The conceptual site model in a phase 1 desk study

The conceptual site model is the part of a phase 1 desk study that the council's contaminated land officer actually reads. Everything else, the historical maps, the geology, the walkover notes, exists to build it. The model sets out which contaminants might be present, how they could reach something that matters, and what that something is, and it ends with a judgement on whether any of those chains is plausible enough to justify digging holes.

Get it right and a self-build plot can sometimes close out the contamination condition without an intrusive investigation at all. Get it wrong, usually by writing a model that lists every conceivable hazard without ranking any of them, and you buy a phase 2 investigation you did not need.

Source, pathway, receptor

A conceptual site model works on pollutant linkages. A linkage needs all three components: a source of contamination, a pathway by which it can reach a receptor, and a receptor that can be harmed. Break any one link and the risk falls away.

That structure comes from the Environment Agency's land contamination risk management guidance, which sets the framework for preliminary risk assessment in England, and it sits behind the preliminary stage described in BS 10175. It is not a formality. A model that identifies a source and a receptor but never explains the pathway has not done the job, and the officer will say so.

The output is normally a table of linkages plus a cross-section sketch. The sketch is worth the hour it takes. A drawing showing made ground over a sand horizon, the water table, the proposed foundations and the garden, with arrows, communicates more than three pages of description.

Building the source list from records and a walkover

The desk study gathers the evidence. Historical Ordnance Survey maps from the mid-nineteenth century onwards show what stood on the site and next to it, and the sequence matters as much as any single sheet. A pond that vanishes between two editions is a filled pond, and filled means made ground of unknown origin.

Beyond the maps, pull the geology and any borehole records from the British Geological Survey, the Environment Agency's records of permits, historical landfill and pollution incidents, the coal mining position where relevant, radon potential, and local authority records of former uses. Then walk the site. Photograph the ground surface, note any staining, stressed vegetation, odours, surface debris, asbestos-containing materials in old outbuildings, fill material, and any tank fill points or vent pipes.

Typical sources on small sites are dull and repetitive: made ground from earlier demolition, a former garage or workshop with a buried fuel tank, agricultural fuel storage, a sheep dip, asbestos cement sheeting, ash and clinker in garden soil, and imported fill of unknown provenance. Say which of these you found evidence for and which you are ruling out, with the reason.

Pathways depend on what you are building

The same site carries different pathways for different end uses, and this is where the self-builder's model differs from the one prepared for a commercial developer.

For a house with a garden, the pathways that matter are direct contact with and ingestion of soil, particularly by children, inhalation of dust and of vapours indoors, uptake of contaminants into home-grown vegetables, ingress of ground gas into the building, attack on buried water supply pipes, and migration to controlled waters. Allotment use is more sensitive again. A commercial car park with a sealed surface and no gardens removes most of the surface pathways and leaves ground gas and groundwater.

That is why the model has to be written against the actual proposal. A desk study prepared for a previous industrial use does not transfer to a residential scheme, and buying a report with the land is not the same as having an assessment for what you intend to build.

Receptors, including the ones that are not people

List them explicitly. Future occupiers, including children and any garden produce. Construction workers during the build. Neighbouring occupiers during and after the works. Buildings, buried services and foundation concrete, which sulfates and hydrocarbons attack. Controlled waters, meaning the aquifer beneath and any watercourse nearby, with the aquifer designation and any source protection zone stated. And designated ecological sites where they are close enough to be relevant.

The controlled waters receptor is the one self-builders overlook, and it is the one that brings the Environment Agency into the consultation. A site over a principal aquifer in a source protection zone attracts a different level of scrutiny from the same site over unproductive strata.

Ranking the linkages rather than listing them

A model that scores every linkage as moderate risk is no use to anyone. Estimate each one on likelihood and consequence, explain the reasoning in a sentence, and reach a category. Then say what you propose to do about each: no further action, further assessment, or a design measure such as a clean cover system or a gas protection layer.

An illustrative example. A plot in a village carries a demolished barn, an area of ash-rich made ground about 400 millimetres deep, and a former above-ground fuel tank on a concrete base. The proposal is one house with a garden. The model identifies three linkages: metals and polycyclic aromatic hydrocarbons in made ground reaching future residents by contact and produce uptake, hydrocarbons beneath the tank base reaching the same receptors, and ground gas from the made ground entering the building. It ranks the first two as requiring confirmation by sampling and the third as low because the made ground is thin and not putrescible. Figures and circumstances here are illustrative.

That is a model an officer can act on. It tells them what you will sample and why, and it explains what you are not doing.

When the model closes the question and when it does not

A phase 1 can conclude that no plausible pollutant linkages exist. Greenfield agricultural land with no history of built development, no filling and no nearby industry can reach that conclusion honestly, and the report should then recommend no further work beyond a watching brief during groundworks.

More often the model identifies linkages that cannot be resolved from records, and the recommendation is a targeted phase 2 investigation. Targeted is the operative word. A phase 1 that hands the next consultant a clear list of what to sample, where and why produces a faster and less costly investigation than one that says further intrusive works are recommended and stops. Our guide to ground gas assessment covers the monitoring side of that, which is the longest lead item.

Either way the report is the evidence for discharging the standard contamination condition, which usually runs in parts: site characterisation, a remediation scheme where needed, implementation with a verification report, and a clause requiring unexpected contamination found during works to be reported. Read the condition before you commission anything, because the wording tells you exactly what the council expects to receive.

A contaminated land assessment is one of the report types where a qualified professional is legally required rather than optional, because the interpretation has to be defensible. Where ground conditions also affect foundations, a Geotechnical Assessment shares much of the same fieldwork, and combining the two visits saves a mobilisation.

Enter your postcode on PlanScope to see whether contamination is flagged for your site and which reports go with it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.