Detailed Site Investigation (DSI): What It Is, When It’s Required and What It Involves

A Detailed Site Investigation (DSI) is an intrusive contaminated site investigation used to determine whether contamination is present, characterise its nature and extent, and assess what it means for the current or proposed use of land.

In NSW, a DSI commonly follows a Preliminary Site Investigation (PSI) where credible contamination sources, potential exposure pathways or important data gaps have been identified. Depending on the site and Conceptual Site Model, the investigation may assess soil, groundwater, soil vapour, surface water or other relevant environmental media using targeted field investigation and laboratory analysis.

The findings provide the technical basis for determining whether further investigation, remediation, management or no further contaminated land assessment is required.

What Is a Detailed Site Investigation?

A Detailed Site Investigation is a detailed contaminated site assessment undertaken where existing information indicates that contamination may be present and intrusive investigation is required to resolve the identified risks or uncertainties.

Unlike a preliminary assessment, which is primarily concerned with understanding the site history and identifying credible contamination scenarios, a DSI collects physical evidence from the site.

The investigation is generally designed to answer several fundamental questions: Is contamination present? What contaminants are present? Where are they located? How far do they extend? Which environmental media are affected? Are relevant migration or exposure pathways present? Does the contamination create an unacceptable risk for the proposed land use? And is further investigation, remediation or management required?

A DSI should therefore do more than produce laboratory results. It should test and refine the Conceptual Site Model (CSM) and provide enough evidence to support a clear decision about what happens next.

Where Does a DSI Fit in Contaminated Site Assessment?

Contaminated land assessment in NSW generally follows a staged, decision-based process.

A Preliminary Site Investigation is usually the first stage. It reviews the site history, current and historical land uses, surrounding properties, environmental setting and site conditions to identify credible potential contamination sources, Areas of Environmental Concern, Contaminants of Potential Concern and relevant source–pathway–receptor relationships.

The term preliminary site assessment is also sometimes used more generally, although Preliminary Site Investigation is the terminology commonly used within the NSW contaminated-land reporting framework.

Where the PSI identifies a material contamination risk or an uncertainty that cannot be resolved from the available information, a DSI may be required.

The DSI then tests those contamination hypotheses through intrusive investigation.

This distinction is important. A DSI should not simply be commissioned because a site has an industrial history. The investigation should have a clear purpose based on what the preliminary assessment has actually identified.

Likewise, not every PSI progresses to a DSI. Where the available information provides sufficient confidence that no credible contamination risk requiring further assessment remains, additional intrusive investigation may not be warranted.

Internal link: link “Preliminary Site Investigation” above to your main PSI service page.

When Is a Detailed Site Investigation Required?

A DSI may be appropriate where a Preliminary Site Investigation, previous environmental assessment or other available information identifies a credible contamination risk that requires intrusive investigation.

Typical examples include former service stations or fuel storage areas, mechanical workshops, industrial land, dry cleaners, historical agricultural or horticultural activities, uncontrolled fill, waste disposal areas, demolition material, underground petroleum storage systems or other potentially contaminating activities.

A DSI may also be required where a site is being redeveloped for a more sensitive land use and the available information is insufficient to demonstrate that the land is suitable, or can be made suitable, for the proposed development.

In NSW, contaminated land is also considered through the planning process. Where a preliminary investigation indicates that further assessment is warranted, a consent authority may require a more detailed investigation before it can determine whether the land is suitable for the proposed use.

The key question is therefore not simply whether contamination could theoretically exist. It is whether the available information identifies a credible contamination scenario or important data gap that requires further investigation.

How Is a Detailed Site Investigation Designed?

A good DSI is a designed investigation, not a predetermined number of samples.

The starting point should be the Conceptual Site Model established during the preliminary investigation.

The CSM identifies potential contamination sources, the contaminants that could reasonably be associated with those sources, the environmental media that may be affected, possible migration or exposure pathways and the receptors that could potentially be impacted.

The investigation then needs to resolve the important uncertainties within that model.

Data Quality Objectives are used to define what decisions need to be made, what information is required to make those decisions, where samples should be collected and what level of data quality is necessary.

That means two sites of the same area may require very different investigation programs.

A relatively uniform area of imported fill may require one form of sampling strategy, while a former workshop containing a fuel tank, wash bay and waste storage area may require a combination of systematic and targeted investigation.

Similarly, groundwater sampling should not be included automatically simply because the project is called a DSI. It should be investigated where groundwater represents a relevant environmental medium, migration pathway or receptor within the CSM.

