Remediation Criteria Are Not Always the Same as Investigation Criteria
A soil result exceeds a Health Investigation Level.
Does that concentration automatically become the target that remediation must achieve?
Not necessarily.
This is an important distinction in contaminated land assessment and remediation.
Investigation criteria are primarily used to identify whether contamination warrants further assessment. Remediation criteria define the conditions that need to be achieved through remediation or management.
Those two functions are related, but they are not the same.
The National Environment Protection (Assessment of Site Contamination) Measure — the ASC NEPM — specifically states that investigation and screening levels are not clean-up or response levels. Their purpose is to identify concentrations above which further investigation and evaluation may be required.
The ASC NEPM also cautions that automatically using investigation and screening levels as remediation criteria can result in unnecessary remediation, unnecessary disturbance, increased development costs and unnecessary consumption of landfill capacity.
For a consultant preparing a Remediation Action Plan, the question is therefore not simply:
“Which investigation criterion was exceeded?”
The more important question is:
“What remediation outcome is required to make this site suitable for its intended use, and how will we demonstrate that outcome?”
That is the basis on which remediation criteria should be developed.
What Are Investigation Criteria?
Investigation criteria are used during contaminated land assessment to determine whether contaminant concentrations require further consideration.
Common examples used under the ASC NEPM include:
Health Investigation Levels (HILs);
Health Screening Levels (HSLs);
Ecological Investigation Levels (EILs);
Ecological Screening Levels (ESLs);
groundwater investigation levels; and
petroleum hydrocarbon management limits.
Different criteria address different questions.
A HIL, for example, is a generic Tier 1 criterion used to assess potential risks to human health associated with chronic exposure to contaminants in soil under defined land-use scenarios.
HSLs are used for selected petroleum hydrocarbons and consider particular exposure pathways, including vapour intrusion, with criteria varying according to factors such as land use, soil type and source depth.
EILs and ESLs are concerned with risks to terrestrial ecosystems rather than directly with human health.
These criteria are therefore not interchangeable.
They are tools used within a broader contaminated land assessment process.
Their appropriate selection should follow the site's Conceptual Site Model, the environmental media being assessed, the relevant receptors and the exposure pathways that may be present.
What Does an Investigation-Level Exceedance Actually Mean?
At its simplest, an exceedance means:
Further consideration is required.
It does not automatically mean:
This soil must be excavated.
Consider a soil sample reporting lead above the applicable Tier 1 Health Investigation Level.
That result might ultimately lead to excavation.
But before reaching that conclusion, the consultant needs to understand:
whether the result represents a localised hotspot or broader contamination;
whether the affected soil is accessible;
what exposure pathways are present;
who the relevant receptors are;
whether other samples support the same conclusion;
how the proposed land use changes the exposure scenario;
whether a more detailed risk assessment is warranted; and
whether remediation, containment or another management response is appropriate.
The exceedance starts the decision-making process.
It does not finish it.
This is also why a Detailed Site Investigation should do more than simply identify which analytical results exceed guideline values.
A good DSI interprets those results within the CSM and determines what they mean for site risk and subsequent management.
Why the ASC NEPM Makes This Distinction
Tier 1 investigation criteria are intentionally designed to provide a conservative screening framework.
Their role is to help identify situations where contamination may warrant further assessment.
If every exceedance were automatically treated as requiring excavation until concentrations were below the relevant screening criterion, there would be little purpose in the subsequent stages of risk assessment.
More detailed assessment can consider site-specific information that generic criteria cannot fully represent.
Depending on the contaminant and pathway, this may include factors such as:
actual contaminant distribution;
contaminant bioavailability;
soil properties;
depth;
exposure frequency;
groundwater conditions;
vapour pathways;
site-specific receptors;
background concentrations;
ecological conditions; and
physical barriers or other exposure controls.
The appropriate level of assessment depends on the complexity and significance of the contamination.
For many straightforward sites, generic criteria may remain entirely appropriate.
The important point is that their use as a remediation endpoint should be deliberate and justified, rather than automatic.
What Are Remediation Criteria?
Remediation criteria are the criteria used to determine whether the objectives of the remediation have been achieved.
They should arise from the remediation objectives and the risk that is being managed.
The NSW EPA's Consultants Reporting on Contaminated Land guideline requires a RAP to identify the selected remediation criteria, provide their references, and explain the rationale for their selection.
Where site-specific remediation criteria have been developed through risk assessment, the RAP should also explain the basis for those criteria.
This makes remediation criteria part of the remediation design, rather than simply a table copied from the preceding investigation.
