When Does Contaminated Soil Need to Be Removed? Excavation, Disposal and Remediation Decisions

Contaminated soil is identified on a development site.

Does it need to be excavated and taken to landfill?

Sometimes.

But not always.

Contaminated soil removal is one of the most direct forms of site remediation. Where contamination is localised, accessible and unsuitable to remain, excavation can remove the source and provide a clear pathway toward validation and redevelopment.

However, contamination does not automatically make soil waste.

Nor does an exceedance of an investigation criterion automatically mean that the affected material must be excavated and disposed of.

The appropriate decision depends on what contamination is present, the risk it creates, where it occurs, how the land will be used, what construction works are proposed and whether other remediation methods can achieve the required outcome.

The National Environment Protection (Assessment of Site Contamination) Measure — the ASC NEPM — specifically establishes a preferred hierarchy that considers treatment and risk reduction before off-site disposal where practicable. It also recognises containment and management strategies where appropriate.

The question for the remediation consultant is therefore not simply:

“Is this soil contaminated?”

It is:

“Does this soil need to be removed to achieve the remediation objective?”

Contamination Does Not Automatically Mean Excavation

The distinction begins with contaminated land assessment.

Investigation and screening criteria are used to identify contamination requiring further assessment. They are not automatically clean-up levels.

The ASC NEPM expressly warns that using investigation levels as default remediation criteria can result in unnecessary remediation, increased development costs, unnecessary disturbance and unnecessary use of landfill capacity.

That has an important practical consequence.

A soil result above a Health Investigation Level, Health Screening Level or Ecological Investigation Level does not automatically translate into:

Excavate this soil and send it to landfill.

The consultant first needs to determine:

• what the result represents;

• whether the contamination is localised or widespread;

• whether the relevant exposure pathway is complete;

• whether the source remains active;

• whether groundwater or other media are affected;

• how the proposed development changes the exposure conditions;

• whether the material can safely remain; and

• what remediation objective needs to be achieved.

That assessment determines whether contaminated soil removal is necessary.

When Is Excavation a Good Remediation Option?

Excavation can be particularly effective where contamination is relatively well defined and physically accessible.

Examples can include:

• a localised hydrocarbon source;

• impacted soil surrounding a former underground storage tank;

• lead contamination around a former building or drip line;

• asbestos-contaminated fill;

• isolated areas of heavily contaminated industrial fill;

• buried waste;

• contamination within a proposed basement or service excavation; or

• soil that would already need to be removed as part of the development.

In these circumstances, excavation may remove the contaminant source relatively quickly.

It can also provide a clear physical remediation endpoint.

The affected material is removed, the excavation is inspected and validated, and suitable replacement material can be placed where required.

For relatively small contamination volumes, this may be considerably more practical than establishing a treatment system or long-term management arrangement.

Our overview of soil remediation methods discusses excavation alongside treatment, stabilisation, containment and in situ management.

Source Removal Can Be More Important Than Simply Removing Soil

There is an important distinction between soil removal and source removal.

Suppose petroleum hydrocarbons are identified around a leaking underground storage tank.

Excavating some contaminated soil while leaving the leaking tank in place would not resolve the underlying contamination problem.

The primary source needs to be addressed.

Similarly, removing shallow contaminated soil may not provide an effective remediation outcome if contamination has already migrated into groundwater.

The remediation strategy should therefore consider the complete source–pathway–receptor relationship.

For petroleum-impacted sites, that can involve integrating source removal with soil and groundwater remediation.

The objective is not simply to move contaminated material.

It is to interrupt the contamination system responsible for the identified risk.

Development Excavation Can Create an Opportunity for Remediation

Sometimes contaminated soil needs to be excavated for construction regardless of whether environmental remediation alone would have required its removal.

Consider shallow contaminated fill within the footprint of a proposed basement.

If construction requires excavation several metres below the contaminated material, there may be little practical value in designing a containment system for soil that will already be removed.