This is one of the most important differences between a defensible contaminated site investigation and simply collecting samples without a clearly defined purpose.

What Does a Detailed Site Investigation Involve?

The scope of a DSI depends on the site history, proposed development, identified Areas of Environmental Concern, contaminants of concern and environmental setting.

Field investigation may involve boreholes, test pits, hand augering, soil sampling, groundwater monitoring wells, soil vapour investigation, surface-water or sediment sampling, or other investigation methods appropriate to the CSM.

Sampling locations and depths should be selected to address the questions identified during investigation planning. This may involve systematic sampling across a broader area, targeted investigation of known or suspected sources, or a combination of both.

Field observations are also important. Soil type, fill, staining, odours, demolition material, buried waste, groundwater conditions and other observations can materially affect the interpretation of laboratory results and the evolving Conceptual Site Model.

The investigation should remain responsive to what is encountered rather than treating the original sampling plan as an inflexible checklist.

What Contaminants Are Tested During a DSI?

There is no universal analytical suite for every Detailed Site Investigation.

The contaminants selected for laboratory analysis should reflect the site's history and the credible contamination sources identified within the CSM.

For example, former fuel infrastructure may warrant investigation for petroleum hydrocarbons and associated compounds, while uncontrolled fill may require consideration of metals, hydrocarbons, PAHs and asbestos. Agricultural land may require a different analytical program again.

Other investigations may need to consider PFAS, solvents, pesticides, asbestos, acid sulfate soil conditions or other site-specific contaminants.

The purpose is not to analyse every sample for every possible contaminant.

It is to investigate the contaminants that could reasonably be associated with the identified contamination scenarios.

Laboratory testing should generally be undertaken by appropriately accredited laboratories, with QA/QC procedures incorporated into the investigation so that the reliability of the resulting data can be assessed.

Does Every DSI Include Groundwater Sampling?

No.

Groundwater investigation should be undertaken where the Conceptual Site Model indicates that groundwater could be affected, could transport contamination, or could represent a relevant pathway or receptor.

For example, groundwater may be particularly important where mobile contaminants have been identified, underground fuel infrastructure has historically been present, contamination extends to depth, sensitive groundwater receptors exist or groundwater could influence the proposed development.

Conversely, a shallow and well-defined soil contamination issue may sometimes be investigated without groundwater monitoring where the available evidence does not identify groundwater as a relevant pathway.

The investigation should respond to the site, not to a generic DSI template.

How Many Samples Are Required for a Detailed Site Investigation?

There is no single sampling number that applies to every DSI.

Sampling density depends on the investigation objective, site area, historical activities, distribution of potential contamination, Areas of Environmental Concern, proposed land use, target hotspot size, environmental media being investigated and the amount and quality of existing information.

Systematic sampling may be appropriate where the objective is to assess broader site conditions or detect previously unidentified contamination.

Targeted or judgemental sampling may be appropriate where specific sources have already been identified.

In many investigations, both approaches are required.

The important issue is whether the sampling strategy can achieve the Data Quality Objectives and adequately address the contamination scenarios identified in the CSM.

A DSI should therefore be able to explain why each part of the sampling program was undertaken and what question it was intended to answer.

How Are DSI Results Assessed?

Laboratory results are interpreted against assessment criteria relevant to the environmental medium, proposed land use and contamination pathway being assessed.

For soil contamination, this may include health-based and ecological investigation or screening criteria from the National Environment Protection (Assessment of Site Contamination) Measure 1999, as varied in 2013, commonly referred to as the ASC NEPM.

Other guidelines may apply to groundwater, surface water, vapour, asbestos, waste or specific contaminant groups.

Importantly, interpretation should not consist solely of identifying whether individual laboratory results exceed numerical screening criteria.

The significance of a result needs to be considered in the context of the CSM, spatial distribution, depth, exposure pathways, surrounding results, data quality and proposed use of the site.

A single elevated result within an otherwise well-characterised site can represent a very different contamination scenario from widespread impacts across several environmental media.

What Should a Detailed Site Investigation Report Include?

A Detailed Site Investigation report should explain the investigation objectives, site history, Conceptual Site Model, investigation design, field methodology, laboratory program, QA/QC, results, data quality assessment and interpretation.

The CSM should be updated using the information generated during the investigation.

The report should then reach clear conclusions regarding the contamination conditions and the implications for the proposed land use.

