You are assessing a site for development. The property's history raises a few questions about possible contamination, but you do not yet know whether those concerns justify detailed sampling.
At this point, you cannot move to designs and approvals before answering an important question: do you start with a Preliminary Site Investigation (PSI), or does the site already warrant a Detailed Site Investigation (DSI)?
Getting that decision right helps you direct the investigation effort where the project actually needs it.
The right approach depends on the site's history, what is already known, the proposed land use and the information still needed to make the next development decision.
That is why the distinction between a preliminary site investigation vs detailed site investigation is less about choosing between two report types and more about establishing what the site needs to determine.
PSI and DSI: What Does Each Investigation Do?
A Preliminary Site Investigation (Phase 1) and a Detailed Site Investigation (Phase 2) are environmental assessments that answer different questions and provide different levels of information.
| Criteria | PSI | DSI |
|---|---|---|
| Primary role | Establishes the site's history, potential contamination concerns and information gaps. | Characterises identified contamination in greater detail, including its nature, extent and concentration. |
| Information used | Historical information, environmental records, site inspection and, where appropriate, sampling and analysis. | More comprehensive investigation, sampling, laboratory analysis and interpretation of the available data. |
| Sampling | Targeted sampling may be undertaken where additional samples can help resolve a contamination concern at the PSI stage. | Sampling is designed to characterise identified contamination and provide sufficient data for detailed assessment. |
| Data interpretation | Determines whether the available information, including any sampling results, is sufficient or whether further investigation is warranted. | Can include statistical assessment such as the 95% UCL, where appropriate, to evaluate the dataset against relevant assessment criteria. |
| Outcome | Resolves identified concerns where possible and determines whether further investigation is required. | Provides the detailed information needed for risk assessment, remediation or ongoing management. |
When Does a Development Site Need a PSI?
A PSI is usually the first step in learning about a site's environmental history and identifying potential contamination risks.
Looking at how the land was used in the past often gives the first clues. Old industrial sites, service stations, chemical storage, waste handling, and intensive farming are examples of activities that might need to be checked.
The current site condition also matters. A site inspection lets the consultant compare visible features with historical information.
A PSI may consider:
- Historical land use and title information
- Environmental records and planning information
- Aerial imagery
- Site inspection and visual observations
- Potentially contaminating activities
- Contaminants of potential concern
- A preliminary conceptual site model
- Recommendations for further investigation
The PSI does not automatically include soil or groundwater sampling. Its purpose is to determine if available information indicates a potential contamination issue and if further assessment is needed.
For developers, this makes the PSI useful early in the project when environmental findings can inform the development concept and investigation strategy.
The decision to progress to a DSI comes from the findings of the PSI and the information still needed for the development.
Where the preliminary assessment identifies potential contamination and the available information cannot resolve the concern, further investigation can be designed around those specific data gaps.
When the Investigation Needs to Go Deeper
A DSI becomes appropriate when the available information cannot answer the questions the development now needs to resolve.
DSI may follow a PSI, but existing evidence can also justify detailed investigation, particularly where the proposed development introduces a more sensitive land use.
Depending on the site, this can involve investigating soil, groundwater or other relevant environmental media for contaminants identified during the earlier assessment.
The proposed development can also create a need for more detailed information. A residential development, for example, may require a clearer understanding of contamination and potential exposure pathways than continuing an existing compatible industrial use.
A DSI can therefore become relevant where:
- A PSI identifies potential contamination.
- Existing information already indicates contamination.
- The proposed land use increases the significance of an identified concern.
- Important data gaps remain.
The practical question is: What information does this development need before the next decision can be made?
What Happens After a DSI?
A DSI provides evidence for the decisions that follow. It does not automatically mean remediation will be required.
The results need to be interpreted against the proposed land use, relevant assessment criteria, and potential exposure pathways.
Where contamination requires management, the project may progress to risk assessment, a Remedial Action Plan, remediation, validation or ongoing environmental management, depending on the findings.
NSW EPA guidance identifies risk assessment, remedial action planning, remediation, and validation as stages that can follow detailed investigation.
The Risk-Based Framework Behind the Assessment
The National Environment Protection (Assessment of Site Contamination) Measure (ASC NEPM) provides the national framework for assessing site contamination in Australia. It takes a risk-based approach, so the investigation should reflect the circumstances of the individual site.
For example, a former industrial site proposed for residential development may require different information from an industrial site continuing with a compatible use. The site's history and proposed use help determine which contamination risks need to be investigated and how much information the project needs.
For contaminated land investigations in NSW, the assessment also needs to align with the relevant state planning and environmental requirements.
Give the Investigation the Right Starting Point
Before commissioning the investigation, provide any previous environmental reports, available site history and relevant development plans.
Previous reports are particularly useful. They can show what has already been investigated, identify limitations or gaps, and help the consultant avoid repeating work unnecessarily.
NSW EPA also recommends providing available information about the site's history, potential contamination sources, and chemical or waste use or storage when engaging a contaminated land consultant.
The aim is simple: give the consultant enough background to build on what is already known and focus the investigation on what still needs to be established.
Read the Scope, Not Just the Label
PSI, DSI, Phase 1 and Phase 2 are useful terms, but the scope of the investigation tells you what the report can actually support.
Two reports carrying the same label can investigate different areas, environmental media or contamination concerns.
Before relying on an existing assessment, check when it was completed, what areas it covered, which potential contaminants it considered, what limitations applied and whether the proposed development has changed.
A previous report may still provide useful background even when it no longer answers every question raised by the current development.
The real distinction is clear: a PSI establishes potential contamination risks, while a DSI develops detailed evidence about identified concerns.
FAQs
What is the difference between a PSI and DSI?
A Preliminary Site Investigation reviews historical information, environmental records, and site conditions to identify potential contamination risks. A Detailed Site Investigation uses targeted sampling and laboratory analysis to investigate the nature and extent of identified contamination and provide information for risk assessment or management.
Is a Phase 1 the same as a PSI?
A Phase 1 Environmental Site Assessment is commonly used to describe a Preliminary Site Investigation. It generally reviews historical land use, environmental records, and site conditions to identify potential contamination risks. Terminology can vary, so always confirm the investigation scope.
When is a DSI required for contaminated land?
A DSI may be required when a PSI identifies potential contamination requiring further assessment, when existing evidence indicates contamination, or when development planning requires detailed information about its nature and extent. The appropriate scope depends on site conditions, proposed land use, and applicable requirements.
What does the NEPM say about contaminated land?
The National Environment Protection (Assessment of Site Contamination) Measure provides a nationally consistent framework for assessing site contamination. It supports a risk-based approach that considers contamination sources, pathways, receptors and potential effects on human health and the environment.
Does every contaminated site need both a PSI and DSI?
No. The investigation stages depend on the information available, site history, contamination risks, and proposed land use. A PSI may provide enough preliminary information for some decisions, while other sites may require a more detailed investigation or progress directly to a more extensive assessment.
