What Is a Conceptual Site Model in a Preliminary Site Investigation?

A Conceptual Site Model (CSM) is one of the most important parts of a Preliminary Site Investigation.

It takes the information collected during the PSI — site history, surrounding land uses, geology, groundwater, site observations and any available analytical data — and brings it together into a logical model of:

Source → Pathway → Receptor

In simple terms, the CSM asks:

What could the contamination source be?

How could someone or something be exposed to it?

Who or what could actually be affected?

This is what allows a consultant to move beyond simply listing every potentially contaminating activity identified during the historical review.

The objective is to determine which issues are credible and relevant to the proposed development.

For more information about the overall assessment process, see our Preliminary Site Investigation services in NSW.

Why Is a Conceptual Site Model Required?

The CSM is not just a preferred reporting style.

It forms part of the contaminated land assessment framework used in NSW.

The NSW EPA's Consultants Reporting on Contaminated Land: Contaminated Land Guidelines requires the CSM to identify existing and potential source-pathway-receptor linkages, contaminants of potential concern and relevant data gaps.

The National Environment Protection (Assessment of Site Contamination) Measure 1999, as amended in 2013 (ASC NEPM) also provides the national technical framework for site contamination assessment, including the use of conceptual site models as part of systematic site characterisation.

The NSW EPA's Contaminated Land Sampling Design Guidelines – Part 1 goes further and explains that a CSM should provide a spatial and temporal overview of contamination, including known and potential:

• sources and contaminants;

• affected environmental media;

• human and ecological receptors;

• migration and exposure pathways;

• uncertainties; and

• data gaps.

The EPA also states that the preliminary CSM developed during a PSI should logically inform any subsequent sampling and investigation.

That last point is particularly important.

The CSM should drive the investigation — the investigation should not be designed first and justified afterwards.

Source → Pathway → Receptor

The easiest way to understand a CSM is through the three basic components.

Source

The source is the activity or material that could have resulted in contamination.

Examples might include:

• imported fill;

• a former underground fuel tank;

• pesticide use within an orchard;

• chemical storage;

• a former workshop;

• asbestos-containing demolition material; or

• contamination migrating from an adjoining property.

Identifying a source does not automatically mean the property is contaminated.

It simply means there is something that needs to be considered.

Pathway

The pathway is how contamination could move from the source to a receptor.

Examples include:

• direct contact with contaminated soil;

• ingestion of soil or dust;

• inhalation of fibres or vapours;

• migration through groundwater;

• surface-water movement;

• vapour intrusion into a building; or

• excavation bringing buried contamination to the surface.

If there is no credible pathway, the presence of a potential source may have limited relevance to the proposed development.

Receptor

The receptor is the person, ecological system or other environmental value that could be affected.

Depending on the project, receptors might include:

• future residents;

• children;

• construction workers;

• commercial workers;

• neighbouring properties;

• groundwater users;

• surface-water ecosystems; or

• future maintenance workers.

This is why the proposed land use matters.

The same contamination source can have a very different significance depending on who will use the site and how they could be exposed.

A CSM Is More Than a Long List of Possible Contamination

One of the easiest ways for a PSI to become unnecessarily complicated is to treat every conceivable historical activity as though it represents the same level of risk.

A good CSM should narrow the assessment.

For example, imagine historical research identifies that a workshop operated on part of a property 50 years ago.

That alone does not tell us whether a significant contamination issue exists.

The CSM should ask:

• What actually occurred in the workshop?

• Where was it located?

• Were vehicles or machinery maintained there?

• Were fuels, oils or chemicals stored?

• Was the floor sealed?

• Were there pits or drains?

• Is the proposed development disturbing that part of the site?

• Could contamination migrate elsewhere?

• Who could be exposed?

Depending on the answers, the former workshop might become a significant Area of Environmental Concern.

Or it may ultimately be considered a relatively low-risk historical activity.

That interpretation is the value of the CSM.

Example 1 – Proposed Childcare Centre

Consider a property proposed for redevelopment as a childcare centre.

Historical information identifies:

• an older dwelling;

• imported fill within part of the property; and

• a former shed containing potential asbestos-containing materials.

Because the proposed land use involves young children, the receptor is particularly sensitive.

The CSM may therefore identify:

Source: Imported fill potentially containing contaminants or asbestos.

Pathway: Direct contact with shallow soil, ingestion of soil or dust, or disturbance during construction.

Receptor: Children, childcare workers and construction workers.

That linkage may justify targeted investigation of the fill.

However, suppose the targeted sampling does not identify contaminants above the applicable assessment criteria, no asbestos is identified within the sampled material, and the site inspection provides no evidence of widespread filling or gross contamination.

The CSM can then be refined.

The fill remains part of the site's history, but the available evidence may support a conclusion that it represents a relatively low contamination risk and does not prevent the site from being considered suitable for the proposed childcare use, subject to any appropriate recommendations.

That is very different to simply writing:

“Fill was identified. Further investigation is recommended.”

The CSM helps explain why further investigation is or is not warranted.