Where important uncertainties remain, the report should identify those data gaps and explain what additional investigation is required.

Where the available evidence is sufficient, the report should state whether remediation, management, further assessment or no further contaminated land investigation is recommended.

A strong DSI report therefore connects the evidence collected in the field with the decision the project needs to make.

What Happens After a Detailed Site Investigation?

A DSI does not automatically lead to remediation.

One possible outcome is that the investigation demonstrates that contamination does not present an unacceptable risk for the proposed use and no further contaminated land assessment is required.

Another possibility is that a specific data gap remains and additional targeted investigation is required.

Where contamination presents an unacceptable risk, a Remedial Action Plan (RAP) may be prepared to establish how the contamination will be remediated or otherwise managed.

Following remediation, validation is generally undertaken to demonstrate whether the remedial objectives have been achieved.

The appropriate pathway is therefore determined by the investigation findings rather than by automatically progressing through a predetermined series of reports.

Internal link: link “Remedial Action Plan” to your RAP service page.

Why the Quality of the DSI Matters

The value of a DSI lies in the reasoning behind the investigation.

Collecting a large number of samples does not necessarily produce a better investigation if those samples do not address the relevant contamination sources or data gaps.

Conversely, an investigation that is too narrow may fail to characterise important contamination and result in further work being requested later.

A defensible DSI should demonstrate a logical connection between the preliminary findings, Conceptual Site Model, investigation objectives, sampling design, analytical program and conclusions.

This becomes particularly important where the work is being considered by Council, an independent technical reviewer or an accredited Site Auditor.

Well-designed investigation can also improve commercial outcomes by identifying contamination constraints before construction, reducing unnecessary investigation and focusing any remediation on the areas that actually require management.

Preliminary Site Investigation vs Detailed Site Investigation

The difference can be summarised simply.

A Preliminary Site Investigation asks whether there is a credible contamination issue that requires further assessment.

A Detailed Site Investigation tests and characterises that issue using intrusive investigation and environmental data.

The PSI establishes the preliminary contamination model.

The DSI tests and refines it.

That is why the quality of the PSI can have a significant influence on the efficiency of the DSI that follows.

Frequently Asked Questions (FAQ):

Q: Does every Preliminary Site Investigation lead to a Detailed Site Investigation?
No. A DSI is generally required where the PSI identifies a credible contamination risk or material uncertainty that needs intrusive investigation. Where the available evidence does not identify a contamination issue requiring further assessment, a DSI may not be necessary.

Q: Does a DSI always require groundwater monitoring?
No. Groundwater should be investigated where it is relevant to the Conceptual Site Model, such as where it may be contaminated, transport contamination or represent an exposure pathway or receptor.

Q: Can a DSI determine that remediation is not required?
Yes. One possible outcome of a DSI is that contamination is absent, sufficiently limited or does not present an unacceptable risk for the proposed land use. In those circumstances, remediation may not be required.

Q: How many samples are required for a DSI?
There is no universal sampling number. The required investigation depends on site area, historical activities, contamination hypotheses, investigation objectives, target media and the sampling strategy necessary to achieve the Data Quality Objectives.

Q: Can a PSI and DSI be combined?
In some circumstances the investigation stages may be combined where sufficient information is already available to appropriately design the intrusive assessment. The reporting approach should still clearly establish the investigation objectives, Conceptual Site Model, data gaps, methods and conclusions.

Q: What happens if contamination is identified during a DSI?
The next step depends on its nature, extent and significance. Further delineation, risk assessment, remediation planning, management or validation may be required.

Detailed Site Investigations in NSW

Confluence Environmental undertakes Detailed Site Investigations across NSW for development, infrastructure, property transactions and contaminated land management.

Our investigations are developed from the available site information, Conceptual Site Model and identified data gaps, allowing sampling and analysis to be directed toward the contamination risks relevant to the project.

Where a Preliminary Site Investigation has already been completed by another consultant, we can review the existing assessment and develop an appropriate scope for the next stage of investigation.

Reports are prepared with reference to the NSW contaminated-land reporting framework and applicable technical guidance, with senior technical review and CEnvP-SC or CPSS CSAM review and endorsement available where required.

Need a Detailed Site Investigation?

If a Preliminary Site Investigation, Council request or previous environmental assessment has identified a potential contamination issue, Confluence Environmental can review the available information and develop an appropriate investigation scope.

Learn more about our Detailed Site Investigation services or contact our team to request a proposal.