The contaminated land strategy can instead be integrated with the development excavation.

This may involve:

• identifying affected material before works begin;

• defining segregation requirements;

• classifying soil for disposal;

• controlling excavation;

• managing stockpiles;

• monitoring unexpected finds;

• validating the completed excavation; and

• documenting material movements.

Integrating remediation with construction can reduce duplicated earthworks and unnecessary handling.

The opposite situation also occurs.

Contaminated fill beneath a permanent hardstand area may not need to be excavated where the risk assessment demonstrates that the material can safely remain and appropriate long-term controls are achievable.

The development itself therefore forms an important part of the remediation decision.

This is discussed further in Designing Remediation Around the Proposed Development.

When Can Contaminated Soil Remain On Site?

Removal is not the only method available for managing contaminated soil.

Depending on the contamination and proposed use, material may potentially be:

• treated;

• stabilised;

• capped;

• contained;

• consolidated within a controlled area;

• managed beneath hardstand;

• retained under an Environmental Management Plan; or

• otherwise managed in situ.

These approaches require appropriate technical justification.

Leaving contaminated soil on site simply because contaminated soil disposal is expensive is not a remediation strategy.

The consultant needs to demonstrate that the remaining contamination does not create an unacceptable risk and that any required controls can be maintained over the long term.

Factors can include:

• contaminant mobility;

• contaminant concentration;

• depth;

• affected area;

• groundwater conditions;

• vapour generation;

• accessibility;

• future excavation;

• ecological considerations;

• proposed land use; and

• long-term site management.

The appropriate response should follow the Conceptual Site Model and remediation objectives rather than a predetermined preference for either excavation or retention.

Sometimes Removal Is the Simplest Long-Term Solution

Although excavation can be expensive, avoiding excavation can also introduce cost and complexity.

A containment strategy may require:

• engineered barriers;

• marker layers;

• survey control;

• minimum cover thicknesses;

• restrictions on future excavation;

• Environmental Management Plans;

• ongoing inspection;

• notation within site-management systems; and

• procedures for future workers.

For a small contamination hotspot, decades of long-term management may make little commercial sense if the material can be removed safely and economically during construction.

This is why remediation decisions need to consider whole-of-project consequences.

The cheapest immediate option is not necessarily the most cost-effective long-term solution.

Likewise, excavation is not automatically superior simply because it permanently removes the material from the property.

The appropriate strategy is site specific.

Excavation Does Not Automatically Mean Landfill

Even where soil needs to be excavated, off-site disposal is not necessarily the only destination.

Depending on the material, contaminants and regulatory framework, excavated soil may potentially undergo:

• on-site treatment;

• off-site treatment;

• lawful resource recovery;

• stabilisation;

• controlled on-site relocation; or

• landfill disposal.

Any proposed reuse or recovery pathway needs to comply with the relevant NSW waste framework.

Waste classification itself does not determine that material is suitable for reuse or application to land. The NSW EPA specifically distinguishes waste classification from resource-recovery requirements.

This distinction is important.

A contaminated land consultant may determine that soil can safely remain somewhere within the site from a risk perspective.

That does not automatically mean the same material can be excavated, transported elsewhere and reused without considering waste legislation.

The site-remediation decision and the waste-management decision are related but separate.

Once Soil Leaves the Site, Waste Requirements Become Critical

Where excavated soil is taken off site as waste, it needs to be appropriately classified.

Under the NSW framework, the waste generator is responsible for classifying the waste, and material can only be taken to a facility lawfully authorised to receive that class or type of waste.

Depending on the material, classification may consider whether the waste is:

• special waste;

• hazardous waste;

• restricted solid waste;

• general solid waste; or

• otherwise subject to a specific waste classification pathway.

This is why waste classification should be considered early during projects involving significant contaminated soil disposal.

