Acid Sulfate Soils NSW: The Excavation And Dewatering Site Test

EXCAVATION COULD EXPOSE MORE THAN ORDINARY SOIL.NSW: check acid sulfate soils before digging or dewatering.

You find a block a few streets back from the water on the NSW coast. The numbers work. Then your planner asks whether you have checked the acid sulfate soils mapping, and whether your excavation will go below the watertable.

That question decides whether this site is straightforward or whether it carries an assessment, a management plan, a disposal problem and a supervision obligation that all sit in front of your consent.

The deal question: is the soil harmless until you dig?

Usually yes, and that is what makes the risk easy to miss. NSW Environment and Heritage explains that acid sulfate soils are natural sediments containing iron sulfides, and that left undisturbed they do not present any risk. Expose them to air and the sulfides react with oxygen to create sulfuric acid, which makes metals such as iron and aluminium more soluble and able to be released in toxic amounts. The same page records that sulfuric acid can corrode concrete, iron, steel and some aluminium alloys.

So the question for your deal is not whether the soil exists. It is what your works do to it.

Where these soils actually sit, and where they do not

They are common along the NSW coast and found in every coastal estuary. Because of their estuarine origin they are usually found at elevations less than one metre above sea level, and they underlie floodplains, levees and backswamps. The published figure is that they may affect more than 260,000 hectares of land. The largest areas are the coastal floodplains of northern New South Wales, particularly the Tweed, Richmond, Clarence, Macleay and Hastings rivers, with the Hunter and Shoalhaven also notable.

The page is equally clear that drainage and excavation of these soils remains a current issue for urban development, infrastructure and sand mining. This is not a historic problem that has been solved.

What triggers the assessment and the management plan

The NSW Acid Sulfate Soils Manual sets out the trigger in two parts. The first is oxidisable sulfur at or above the Action Criteria in Table 4.4, which is a sampling result. The second is groundwater. The manual states that the second principal issue with regard to triggering the need for a management plan and development consent is whether the groundwater level is likely to be lowered and by how much, and that if the concentration meets or exceeds the action criteria then an acid sulfate soils management plan must be prepared and development consent must be obtained from council.

The manual also explains that works which lower groundwater levels include excavation to or below the watertable and dewatering of construction sites. It assigns a figure of 500 metres from Class 1-4 land as the threshold beyond which proposed works in Class 5 land will not trigger the local environmental plan, because the concern in Class 5 is the potential to alter groundwater levels in adjacent higher-risk land.

Read that back into a project. A basement, a semi-basement, a stormwater basin, deep services trenches, a driveway cut, or a subdivision that needs a new drainage line can all be the event that matters.

What it costs, and where the cost hides

Three cost lines appear, and only one of them is obvious.

• Assessment and reporting. Sampling to the required depths, laboratory analysis against the Action Criteria, and a management plan prepared by a qualified consultant.
• Construction method and materials. Corrosion resistance, foundation design and the sequencing of earthworks and drainage can all change once the soil is in scope.
• Disposal. The NSW EPA Waste Classification Guidelines define virgin excavated natural material as natural material that does not contain sulfidic ores or soils. Material that fails that test does not travel as clean fill, so it attracts different handling and a different gate fee.

National guidance through the Australian and New Zealand Guidelines for Fresh and Marine Water Quality records that acid sulfate soil can lead to reduced pH, decreased oxygen concentration in water and the release of heavy metals such as cadmium and lead and metalloids such as arsenic, and that acid and contaminants enter waterways when soils are rewetted. That is why the sequencing of earthworks and the control of water leaving the site are conditions rather than choices.

The trap: assuming the drainage channel is a given

The quiet trap on coastal sites is older drainage and flood mitigation works. Major drainage works were built on floodplains from the Tweed to the Shoalhaven from the late 1800s until the mid-1970s, and they lowered watertables across land that now carries housing. A site can look dry and settled while the material beneath it sits in a permanently lowered watertable that your excavation will disturb further. Existing drains are also the route by which disturbed acid and metals reach a waterway, which brings the drainage design and the receiving environment into the same assessment.

The second trap is the reverse: treating a mapped risk class as a conclusion. Mapping identifies probability and risk; the manual requires sampling to establish whether the Action Criteria are exceeded. Neither the mapping nor a neighbouring approval answers it for your site.

The site test a student would run, in order

1. Check the mapping and the planning instrument. Identify the risk class and read the acid sulfate soils clause in the applicable local environmental plan before you design anything.
2. Establish the excavation profile. How deep, where, and does any part of the works reach or fall below the watertable — including services, drainage and any basin or basement.
3. Ask whether groundwater will be lowered, even indirectly, and by how much. The manual treats this as the second trigger, so answer it explicitly rather than by assumption.
4. Commission sampling to the required depths, designed by a consultant, and interpret the results against the Action Criteria.
5. Price the method, not just the report. Corrosion, foundation design, treatments, disposal classification and gate fees all belong in the feasibility with a contingency.
6. Map the programme. Assessment, management plan, any concurrence or condition, and construction-phase monitoring obligations.

What this does to the offer

Run the feasibility twice: once with the site cleared of acid sulfate soils scope, and once with the assessment, the management plan, the disposal route and the supervision built in. If only the second version works, that second version is your real deal, and the difference belongs in your offer and your contingency rather than in a hope that the condition will not be imposed. Coastal sites are also where a condition can add weeks to a programme, and weeks are holding costs.

Practical next steps

You are not expected to solve this yourself. You are expected to recognise the issue, ask the right questions and brief the right people. A town planner confirms which local environmental plan clause applies to the land and whether the provisions are triggered. A qualified environmental consultant or soil scientist designs the sampling, interprets the results against the Action Criteria and prepares the management plan. A civil or geotechnical engineer confirms the excavation depth, the dewatering design and the drainage implications. A licensed waste contractor confirms the disposal route for anything leaving the site. Do that before you exchange, not after your consent is conditioned.

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