When Is an Acid Sulfate Soil Assessment Required in NSW?

An acid sulfate soil assessment is generally required in NSW where proposed excavation, earthworks, drainage or groundwater lowering could disturb acid sulfate soils identified by the applicable planning controls, or where Council or another approval authority requires the risk to be investigated.

Whether an assessment is required depends primarily on:

• the Acid Sulfate Soil class or risk mapping applying to the land;

• the depth and extent of the proposed disturbance;

• whether the works could lower the groundwater table;

• the site's environmental setting; and

• the requirements of the applicable Local Environmental Plan, development consent or other approval.

The purpose of the assessment is to determine whether the proposed works are actually likely to encounter or affect acid sulfate soils and, if so, whether an Acid Sulfate Soil Management Plan (ASSMP) or other management measures are required.

A mapped acid sulfate soil risk does not automatically mean that acid sulfate soils will be encountered or that treatment will be necessary.

Acid Sulfate Soil Requirements in NSW Planning

Acid sulfate soil controls in NSW are generally implemented through Local Environmental Plans (LEPs) and associated Acid Sulfate Soil Maps.

The objective of these provisions is to prevent development from disturbing, exposing or draining acid sulfate soils in a way that causes environmental harm. For example, the acid sulfate soil clause in the Maitland Local Environmental Plan requires consent for specified works according to the mapped ASS class and provides a pathway for a preliminary assessment to demonstrate that an ASS Management Plan is not required.

The exact clause and mapping applying to a property should therefore be checked against the relevant Council LEP rather than assuming the same requirement applies uniformly across NSW.

NSW Government guidance also notes that Councils use local environmental planning instruments to specify how acid sulfate soils are to be managed for development and construction.

How Acid Sulfate Soil Classes Affect Development

Many NSW LEPs use a Class 1 to Class 5 Acid Sulfate Soil mapping framework.

The classes broadly reflect the likelihood that particular types of works could disturb acid sulfate soils or alter groundwater conditions.

Using the common model-style LEP provisions as an example:

Class 1 Acid Sulfate Soils

Class 1 land generally has the highest sensitivity to disturbance.

Under the model-style provisions, any works may trigger the acid sulfate soil provisions.

This commonly applies to very low-lying estuarine and tidal environments where ASS may occur close to the ground surface.

Class 2 Acid Sulfate Soils

Class 2 controls commonly apply to:

• works below the natural ground surface; and

• works that are likely to lower the groundwater table.

This means even relatively shallow excavation can be relevant.

Class 3 Acid Sulfate Soils

Class 3 provisions commonly apply to:

• works more than approximately 1 metre below natural ground level; or

• works that lower the groundwater table by more than approximately 1 metre.

Class 4 Acid Sulfate Soils

Class 4 provisions commonly apply to:

• works more than approximately 2 metres below natural ground level; or

• works that lower the groundwater table by more than approximately 2 metres.

Class 5 Acid Sulfate Soils

Class 5 land is generally treated differently.

The issue is commonly whether works on the Class 5 land could lower the groundwater table in nearby Class 1–4 land, particularly low-lying land where ASS is more likely to occur.

These thresholds are reflected in commonly adopted NSW LEP provisions, but the applicable LEP should always be checked for the individual site.

Does Being on an Acid Sulfate Soil Map Mean I Need Testing?

Not automatically.

The mapping is a risk-screening tool.

NSW Acid Sulfate Soil Risk Maps predict matters including:

• the likelihood of ASS occurring;

• the approximate depth at which it may occur;

• the environmental risk associated with disturbance; and

• the types of landforms associated with ASS.

They do not confirm the actual soil conditions beneath an individual property.

The more important question is:

Will the proposed works interact with the soil or groundwater conditions that create the ASS risk?

For example, a property may be mapped as Class 4, but a proposed shallow footing excavation may remain well above the depth at which ASS could reasonably occur.

In another case, a basement, pool, service trench or dewatering system may extend into the relevant soil profile and require site-specific investigation.

This is where a preliminary ASS assessment becomes important.

When Can a Preliminary Assessment Be Enough?

In some circumstances, the applicable LEP allows a preliminary assessment to demonstrate that an Acid Sulfate Soil Management Plan is not required.

For example, the Maitland LEP provides that development consent under the ASS clause is not required where a preliminary assessment prepared in accordance with the Acid Sulfate Soils Manual indicates that an ASSMP is unnecessary and the consent authority confirms that assessment.

The preliminary assessment may consider matters such as:

• mapped ASS class;

• proposed excavation depth;

• earthworks volumes;

• site elevation;

• geology and geomorphology;

• expected groundwater conditions;

• previous site investigations;

• construction methodology; and

• whether groundwater lowering is proposed.

Where those factors demonstrate that the proposed works are unlikely to encounter or affect ASS, intrusive testing may not always be necessary.

Where uncertainty remains, however, soil investigation may be required.

When Is Acid Sulfate Soil Testing Required?

Site-specific testing is generally appropriate where the available information cannot reliably determine whether the proposed works will encounter acid sulfate soils.

This may include situations where:

• excavation extends into the mapped or expected ASS horizon;

• the proposed development involves substantial earthworks;

• the site is low-lying and close to an estuary, river, wetland or coastal floodplain;

• groundwater will be lowered;

• previous investigations are incomplete;

• field conditions are inconsistent with the available mapping;

• Council specifically requests ASS investigation; or

• an ASS Management Plan requires confirmation of soil conditions.

Testing commonly involves advancing boreholes or other investigation points through the proposed disturbance zone and collecting samples at appropriate depth intervals.

