sulfate Soils and Development in NSW
Acid sulfate soil requirements in NSW are commonly implemented through Local Environmental Plans and associated acid sulfate soil mapping.
The applicable controls vary according to the location, mapped ASS class or risk, proposed depth of disturbance and nature of the works.
A project may therefore require an ASS assessment where works involve excavation or groundwater lowering within an identified risk area.
The NSW Government advises that local councils' environmental planning instruments specify how acid sulfate soils are to be managed for development and construction.
For developers, builders and project managers, ASS should ideally be considered before the excavation methodology is finalised.
Early assessment can establish whether ASS is actually relevant to the proposed works and, where it is, allow soil treatment, dewatering and management requirements to be incorporated into construction planning.
Acid Sulfate Soil Assessment and Management
The important distinction with acid sulfate soils is that their presence does not automatically constitute a problem.
Risk arises when site activities cause sulfide-bearing materials to oxidise or allow acidic products to migrate into the surrounding environment.
A well-designed ASS assessment therefore focuses on the relationship between:
the soil conditions + the proposed disturbance + the required management outcome.
Where investigation or management is required, Confluence Environmental provides acid sulfate soil assessment, testing, Acid Sulfate Soil Management Plans, construction support and verification across NSW.
Our ASS investigations are developed around the actual depth and nature of the proposed works, with environmental personnel undertaking ASS work having completed specialist training delivered by a Registered Soil Practitioner – Acid Sulfate Soils (RSP-ASS).
For projects involving acid sulfate soil risk, see our Acid Sulfate Soil Assessment and Management services.