Depending on the project, remediation criteria might include:
nominated contaminant concentrations;
removal of a specific contaminant source;
removal of a defined material or fill unit;
treatment performance requirements;
removal of asbestos-containing material;
construction of a specified clean-soil cover;
construction of an engineered containment system;
groundwater performance criteria;
vapour-management requirements; or
a combination of numerical and physical criteria.
The appropriate remediation criteria therefore depend on what the remediation is intended to achieve.
Sometimes the Investigation Criteria Will Still Be Used
Saying that investigation criteria are not automatically remediation criteria does not mean they can never be used that way.
On many projects, an applicable Tier 1 investigation criterion may provide a practical and appropriately conservative remediation or validation criterion.
For example, a small localised area of metal-contaminated soil on a straightforward residential development may be excavated and the resulting validation samples compared against relevant residential assessment criteria.
That may be entirely appropriate.
A separate quantitative human health risk assessment would rarely be justified merely for the purpose of avoiding a small excavation.
The important distinction is that the criterion has been selected as an appropriate remediation endpoint for the site, rather than being treated as an automatic statutory clean-up value simply because it appeared in the investigation.
The RAP should make that reasoning clear.
Remediation Criteria Can Be Site-Specific
More complex sites may justify the development of site-specific remediation criteria.
This can arise where generic assumptions embedded within Tier 1 criteria do not adequately represent the site.
For example, site-specific risk assessment may consider:
actual exposure frequency;
contaminant accessibility or bioavailability;
site-specific vapour conditions;
groundwater behaviour;
particular receptor populations;
ecological conditions; or
other site-specific exposure assumptions.
Where supported by an appropriate risk assessment, these factors can be used to establish criteria that are better aligned with the actual source-pathway-receptor relationships.
This requires considerably more technical justification than simply choosing a different number.
The risk assessment needs to be scientifically defensible, based on suitable data and consistent with the applicable contaminated land assessment framework.
For projects undergoing review under the NSW EPA Site Auditor Scheme, the assumptions supporting site-specific criteria should also be expected to receive detailed scrutiny.
Remediation Criteria Can Also Be More Conservative
Site-specific assessment does not necessarily produce a higher acceptable concentration.
Sometimes the appropriate remediation criterion may be more conservative than a generic Tier 1 human-health criterion.
That can occur because remediation decisions need to consider more than a single exposure pathway.
For example:
ecological protection may drive the assessment rather than human health;
contamination may threaten groundwater;
a volatile contaminant may create a vapour pathway;
free product or highly contaminated source material may require removal;
petroleum management limits may become relevant;
contamination may create aesthetic or odour issues; or
the contaminant may continue to migrate if the source remains.
A concentration below a HIL therefore does not automatically mean that no remediation or management is required.
The CSM remains critical.
A contaminant can satisfy one assessment criterion while still creating an unacceptable risk through another pathway.
Example: Lead in Commercial Soil
Consider a commercial property where a DSI identifies elevated lead in shallow fill.
One location reports lead above the applicable commercial/industrial HIL.
A simplistic remediation approach might be:
Excavate soil until every validation sample is below the HIL.
That may ultimately be the selected strategy, but the exceedance alone does not establish it.
The consultant should first consider:
What caused the lead impact?
Is it a discrete hotspot or characteristic of the broader fill?
Is the soil accessible?
Is direct contact the primary exposure pathway?
Does the proposed development retain accessible soil in this area?
Are ecological receptors relevant?
Could the affected material be retained beneath an appropriate engineered surface?
Would retaining it create unreasonable long-term management requirements?
For a small hotspot in an accessible area, excavation to the nominated criterion may provide the simplest and most defensible outcome.
For widespread low-level fill beneath a permanent commercial hardstand, a different management response may be appropriate if supported by the risk assessment and remediation objectives.
The analytical concentration is the same.
The remediation decision is shaped by the CSM.
Example: Petroleum Hydrocarbons Beneath a Building
Petroleum contamination provides another useful example.
Suppose hydrocarbon concentrations in soil exceed an applicable HSL for vapour intrusion.
That exceedance identifies a potential vapour risk.
The next step is not necessarily to excavate all soil until every concentration falls below that HSL.
The consultant may need to consider:
depth of contamination;
soil type;
building configuration;
vapour migration;
source characteristics;
groundwater impacts;
whether free product is present;
whether the petroleum source remains; and
whether further vapour assessment is required.
Further investigation may show that the exposure pathway is incomplete.
Alternatively, it may demonstrate that the source requires remediation even more urgently than the original screening result suggested.
The screening criterion tells us that the pathway warrants attention.
The subsequent assessment determines what management response is appropriate.
