5 Soil Remediation Methods for Managing Contaminated Soil
Contaminated soil does not always need to be excavated and sent to landfill.
There are several soil remediation methods that can be used to manage contaminated land, depending on the type and extent of contamination, the proposed land use, site conditions and the risks identified during the contaminated land assessment.
In some cases, contaminated soil may be excavated and removed. In others, it may be treated, contained or safely managed in place.
Selecting the right soil remediation technique is therefore an important part of the remediation planning process.
Below are five common methods used to remediate or manage contaminated soil.
1. Excavation and Off-Site Disposal
Excavation and off-site disposal is one of the most common soil remediation methods.
Contaminated soil is excavated, appropriately classified and transported to a facility that is legally able to receive the material.
This method can be suitable where contamination is:
confined to a relatively small area;
located within a proposed excavation footprint;
unsuitable for treatment or reuse;
associated with asbestos or other contaminants requiring controlled removal; or
likely to interfere with the proposed development.
For example, hydrocarbon-contaminated soil surrounding former fuel infrastructure may be excavated where the affected area is relatively small and removal provides a practical way to eliminate the source.
Following excavation, the area will often require validation to confirm that the remediation objectives have been achieved.
Excavation and disposal can provide a relatively direct solution, but it can also become expensive where large volumes of soil are involved.
Transport, waste classification, disposal fees, imported replacement material and construction delays can all increase the cost of remediation.
For this reason, contaminated soil removal should not automatically be assumed to be the only remediation option.
2. Treatment of Contaminated Soil
Some contaminated soils can be treated rather than removed from the site.
Treatment aims to reduce contaminant concentrations, destroy contaminants or otherwise reduce the risk they present.
A range of soil remediation techniques may be used depending on the contaminants and site conditions.
These can include:
bioremediation;
landfarming;
soil washing;
chemical oxidation or reduction;
thermal treatment; and
other physical or chemical treatment processes.
Bioremediation, for example, may be suitable for some petroleum hydrocarbon contamination.
Naturally occurring microorganisms can be encouraged to break down certain hydrocarbons over time, reducing contaminant concentrations within the soil.
Soil washing is another remediation method that separates contaminants from soil particles through physical or chemical processes.
Treatment can potentially reduce the volume of soil requiring off-site disposal and may provide a more sustainable solution on larger remediation projects.
However, treatment is not appropriate for every contaminant.
Its feasibility depends on factors including soil type, contaminant concentrations, available space, treatment timeframe, remediation objectives and construction program.
3. Stabilisation and Immobilisation
Another method of contaminated soil remediation is stabilisation or immobilisation.
Rather than removing or destroying the contaminant, this approach aims to reduce its mobility or availability within the soil.
The affected material may be mixed with binding agents or other amendments that reduce the ability of contaminants to migrate or leach into the surrounding environment.
This method can be particularly relevant for some forms of inorganic contamination, including certain metals.
Stabilisation may allow contaminated material to remain within an appropriately controlled area of the site rather than being removed and disposed of.
However, the contamination remains present.
The remediation strategy therefore needs to consider the long-term stability of the treated material, potential future disturbance and whether ongoing management measures are required.
4. Capping and Containment
Soil capping remediation involves placing a physical barrier between contaminated material and future site users or environmental receptors.
Rather than removing the contaminated soil, exposure pathways are controlled.
Depending on the proposed development, the cap may consist of:
clean soil;
concrete slabs;
roads;
car parks;
buildings;
engineered pavement; or
a specifically designed containment system.
Capping can be an effective soil remediation method where contamination extends across a large area but can remain safely in place provided exposure is prevented.
For example, contaminated fill beneath a proposed commercial hardstand may sometimes be able to remain beneath the development where the contaminated land assessment demonstrates that the material does not present an unacceptable risk under the proposed land use.
This can significantly reduce the amount of contaminated material requiring excavation and disposal.
However, because the contamination remains on the property, future excavation or redevelopment may disturb the affected material.
Long-term controls, such as an Environmental Management Plan, may therefore be required.
5. In Situ Management
Not every contaminated site requires soil to be physically removed or treated.
In situ soil remediation and management involves addressing contamination while it remains in its existing location.
The appropriate strategy depends on the contaminant and the potential exposure pathways.
In some cases, active in situ treatment may be undertaken. In others, the contamination may remain in place with controls that prevent exposure.
This reflects a fundamental principle of contaminated land assessment.
Risk depends on the relationship between:
Source → Pathway → Receptor
The presence of contamination alone does not automatically mean that soil must be excavated.