Waiting until hundreds of tonnes of soil have already been excavated into a stockpile can create unnecessary delays and commercial pressure.

Where practicable, remediation planning should establish:

• anticipated material types;

• likely waste classifications;

• sampling requirements;

• potential receiving facilities;

• stockpile areas;

• truck access; and

• documentation requirements

before bulk excavation begins.

Contaminated Land Criteria and Waste Criteria Are Not the Same

This is another area where confusion can occur.

A contaminated-land criterion answers:

Does this contamination represent an unacceptable risk at this site?

Waste classification answers:

How must this excavated material be managed once it becomes waste?

Those are not the same question.

Soil might require removal because it is unsuitable to remain within a future residential garden, yet still classify within a relatively common waste category for disposal.

Conversely, soil located where the contamination does not create an unacceptable site-specific exposure pathway may nevertheless require appropriate waste classification if it is excavated and taken off site for construction reasons.

The analytical datasets may overlap.

The decision frameworks do not.

This is why remediation criteria should not be confused with disposal thresholds.

For a deeper discussion of this distinction, see Remediation Criteria Are Not Always the Same as Investigation Criteria.

Segregation Can Have a Major Effect on Disposal Volumes

One of the most important practical decisions during contaminated soil excavation is whether different material types can be reliably separated.

Imagine a site containing:

• a localised heavily contaminated hotspot;

• surrounding lower-risk fill; and

• underlying natural soil.

If all three materials are excavated together and mixed into one stockpile, the entire combined volume may need to be assessed and managed as one waste stream.

A relatively small contamination problem can therefore become a much larger contaminated waste disposal problem.

Where the site conditions allow it, segregation can help maintain separate material streams.

This might involve separation based on:

• source area;

• fill type;

• depth;

• visual observations;

• contaminant distribution;

• asbestos occurrence;

• field screening; or

• previously established investigation data.

However, segregation needs a defensible technical basis.

It should not be used to dilute contamination.

The NSW EPA Waste Classification Guidelines expressly state that wastes should not be mixed to reduce contaminant concentrations and encourage separation of different waste classes where safe, practical and appropriate.

Good remediation supervision therefore has both environmental and commercial value.

In Situ Waste Classification Can Help Before Excavation Starts

Where there is sufficient site information, waste classification can sometimes be undertaken while soil remains in situ.

That can provide earlier information about likely disposal pathways and costs.

For example, a defined soil unit proposed for bulk excavation might be investigated and characterised before civil works commence.

This can allow:

• tenderers to understand disposal requirements;

• receiving facilities to be identified;

• disposal allowances to be developed;

• material segregation requirements to be established; and

• programme risks to be reduced.

But in situ classification needs to represent the material that will actually be excavated.

The sampling design, material boundaries and existing analytical information all need to support that conclusion.

If unexpected contamination is encountered during excavation, the classification may need to be reconsidered.

The objective is to establish a representative classification, not simply obtain laboratory results as early as possible.

Stockpile Classification Has Different Advantages and Limitations

Where material has already been excavated, classification can also be performed on stockpiles.

This can provide direct access to the excavated waste stream.

However, it can create logistical problems if classification was not anticipated.

Large stockpiles require:

• physical space;

• appropriate environmental controls;

• representative sampling;

• separation between material types;

• truck access; and

• sufficient time for laboratory analysis and reporting.

On constrained or operational sites, stockpiles can quickly interfere with construction.

This is one reason the waste-management strategy should be developed alongside the remediation strategy.

The consultant should understand what will happen to the soil after the excavator removes it before excavation begins.

Asbestos-Contaminated Soil Requires Particular Management

Asbestos can materially change how excavated soil is handled.

Soil containing asbestos may fall within the NSW special-waste framework, and the asbestos risk also affects excavation controls, worker protection, transport, disposal and validation.