Example: Ecological Criteria on a Developed Site
Ecological criteria can also illustrate why interpretation matters.
A soil sample may exceed an EIL or ESL.
The significance of that exceedance depends partly on whether a relevant terrestrial ecosystem exists or will exist at the location.
The ecological assessment for a landscaped area may be very different to that for soil several metres beneath a permanent building or heavily engineered pavement.
That does not mean ecological criteria can simply be disregarded whenever development is proposed.
It means their relevance needs to be assessed within the CSM and the intended site condition.
This is one reason remediation should be designed in conjunction with the proposed development rather than as a completely separate exercise.
For a more detailed discussion, see Designing Remediation Around the Proposed Development.
The Remediation Objective Comes First
One of the most effective ways to select remediation criteria is to stop thinking initially about the numerical criterion and instead ask:
What exactly are we trying to achieve?
A remediation objective might be:
Remove a localised lead hotspot from a future residential garden.
Or:
Remove the petroleum source responsible for groundwater impacts.
Or:
Prevent direct contact with residual contaminated fill.
Or:
Remove asbestos-impacted material from an area that will remain accessible.
Or:
Manage residual contamination beneath an engineered containment system.
Once the remediation objective is clear, the appropriate criteria can be selected to demonstrate whether that objective has been achieved.
This produces a much stronger RAP than starting with a table of guideline values and attempting to design the remediation around them.
Numerical Criteria Are Only One Type of Remediation Endpoint
This distinction is particularly important.
Contaminated land consultants often talk about remediation criteria as though remediation always finishes when a laboratory concentration falls below a number.
That is not always the case.
Consider removal of a former underground storage tank.
The remediation objective may include removing the tank, associated infrastructure and identifiable source-zone contamination.
Laboratory results remain important, but successful remediation may also depend on:
physical removal of infrastructure;
field observations;
removal of grossly impacted soil;
assessment of excavation walls and base;
groundwater conditions; and
demonstrating that no unacceptable residual risk remains.
Similarly, capping remediation might be validated through:
confirmation of material placement;
minimum cover thickness;
marker layers;
survey data;
imported material quality; and
construction records.
The criterion is partly physical rather than simply chemical.
This is why the soil remediation method and the remediation criteria need to be developed together.
Remediation Criteria and Validation Criteria
These terms are often used interchangeably, but it is useful to understand the distinction.
Remediation criteria define the outcome the remediation strategy needs to achieve.
Validation criteria are the criteria used to demonstrate that the completed works achieved that outcome.
In a straightforward excavation, they may effectively be the same numerical concentrations.
But not always.
A remediation strategy involving containment may have criteria relating to which material can remain and where, while validation also needs to confirm that the required cap was actually constructed.
A treatment strategy may establish performance criteria for treated soil, while validation may include both chemical testing and confirmation that the treatment process was implemented across the required material.
An asbestos remediation strategy may involve physical removal, inspection and soil assessment rather than relying on a single laboratory concentration.
The remediation and validation process should therefore be designed as a continuous framework.
The consultant should know how the remediation will be validated before the remediation works begin.
The Same Criterion Should Not Be Applied Blindly Across the Entire Site
Another common problem occurs when a single criterion is applied uniformly to every area and depth of a complex site.
The CSM may identify different:
land uses;
environmental media;
depth intervals;
exposure pathways;
remediation areas;
contaminant sources; and
receptors.
A remediation plan may therefore require different decision rules in different areas.
For example, shallow accessible soil may be assessed differently from deeper soil that will remain beneath a permanent structure.
Soil adjacent to groundwater may require consideration of contaminant leaching and migration.
A landscaped ecological area may require different consideration from an industrial hardstand.
The appropriate approach should be defined within the RAP rather than improvised during excavation.
Background Concentrations Can Also Matter
Not every naturally elevated concentration represents contamination requiring remediation.
For some contaminants, particularly metals, naturally occurring background concentrations can be relevant to ecological assessment and site interpretation.
The ASC NEPM's ecological investigation framework explicitly considers ambient background concentrations when deriving certain EILs.
This matters because attempting to remediate soil below naturally occurring background conditions would be technically inappropriate and potentially impossible.
Site history, geology and appropriate background assessment can therefore become important when evaluating apparently elevated concentrations.
Again, the numerical laboratory result only makes sense when interpreted in its environmental context.
Waste Classification Criteria Are a Different Question Again
Another distinction worth making is between remediation criteria and waste classification criteria.
A soil may require remediation because of its risk at the site, but its legal waste classification for off-site disposal is determined under the NSW waste framework.
Conversely, material may satisfy the remediation requirements for a particular land-use scenario but still require appropriate classification if it is excavated and taken off site.
