Phase 1 desk study or Phase 2 site investigation

In the phase 1 and phase 2 contaminated land sequence, the phase 1 desk study is a preliminary risk assessment that identifies whether plausible pollutant linkages exist on the site, and the phase 2 site investigation is intrusive work that measures whether they are real. You commission phase 2 only when phase 1 says you need to. Doing both at once is the commonest way to overspend on a small site, and skipping phase 1 is the commonest way to sample in the wrong places.

Land contamination reports must be prepared by suitably qualified and experienced persons in accordance with BS 10175, and a competent professional is legally required for this work. An AI-assisted draft is a starting point for that professional, never a substitute for the investigation.

What a desk study can conclude on its own

A phase 1 assessment gathers historical Ordnance Survey maps at successive editions, environmental database records, geological and hydrogeological information, landfill and permit records, and any previous site investigation reports. It is completed with a site walkover.

From that it builds a conceptual site model: the sources of contamination that may be present, the pathways by which they could reach a receptor, and the receptors themselves, which on a residential scheme means future occupants, plant growth, groundwater, surface water, building materials and construction workers.

Where the model shows no plausible linkage, the phase 1 can conclude that no further work is required, and councils accept that. A greenfield paddock with no history of made ground, no landfill within the relevant distance and no sensitive groundwater beneath it does not need boreholes. Saying so clearly, with the evidence attached, is a legitimate and valuable output.

Where the model shows a plausible linkage, the phase 1 sets the objectives for the next stage. That is its second function and the one people forget.

The conceptual site model and its role

The model is the spine of the whole exercise and it is revised at every stage. Government guidance on land contamination risk management, which replaced the older model procedures, structures the process as risk assessment, then options appraisal, then remediation and verification, with the conceptual site model updated as information arrives.

A weak model produces a weak investigation. If the phase 1 says only that the site was formerly industrial, the phase 2 will be a grid of boreholes with a standard laboratory suite, which is expensive and often misses the thing that matters.

A good model says where the fuel tank was in 1968, where the made ground is likely to be deepest, which direction groundwater flows, and which of the proposed gardens sits over the former works. The investigation then targets those, and the cost falls even as the confidence rises.

When intrusive investigation becomes unavoidable

Phase 2 is required where the phase 1 identifies a plausible linkage that cannot be discounted from desk information alone. In practice that means most brownfield sites and a good number of apparently innocent ones.

Two triggers are worth planning around. Ground gas monitoring needs several visits over weeks or months, ideally including periods of falling atmospheric pressure, so it cannot be compressed into a fortnight before a committee date. And where asbestos is suspected in made ground, the sampling and analysis strategy is different from a standard suite, and it needs to be specified at the outset.

Sampling strategy and laboratory suites

BS 10175 sets out the approach to investigating potentially contaminated sites, with BS 5930 covering the geotechnical side. Sampling is either targeted at identified sources, systematic on a grid where contamination could be anywhere, or a combination, which is what most real investigations use.

The laboratory suite should follow the conceptual site model. A standard suite covering metals, polycyclic aromatic hydrocarbons, total petroleum hydrocarbons speciated into bands, pH and asbestos screening is the usual baseline, with additional determinands where the history points to them. Solvents, cyanide and sulphate each need to be asked for.

Results are compared first against generic assessment criteria in a generic quantitative risk assessment. Where results exceed those criteria, a detailed quantitative risk assessment with site-specific parameters often shows the risk is acceptable without remediation, and that step is worth taking before pricing a dig and dump exercise.

Ground conditions also drive foundation design, and running the contamination and geotechnical investigations together saves a mobilisation. A Geotechnical Assessment and the contaminated land assessment usually share the same boreholes.

Remediation strategy and verification reporting

Where remediation is needed, the strategy sets out the objectives, the options considered, the chosen approach and the criteria against which success will be judged. Options run from removal, through cover systems and gas protection measures designed to BS 8485, to bioremediation and stabilisation.

The verification report then demonstrates that the strategy was carried out and the criteria met, with validation sampling results, records of materials imported and their testing, photographs, and as-built details of any protection system. It is submitted to discharge the condition, and it is the document a purchaser's solicitor will ask for in ten years time.

Keep it. Sites change hands, consultants close, and a missing verification report turns into a retrospective investigation at the worst possible moment.

Conditions that sequence the two stages

Most English councils use a four-part contaminated land condition, and recognising it saves argument.

1. A site investigation and risk assessment to be submitted and approved before development begins, which is where the phase 1 and, if required, the phase 2 land.
2. A remediation scheme, where the risk assessment shows unacceptable risk.
3. Implementation of the approved scheme and submission of a verification report before occupation.
4. A reporting requirement for unexpected contamination found during works, with development halted until a remediation strategy for it is approved.

Part 4 is the one that catches programmes. A driver hitting a buried tank on a Tuesday stops the job until the authority approves a way forward, so a contingency in the programme is sensible on any site with made ground.

Budgeting a phased investigation

The economics favour phasing. A phase 1 is a fraction of the cost of a phase 2, and it either removes the need for intrusive work or makes that work cheaper by targeting it. Our note on budgeting a contaminated land investigation goes into the cost structure, and the question of who may sign the reports is covered in suitably qualified persons for land contamination reports.

Three budget items are routinely forgotten: the extended ground gas monitoring visits, the detailed quantitative risk assessment that can avoid remediation altogether, and the verification report at the end, which is often priced as an afterthought and then written under time pressure.

Commission the phase 1 before you exchange on the land if you can. It is the cheapest piece of due diligence available, and it occasionally saves the whole transaction.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.