Does a Preliminary Site Investigation Need Soil Sampling?

No — a Preliminary Site Investigation (PSI) does not always require soil sampling.

The purpose of a PSI is to understand the site's history, identify potential contamination sources and determine whether those issues are likely to matter to the proposed land use or development.

Sometimes that can be achieved through historical research, Council records, a site inspection and a well-developed Conceptual Site Model.

In other cases, a small amount of targeted soil sampling can provide useful additional evidence and reduce an uncertainty that would otherwise remain unresolved.

The important distinction is that limited sampling during a PSI is not the same as comprehensively characterising contamination through a Detailed Site Investigation (DSI).

For an overview of the broader process, see our Preliminary Site Investigation services in NSW.

Why Include Soil Sampling in a PSI?

Targeted sampling can be useful where the PSI identifies a relatively simple and defined uncertainty that can reasonably be investigated without undertaking a full detailed investigation.

A common example is imported fill.

A site may contain a small amount of fill beneath a driveway or within a localised area of the property. Historical research may not establish where that material originated.

Without sampling, the consultant may need to retain that uncertainty within the Conceptual Site Model.

However, one or two appropriately targeted samples may provide additional evidence about the condition of the material.

If the analytical results are below the relevant assessment criteria, there are no indications of gross contamination and the broader weight of evidence is consistent with the findings, the consultant may be able to conclude that the fill represents a relatively low contamination risk in the context of the proposed development.

This does not mean that one or two samples have comprehensively characterised all of the fill.

They have simply provided additional evidence that helps resolve a particular uncertainty.

What Can PSI Sampling Actually Achieve?

Sampling during a PSI is most useful when it answers a specific question.

For example:

• Does a small area of imported fill show evidence of contamination?

• Does a localised soil stain contain elevated petroleum hydrocarbons?

• Is a suspected material actually asbestos-containing material?

• Do shallow soils within a clearly defined former activity area show evidence of the contaminants expected from that activity?

The objective should not be:

“We are doing a PSI, so we should collect some samples.”

It should be:

“There is an identified uncertainty. Will sampling help answer it?”

If the answer is yes, targeted sampling can strengthen the PSI and provide the client and reviewing authority with additional evidence supporting the consultant's conclusions.

When Is Soil Sampling During a PSI Less Appropriate?

Limited PSI sampling becomes much less useful when the investigation question is no longer preliminary.

For example, consider a large industrial property containing:

• multiple workshops;

• historical fuel storage;

• chemical handling;

• waste disposal;

• extensive imported fill; and

• several potentially contaminating activities.

Collecting a handful of samples across that site may confirm that contamination exists — or may fail to encounter it altogether.

Either way, those few samples are unlikely to adequately determine the nature and extent of contamination across the property.

If the information required is:

• whether contamination is actually present across an Area of Environmental Concern;

• its lateral and vertical extent;

• whether groundwater has been affected;

• whether contamination presents an unacceptable risk;

• whether remediation or management is required; or

• whether the site is suitable for the proposed use following assessment,

then the project has generally moved beyond a limited preliminary investigation.

A properly designed Detailed Site Investigation is usually more appropriate.

Sampling During a PSI Does Not Guarantee That a DSI Will Not Be Required

This is an important expectation to establish before sampling begins.

Including soil sampling in a PSI does not guarantee that the project will avoid a Detailed Site Investigation.

Sampling can reduce uncertainty.

It can provide additional confidence.

It can sometimes demonstrate that a relatively minor potential source appears to represent a low risk.

But if the results identify contamination, or if significant uncertainty remains regarding the distribution and extent of potential contamination, further investigation may still be required.

In some cases, preliminary sampling actually helps design the next investigation more efficiently because the consultant now has better information regarding:

• contaminants of concern;

• likely affected areas;

• soil conditions;

• investigation depths; and

• the remaining data gaps.

So the value of PSI sampling is not necessarily that it eliminates the next stage.

The value is that it helps make the next decision better informed.

How Many Soil Samples Should Be Collected During a PSI?

There is no single number that applies to every PSI.

The amount of sampling should respond to the Conceptual Site Model and the question the sampling is intended to answer.

Factors can include:

• the size of the site;

• the size of the area being investigated;

• the number of potential Areas of Environmental Concern;

• the nature of the potential contamination source;

• whether contamination is expected to be localised or widespread;

• the depth at which contamination could occur;

• the contaminants being investigated;

• accessibility; and

• the level of confidence required from the data.

A small, clearly defined area of fill may only justify a limited number of targeted samples during a PSI.

That does not mean the same sampling density would be appropriate for several hectares of imported fill or a former industrial area.

Where the objective becomes to characterise an area rather than simply investigate a defined uncertainty, a more systematic sampling design may be required as part of a DSI.

For more information about how these decisions are made, see What Is a Conceptual Site Model in Contaminated Land Assessment?

Could Sampling During the PSI Save Money?

Sometimes.

Adding sampling increases the initial cost of the PSI because it involves additional fieldwork, laboratory analysis and reporting.

However, there are situations where a relatively small amount of targeted sampling can be more efficient than leaving an easily resolved uncertainty open.

For example, if a site inspection identifies a small area of fill and targeted sampling can reasonably be completed during the same site visit, it may be more economical to investigate it then rather than mobilise again later.

Where geotechnical drilling or other intrusive investigation is already being undertaken, there may also be opportunities to coordinate environmental sampling with the existing field programme.

The key is that the additional sampling should have a clear purpose.

For more information about how sampling and fieldwork affect project fees, see How Much Does a Preliminary Site Investigation Cost in NSW?

When Would Confluence Recommend Sampling With a PSI?

We generally consider targeted PSI sampling where it can efficiently provide useful evidence about a specific, relatively limited uncertainty.

We would be less likely to recommend limited PSI sampling where the site's history already indicates that comprehensive characterisation will probably be required.

For example, collecting a handful of preliminary samples from a complex former industrial facility may simply confirm what the historical assessment has already established: that a properly designed DSI is required.

In that situation, it may be more efficient to complete the PSI, develop the Conceptual Site Model and then design the detailed investigation around the identified data gaps.

Our approach is therefore not:

“PSIs should always include sampling.”

Nor is it:

“PSIs should never include sampling.”

It is:

“What uncertainty are we trying to resolve, and will the proposed sampling actually resolve it?”

Need a PSI With or Without Soil Sampling?

If you require a Preliminary Site Investigation, we can review the property history, proposed development and available Council information before determining whether soil sampling is likely to add value.

Send us:

• the property address;

• proposed development;

• Council correspondence;

• previous environmental reports; and

• any information regarding known fill, fuel infrastructure or historical activities.

We can then develop a project-specific PSI scope, including targeted soil sampling where it is warranted.

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