For unacceptable risk to occur, there generally needs to be a contaminant source, a pathway through which exposure can occur and a receptor that could be affected.
Where a pathway can be reliably removed or controlled, contamination may sometimes remain in place without presenting an unacceptable risk.
Management measures may include:
maintaining existing pavement or buildings;
restricting excavation within affected areas;
controlling groundwater use;
establishing procedures for future subsurface works;
maintaining containment systems; and
implementing a long-term Environmental Management Plan.
This approach can avoid unnecessary disturbance of contaminated material while still protecting human health and the environment.
How Do You Choose the Right Soil Remediation Method?
There is no single soil remediation technique that is appropriate for every contaminated site.
The most suitable method depends on the findings of the contaminated land investigation and the proposed future use of the property.
Important considerations can include:
contaminant type;
contaminant concentrations;
horizontal and vertical extent of contamination;
soil and geological conditions;
groundwater conditions;
current and proposed land use;
potential exposure pathways;
construction requirements;
waste classification;
availability of treatment areas;
remediation timeframe;
environmental impacts;
worker and community safety; and
overall remediation cost.
Several soil remediation methods may also be used on the same site.
For example, a highly contaminated hotspot may be excavated and removed, while lower-level contamination elsewhere on the site is retained beneath a hardstand or managed through an Environmental Management Plan.
The objective is to develop a remediation strategy that responds to the actual contamination risks rather than unnecessarily removing soil across the entire property.
What Is the Soil Remediation Process?
Before deciding how to remediate contaminated soil, the nature and extent of contamination generally needs to be sufficiently understood.
This may involve a Preliminary Site Investigation, followed where necessary by a Detailed Site Investigation.
These investigations are used to develop and refine the Conceptual Site Model and determine:
what contaminants may be present;
where contamination is located;
how far contamination extends;
what exposure pathways may exist;
who or what could potentially be affected; and
whether remediation or management is required.
Where remediation is necessary, the preferred soil remediation methods are typically documented within a Remedial Action Plan.
The Remedial Action Plan establishes how contamination will be managed, the remediation objectives, environmental and safety controls, and the validation requirements that will apply following the works.
This planning stage is important.
Beginning excavation before contamination has been adequately characterised can result in unexpected waste costs, unnecessary soil disposal or remediation works that fail to address the actual contamination risk.
Does All Contaminated Soil Need to Be Removed?
No.
One of the most important principles of contaminated land remediation is that contamination does not automatically need to be completely removed from a site.
The appropriate response depends on whether the contamination presents an unacceptable risk under the proposed land use.
Where contaminated material can be safely treated, contained or managed in place, excavation and disposal may not be necessary.
For example, contamination beneath a proposed building or pavement may sometimes be able to remain in place where exposure pathways are effectively controlled and the arrangement is appropriate for the long-term use of the site.
Conversely, contamination within proposed garden areas, utility trenches or other locations where people could be exposed may require a different remediation approach.
This is why the remediation strategy should be based on the site's Conceptual Site Model rather than simply the presence or absence of contamination.
What Happens After Soil Remediation?
Once remediation works are complete, the site generally needs to be validated.
Validation provides evidence that the objectives established for the remediation works have been achieved.
Depending on the remediation method, this may include:
inspection of remediated areas;
validation soil sampling;
laboratory analysis;
confirmation of excavation boundaries;
review of waste disposal documentation;
assessment of imported material;
surveying of containment areas; and
preparation of a Site Validation Report.
Where contamination remains on site, additional long-term management measures may also be required.
The final documentation should provide a clear record of what contamination was identified, how it was managed and whether the site is suitable for its intended use.
Soil Remediation Services in NSW
Confluence Environmental provides contaminated land investigation, remediation planning and validation services across NSW.
Our contaminated land services include:
Preliminary Site Investigations;
Detailed Site Investigations;
Remedial Action Plans;
contaminated soil assessment;
remediation supervision;
waste classification;
asbestos in soil assessment;
Environmental Management Plans; and
Site Validation Reports.
Our remediation strategies are developed around the specific contamination risks, proposed development and site conditions, with the aim of identifying practical and proportionate ways to manage contaminated soil.
For projects requiring independent review, contaminated land investigations and remediation documentation can also be prepared for review under the NSW EPA Site Auditor Scheme.
If contaminated soil has been identified on your site, Confluence Environmental can assess the available soil remediation methods and develop an appropriate strategy for the proposed development.