The remediation strategy needs to consider the form and distribution of asbestos, including whether impacts involve:

• isolated bonded ACM fragments;

• widespread bonded ACM;

• asbestos fines;

• fibrous asbestos; or

• mixed asbestos and other contamination.

For localised non-friable asbestos-impacted soil, targeted removal may provide a practical remediation outcome.

For widespread heterogeneous fill, the strategy may require a broader approach.

Simply identifying “asbestos in soil” is not enough to determine the remediation method.

The material distribution, risk profile, proposed development and ability to safely segregate affected soil all matter.

Where excavation occurs, contaminated-land validation and asbestos clearance requirements also need to be understood as related but distinct project requirements.

Petroleum-Contaminated Soil May Have a Groundwater Component

Petroleum-impacted soil also requires caution before assuming excavation will resolve the contamination issue.

Removing a petroleum source can substantially reduce contaminant mass.

But if contamination has reached groundwater, contaminated soil removal may be only one component of the remediation program.

Residual hydrocarbons may remain around the water table.

LNAPL may be present.

Dissolved contamination may have migrated downgradient.

Vapour may also represent a pathway.

The soil excavation should therefore be considered within the broader contamination system.

Our article Integrating Soil and Groundwater Remediation on Petroleum-Impacted Sites examines this in greater detail.

Excavation Boundaries Should Be Technically Defined

Another common mistake is allowing the waste-disposal process to define the remediation boundary.

For example:

“This sample exceeded, therefore excavate five metres around it.”

That may produce a convenient excavation shape.

It does not necessarily have a technical basis.

Remediation extents should follow from:

• the contaminant source;

• contaminant distribution;

• lateral and vertical delineation;

• the CSM;

• remediation objectives;

• remediation criteria; and

• the validation strategy.

The physical excavation may ultimately extend beyond the interpreted contamination boundary for constructability or safety reasons.

But that distinction should be understood.

Contamination extent, remediation extent and excavation footprint are not necessarily identical.

This becomes particularly important when estimating contaminated soil disposal volumes.

Over-Excavation Can Be Expensive

Excavating soil beyond what is required can have a substantial cost.

Each additional cubic metre may involve:

• excavation;

• loading;

• waste classification;

• transport;

• landfill charges;

• replacement material;

• placement;

• compaction;

• additional validation; and

• programme impacts.

This means poor delineation can directly affect remediation cost.

If the contamination extent is highly uncertain, the contractor may need to carry a larger allowance.

If clean and contaminated materials are unnecessarily mixed, disposal volumes may increase.

If validation is poorly planned, excavation may need to remain open while additional testing occurs.

Good remediation design therefore has a commercial function as well as an environmental one.

The objective is not to minimise excavation regardless of risk.

It is to ensure the excavation is proportionate to the remediation objective.

Under-Excavation Creates a Different Problem

Trying too hard to minimise contaminated soil removal can create the opposite risk.

If contamination remains above the established remediation criteria or an identified source is incompletely removed, the project may fail validation.

That can result in:

• further mobilisation;

• reopening excavations;

• additional laboratory testing;

• removal of completed backfill;

• programme delays;

• revised reporting; and

• loss of confidence in the remediation process.

The appropriate remediation extent therefore needs enough conservatism to account for investigation uncertainty without simply assuming that every potentially affected area must be removed.

This is where professional judgement and a well-developed RAP become important.

Validation Should Be Planned Before Excavation

If contaminated soil is being removed as part of remediation, the project should know in advance how successful removal will be demonstrated.

The NSW EPA contaminated-land reporting guidance requires remediation works to be validated against the objectives and criteria established in the RAP and expects the validation report to document the works, results and final condition of the site.

For excavation, validation may involve:

• inspection of excavation surfaces;

• soil sampling from walls and bases;

• assessment of residual soil;

• confirmation of physical source removal;

• survey information;

• groundwater assessment where relevant;

• waste disposal records; and

• documentation of unexpected finds.

The validation strategy should inform the excavation sequence.