The sampling program should be developed around the depth and volume of the proposed works, rather than simply collecting an arbitrary number of samples.

Excavation Depth Is Critical

One of the most important factors in determining whether ASS assessment is required is the depth of disturbance.

This is because acid sulfate soils often occur below the surface within saturated soil horizons.

A development involving only shallow excavation may present little ASS risk even where the property is mapped.

By contrast, works such as:

• swimming pools;

• basements;

• deep footings;

• sewer and drainage trenches;

• service installations;

• bored or piled foundations; and

• civil earthworks

may extend into deeper soil horizons where ASS is more likely to occur.

An effective assessment therefore considers not simply the property boundary, but the actual vertical and horizontal extent of the proposed works.

Dewatering Can Trigger ASS Risk Without Excavating the Soil

Excavation is not the only activity that can create an ASS issue.

Lowering the groundwater table can expose previously saturated sulfide-bearing soils to oxygen, potentially initiating oxidation even outside the immediate excavation footprint.

This can occur during:

• basement dewatering;

• deep excavations;

• trench dewatering;

• groundwater pumping;

• drainage installation; and

• other works that alter groundwater levels.

NSW Government guidance specifically identifies drainage and excavation as key mechanisms by which ASS can be exposed, and national ASS guidance includes dedicated provisions for groundwater dewatering.

Projects involving dewatering should therefore consider both the excavated materials and the potential zone of groundwater drawdown.

Common Development Projects That May Require ASS Assessment

Acid sulfate soil assessment is commonly relevant to:

• new residential dwellings;

• alterations and additions;

• swimming pools;

• subdivisions;

• basements;

• apartment developments;

• industrial and commercial developments;

• roads and civil infrastructure;

• utility trenches;

• stormwater and drainage works;

• sewer infrastructure;

• piling and drilling;

• dredging and foreshore works;

• jetties and marine infrastructure; and

• projects requiring groundwater dewatering.

Whether an assessment is actually required depends on the site-specific combination of mapping, excavation depth, groundwater interaction and planning controls.

The scale of the development alone is not the deciding factor.

A relatively small residential excavation can require assessment if it extends into a relevant ASS horizon, while a much larger project may avoid significant ASS disturbance through its design.

Acid Sulfate Soils and Development Applications

ASS assessment is frequently undertaken as part of a Development Application where the applicable LEP identifies an ASS constraint relevant to the proposed works.

Council may require sufficient information to determine:

• whether ASS is likely to be disturbed;

• whether groundwater will be affected;

• whether an ASSMP is required;

• whether soil treatment may be necessary; and

• whether the proposed works can be appropriately managed.

For planning proposals involving land with a probability of containing ASS, NSW Local Planning Direction 4.5 also requires planning authorities to consider acid sulfate soil guidance and, for certain intensification proposals, an ASS study addressing the suitability of the proposed change in land use. Planning NSW

The appropriate level of assessment therefore depends on the planning stage as well as the physical works.

What Happens During an Acid Sulfate Soil Assessment?

Where site-specific investigation is required, the assessment generally begins with review of:

• the applicable LEP and ASS map;

• project plans and excavation levels;

• site topography and elevation;

• geology and soil landscape;

• groundwater conditions; and

• previous ASS or geotechnical investigations.

An intrusive investigation may then be designed around the proposed disturbance.

Depending on the project, this can involve:

• hand augering or drilling;

• soil-profile logging;

• field pH and peroxide screening;

• laboratory analysis using recognised ASS methods;

• interpretation of acid-generating characteristics; and

• assessment against relevant action criteria.

The investigation should ultimately determine:

Is ASS present within the zone affected by the proposed works, and if so, what management is required?

What Happens if Acid Sulfate Soils Are Identified?

Identification of ASS does not automatically prevent development.

Where disturbance cannot be avoided, an Acid Sulfate Soil Management Plan may be prepared to establish how the material will be handled and how environmental impacts will be prevented.

Depending on the project, the ASSMP may address:

• excavation and segregation;

• temporary stockpiling;

• treatment and neutralisation;

• lime application rates;

• verification sampling;

• material reuse;

• off-site disposal;

• groundwater and dewatering;

• runoff and leachate;

• monitoring;

• contingency procedures; and

• construction records.

In other cases, the investigation may show that ASS occurs below the proposed excavation depth and therefore does not require management for the currently proposed works.

This distinction is important: the purpose of assessment is to establish the actual project risk, not to assume that every mapped site requires treatment.

What if Excavation Has Already Occurred?

ASS assessment can also be required retrospectively where excavation has occurred without the necessary investigation or management documentation.

In that situation, the consultant may need to consider:

• the depth and extent of the completed excavation;

• remaining soil conditions;

• ASS characteristics at comparable depths;

• whether excavated material remains on site;

• whether material has been reused or removed;

• potential evidence of oxidation;

• groundwater conditions; and

• whether further management is required.

Retrospective investigation can sometimes provide the information needed by a certifier, Council or project team, but it is generally preferable to assess ASS before disturbance occurs so that appropriate controls can be incorporated into the works.

The Key Question: Will the Proposed Works Disturb ASS?

The presence of an ASS planning constraint does not necessarily mean a development has a significant acid sulfate soil problem.

The key question is whether the proposed excavation, earthworks or groundwater changes are capable of disturbing acid sulfate soils.

A proportionate assessment should establish:

where the risk occurs + whether the works interact with it + what management response is actually required.

Where further assessment is required, Confluence Environmental undertakes acid sulfate soil assessment, testing, Acid Sulfate Soil Management Plans, construction support and verification across NSW.

Our ASS investigations are developed around the proposed disturbance rather than applying the same investigation program to every property.