Example 2 – Mixed-Use Development

Now consider a larger property proposed for mixed commercial and residential development.

Historical information identifies:

• several former workshops;

• underground petroleum storage;

• imported fill;

• chemical storage;

• an infilled drainage feature; and

• industrial activity on an adjoining property.

The CSM is immediately more complex.

Potential pathways may include:

• direct contact with contaminated soil;

• groundwater migration;

• vapour intrusion;

• excavation during basement construction; and

• migration from neighbouring land.

Potential receptors may include:

• future residents;

• commercial workers;

• construction workers;

• neighbouring properties; and

• groundwater or surface-water environments.

In this situation, a small number of preliminary soil samples would be unlikely to resolve the identified uncertainties.

The CSM may instead demonstrate that a Detailed Site Investigation is required to characterise the nature and extent of contamination before conclusions can be reached about site suitability or remediation.

The CSM has therefore shaped the outcome of the PSI.

It has identified not only that further investigation is required, but why it is required and what the next investigation needs to address.

The Proposed Development Can Change the CSM

A Conceptual Site Model is site-specific, but it is also land-use specific.

Consider shallow lead contamination beneath an existing sealed commercial car park.

For continued commercial use, the current exposure pathway may be limited because the soil is covered by hardstand.

If the same site is proposed for residential development with private gardens, the exposure scenario changes significantly.

Likewise:

• a former fuel tank may become particularly relevant where basement excavation is proposed;

• asbestos-containing fill may become relevant where extensive earthworks are planned;

• groundwater contamination may be less relevant where no pathway to the proposed development exists; or

• an off-site source may become important where groundwater flow or vapour migration creates a credible pathway onto the site.

This is why contaminated land assessment cannot be based on the site's history alone.

The consultant must understand what is being proposed.

How Does the CSM Influence Soil Sampling?

The CSM should help answer two questions:

Where should we sample?

and

Why are we sampling there?

The NSW EPA Sampling Design Guidelines state that the preliminary CSM and identified data gaps should logically feed into the sampling and analysis programme, and that the model should continue to be updated as new information becomes available.

For example:

If the CSM identifies a former underground fuel tank, sampling may be targeted around the tank pit, fuel lines and bowser area.

If the issue is widespread historical fill, a broader sampling approach may be necessary.

If the potential source is a small area of imported fill beneath a driveway, a limited targeted investigation may be sufficient to reduce the identified uncertainty.

The sampling design therefore follows the contamination model, rather than simply applying an arbitrary number of sample locations.

For more information, see Does a Preliminary Site Investigation Need Soil Sampling?

What Is a Data Gap?

A data gap is not simply something that the consultant does not know.

It should be an uncertainty that matters to the objectives of the assessment.

For example:

We don't know exactly when a shed was constructed.

That may not materially affect the PSI.

But:

Historical records indicate that an underground fuel tank existed, but its location, removal status and validation history cannot be confirmed.

That may be a significant data gap.

The EPA reporting guidelines specifically link data quality objectives and further investigation to critical data gaps within the CSM.

This is an important distinction because not every uncertainty needs to trigger more investigation.

The question is whether the uncertainty affects our ability to reach a reliable conclusion about the property.

What Does a Good CSM Look Like?

A good Conceptual Site Model should make the logic of the assessment easy to follow.

It should explain:

• what contamination sources are credible;

• which contaminants are associated with those sources;

• where those sources are located;

• how contaminants could migrate or create exposure;

• who or what could be affected;

• which pathways appear complete or incomplete;

• what evidence supports those conclusions; and

• what important uncertainties remain.

The EPA recognises that CSMs can be presented using text, tables, plans, figures, cross-sections or flow diagrams, depending on the complexity of the site and the intended audience.

For a simple property, the CSM may be relatively concise.

For a major industrial site, it may require separate areas, environmental media, migration pathways and graphical representations.

The complexity should reflect the site.

Why the CSM Matters to the Outcome of a PSI

Ultimately, the CSM is what allows a PSI to answer the question that matters:

Does the available evidence identify a credible contamination issue that affects the proposed development?

A good CSM may support:

• no further contamination investigation, where potential sources and pathways have been adequately addressed;

• limited targeted investigation, where a specific uncertainty can be efficiently resolved;

• a Detailed Site Investigation, where contamination needs to be characterised;

• management recommendations for future earthworks; or

• consideration of an off-site contamination source.

The objective is not to identify as many potential contamination issues as possible.

It is to determine which issues actually matter and what needs to happen next.

Need a Preliminary Site Investigation?

Confluence Environmental prepares Preliminary Site Investigations for development, planning, property acquisition and environmental due diligence across NSW.

Our approach is based on developing a clear, site-specific Conceptual Site Model and using it to determine whether additional investigation is genuinely warranted.

If you require a PSI, send us:

• the property address;

• proposed development;

• Council correspondence;

• available plans; and

• previous environmental reports.

We can review the information and develop an appropriate investigation scope.

Learn more about our Preliminary Site Investigation services in NSW or contact Confluence Environmental for a project-specific quote.