Proof in the Soil: PSI and Screening for a Former Sawmill

Overview

This case study describes a focused Preliminary Site Investigation (PSI) and targeted soil screening completed for a former sawmill yard in regional Queensland. The work identified and assessed priority risks — timber treatment residues, unknown fill, and petroleum from machinery areas — and produced a defensible, commercially usable baseline for sale, planning and due diligence.

Why a PSI matters

A PSI gives purchasers, vendors and advisors a fast, evidence-based view of likely contamination issues so commercial decisions can proceed with confidence. It:

• identifies obvious sources and hotspots;
• defines a conceptual site model (CSM) to guide sampling;
• establishes whether a Detailed Site Investigation (DSI) or groundwater assessment is needed;
• produces reporting suitable for negotiation and approval pathways.

What iEnvi delivered

• Records review: historic aerial imagery, council and state records to locate past processing areas and fill sources.
• Detailed site walkover and services checks to map old sheds, stockpiles, machine bays, points of staining/odour and suspected unauthorised fill.
• Targeted soil investigation at priority locations informed by the CSM.
• Sampling and analysis tailored to observed risks — metals suite, CCA indicators (copper/chromium/arsenic), PAHs where ash/building waste was present, petroleum indicators where staining/odour suggested hydrocarbons.
• Field screening of suspect material for asbestos with laboratory confirmation for any suspect fragments.
• Assessment of results against nationally-accepted investigation levels to refine risk ratings and recommend next steps.

Technical approach (plain English)

We prioritised safe, practical sampling to produce legally-defensible results quickly:

• Use records and a walkover to map likely contaminant sources and select targeted sampling locations rather than grid-sampling the whole site.
• Sample matrices and analytes are driven by observed evidence (e.g., CCA testing where treated timber residues are suspected; PAHs where ash or burnt building material is present; petroleum hydrocarbon indicators where staining or odour suggests machine-area leaks).
• Field screening for gross contamination and asbestos fragments reduces lab costs while ensuring suspect material is confirmed in an accredited laboratory.

How results are assessed

Laboratory results are compared to nationally recognised investigation levels used in contaminated land practice in Australia to decide whether contamination is likely to pose human health or ecological risk and whether further investigation or management is needed. The assessment is used to update the CSM, score risk pathways (exposure, migration, receptors) and set clear triggers for a DSI or groundwater assessment.

Outcomes and commercial benefits

• Defensible baseline for negotiations and due diligence — reporting written in clear, commercial language so findings translate directly into action for vendors, buyers and financiers.
• Time and cost control — targeted sampling reduces unnecessary testing and shortens turnaround compared with full-site DSIs.
• Clear decision triggers — when results exceed investigation levels, we specify what further work is needed (DSI, excavation trials, groundwater monitoring) and provide an indicative scope and timing.
• Reduced transaction uncertainty — buyers and lenders can price risk accurately or include conditions in sale contracts with confidence.

Practical takeaways

1. If visible timber-treatment residues, ash or staining are present, include CCA, PAHs and petroleum indicators in the initial sample suite.
2. Screen suspect soil for asbestos fragments in the field and confirm any suspect fragments in the lab to avoid surprises later in remediation or demolition.
3. Use the PSI to set clear, contractual triggers for a DSI — this keeps the sale process moving while protecting both parties.

Next steps we often recommend

• Prepare a concise remedial options note if limited hotspots are identified (cost ranges, likely approvals and estimated timeframes).
• If results suggest leachable contamination or perched/seasonal water, scope early groundwater monitoring to define migration risk.
• Include a plan for safe handling and disposal of any asbestos-impacted soils discovered during works.

Want the same clarity for your site? Contact our expert team on 13000 43684 or visit /contact/ to discuss a pragmatic PSI and targeted soil screening program.