An excavation should not normally be backfilled simply because the contractor believes all affected material has been removed.

The evidence needed to demonstrate completion should be collected while the excavation remains accessible.

Waste Dockets Are Part of the Remediation Evidence

Where contaminated soil disposal forms part of remediation, the final validation record is not limited to laboratory results.

The project should also be able to demonstrate where the excavated material went.

The NSW contaminated-land framework expects documentary evidence confirming that contaminated soil removed for off-site disposal or reuse was dealt with in accordance with the relevant requirements.

Depending on the project, records can include:

• waste classification reports;

• landfill or facility acceptance;

• weighbridge dockets;

• transport records;

• disposal receipts;

• material tracking records; and

• photographs or site diaries.

These documents establish the chain between contaminated material leaving the excavation and its lawful destination.

For large remediation projects, material tracking can become an important component of the overall validation dataset.

Operational Sites Require an Additional Layer of Planning

Contaminated soil removal can become more difficult where the facility needs to remain operational.

Excavation may need to occur around:

• traffic;

• customers;

• operational buildings;

• services;

• workers;

• production activities; or

• critical access routes.

Stockpile space may also be limited.

In these circumstances, removal may need to be staged, with classification and disposal closely coordinated to minimise the amount of excavated material sitting on site.

Our article on remediation at operational sites explores these project constraints in more detail.

The Disposal Cost Should Not Determine the Risk Conclusion

Landfill disposal can represent a major proportion of remediation cost.

That inevitably creates pressure to reduce disposal volumes.

Reducing unnecessary disposal is a legitimate project objective.

Changing the technical interpretation simply because disposal is expensive is not.

The process should remain:

Characterise the contamination

↓

Assess the risk

↓

Define the remediation objective

↓

Select the remediation method

↓

Determine what material actually needs to be removed

↓

Classify and manage excavated waste appropriately

Cost and sustainability are relevant to selecting between technically appropriate options, but they do not replace the risk assessment.

Likewise, the ASC NEPM explicitly recognises practicality, cost, effectiveness and sustainability as considerations in site-management decisions while cautioning against unnecessary remediation.

Removal Is One Tool Within the Remediation Strategy

Contaminated soil removal remains one of the most useful remediation methods available.

It can:

• remove contaminant mass;

• eliminate localised sources;

• simplify redevelopment;

• reduce long-term management;

• resolve asbestos-contaminated fill;

• integrate with construction excavation; and

• provide a readily verifiable remediation outcome.

But excavation should be selected because it solves the contamination problem — not simply because contaminated soil has been identified.

For some sites, treatment may be more appropriate.

For others, containment can achieve the required risk-management outcome.

For petroleum sites, source removal may need to be combined with groundwater remediation.

For operational facilities, a staged strategy may reduce disruption.

For widespread contaminated fill, unnecessary disposal can significantly increase project cost without providing a proportionate environmental benefit.

The decision needs to follow the site.

Contaminated Soil Removal, Disposal and Validation in NSW

Confluence Environmental provides soil and groundwater remediation and validation services for development, infrastructure, industrial and government projects across NSW.

Our contaminated-land team can support projects through:

• Detailed Site Investigation;

• remediation planning;

• Remediation Action Plans;

• contaminated soil delineation;

• contaminated soil removal;

• remediation supervision;

• soil segregation and material management;

• asbestos-contaminated soil management;

• waste classification;

• contaminated soil disposal planning;

• validation sampling; and

• Site Validation Reports.

Where soil needs to be removed, our approach is to establish a clear technical basis for the excavation, integrate waste requirements with the remediation program and collect the evidence required to validate the completed works.

Where removal is not necessary, we can assess whether treatment, containment or another risk-based remediation method provides a more appropriate project outcome.

The objective is not to excavate the greatest possible volume of soil.

It is to manage the contamination effectively, lawfully and in a way that allows the site to progress.