These are different regulatory questions.
One determines whether contamination can remain or requires management in the context of the site.
The other determines how excavated material must be legally managed as waste.
Where excavation forms part of remediation, waste classification should therefore be integrated with the remediation planning process rather than confused with the remediation criteria themselves.
Why This Matters Commercially
Using screening criteria automatically as clean-up criteria can have significant practical consequences.
Imagine a large development containing widespread fill with occasional marginal Tier 1 exceedances.
If every exceedance automatically triggers excavation and disposal until all validation samples fall beneath the screening level, the project may incur:
unnecessary excavation;
unnecessary landfill disposal;
imported replacement fill;
additional truck movements;
increased carbon impacts;
extended remediation programs;
higher contractor costs; and
construction delays.
That may provide no meaningful additional reduction in risk if the original screening exceedance did not represent an unacceptable site-specific risk.
The opposite is also true.
Choosing a permissive criterion simply to reduce remediation cost is not risk-based remediation.
The objective is not to minimise remediation.
It is to undertake the remediation necessary to achieve a defensible site-suitability outcome without unnecessary remediation.
The RAP Needs to Explain the Logic
A strong Remediation Action Plan should make the relationship between investigation results, remediation objectives and remediation criteria clear.
For each significant contaminant or remediation area, the RAP should allow a reviewer to understand:
What contamination was identified?
Why does it require remediation or management?
What risk or source is being addressed?
What remediation outcome is required?
Which criteria will demonstrate that outcome?
Why are those criteria appropriate?
How will compliance be validated?
If that logic is clear, the selected remediation criteria become defensible.
If the RAP simply reproduces the DSI's investigation criteria and labels them “remediation criteria” without further explanation, an important step in the assessment has been skipped.
When Is Site-Specific Risk Assessment Worthwhile?
Site-specific risk assessment is not automatically the next step whenever a generic criterion is exceeded.
Sometimes remediation is simpler, cheaper and more defensible.
For example, undertaking a complex human health risk assessment to avoid removing 5 m³ of clearly impacted soil is unlikely to represent proportionate investigation.
Site-specific assessment becomes more valuable where the decision materially affects the project.
Examples might include:
widespread contamination where excavation would involve very large volumes;
contamination beneath critical infrastructure;
inaccessible contamination;
complex vapour pathways;
unusual exposure conditions;
naturally elevated background concentrations;
large operational industrial sites;
groundwater contamination;
ecological constraints; or
sites where generic assumptions poorly represent actual conditions.
The decision should therefore consider the scale of the problem, the uncertainty and whether further assessment could realistically change the remediation strategy.
This is closely related to the question of when investigation has progressed far enough to design remediation, which we discuss in From DSI to RAP: When Is There Enough Information to Design Remediation?.
An Investigation Criterion Is a Trigger, Not an Excavator Instruction
The simplest way to understand the distinction is:
Investigation criteria help identify where further assessment may be required.
Remediation criteria define the outcome that remediation must achieve.
Sometimes the same numerical value will appropriately perform both functions.
Sometimes it will not.
The decision depends on the contaminant, environmental media, CSM, exposure pathways, proposed land use, remediation objectives and level of assessment undertaken.
The ASC NEPM deliberately separates screening from risk management for this reason.
An exceedance should prompt interpretation.
It should not automatically prompt excavation.
Developing Defensible Remediation Criteria
A technically defensible remediation framework generally follows a logical sequence:
Investigation results
↓
Refined Conceptual Site Model
↓
Risk evaluation
↓
Remediation objectives
↓
Selection of remediation strategy
↓
Remediation criteria
↓
Validation requirements
Each stage should follow from the one before it.
Skipping directly from an analytical exceedance to an excavation criterion removes much of the professional judgement that contaminated land assessment is intended to provide.
The result may still be conservative, but it is not necessarily proportionate, technically justified or cost-effective.
Remediation Action Plans and Contaminated Land Remediation in NSW
Confluence Environmental provides contaminated land investigation and remediation services across NSW, including Detailed Site Investigations, Remediation Action Plans, remediation support and validation.
Our Remediation Action Plans establish remediation objectives and criteria based on the available investigation data, Conceptual Site Model, relevant exposure pathways and the outcomes required for the proposed land use.
Where generic Tier 1 criteria are appropriate, they can provide practical and conservative remediation endpoints.
Where more detailed assessment is warranted, remediation criteria can be developed around the site-specific contamination risks and remediation strategy.
The objective is not to remediate a site to an arbitrary number.
It is to establish and achieve a technically defensible outcome that protects human health and the environment and supports the site's intended use.