Conceptual site model

A conceptual site model is the document that decides how much your ground investigation costs. It sets out what contamination might be present, how it could reach something that matters, and what that something is, and from those three lists it produces the exploratory hole positions, the depths, the monitoring wells and the analytical suite. A model built from a thorough desk study produces a targeted investigation. A generic model produces a grid of holes, a standard suite and a second visit.

The structure comes from the land contamination risk management framework and from BS 10175, and the model is revised at every stage rather than written once.

What the model sets out

A conceptual site model is a description and a diagram, not a calculation. It records the site setting, the history, the geology and hydrogeology, the plausible contamination sources, the people and things that could be affected, and the routes between them.

What makes it usable is that every element is specific to the site. "Made ground may be present" tells a reviewer nothing. "Made ground of unknown origin to approximately 1.5m across the northern third of the site, consistent with infilling of the former clay pit shown on the 1912 map" tells them where to dig and what to test for.

The model also records the uncertainties, which is its most underused function. Listing what is not yet known, and what would resolve each point, is how you justify the investigation you are proposing rather than the one the officer imagines.

Source, pathway and receptor

The framework turns on pollutant linkages, and a linkage needs all three components present. Break any one and the risk is not realised.

Sources on a typical brownfield site include made ground, underground tanks and their pipework, interceptors and sumps, asbestos in demolition arisings, hydrocarbon staining in workshop floors, ash and clinker, and landfill gas from adjoining land. Receptors include future occupiers and their gardens, construction workers, buildings and buried services, controlled waters, and plants. Pathways are the routes: ingestion of soil, inhalation of dust or vapour, dermal contact, uptake into produce, leaching to groundwater, and lateral migration of gas along service trenches.

Our note on how pollutant linkages work goes through the linkage logic in detail. The point for this document is that the linkage list, not the source list, drives the investigation.

Where the information comes from

The desk study is where the value is added, and the sources are largely public.

• Historic Ordnance Survey mapping across the full available sequence, which is the single most informative source, as our note on using historic mapping in a contamination desk study sets out.
• Environment Agency records: historic landfill, permitted sites, discharge consents, pollution incidents, groundwater vulnerability and source protection zones.
• British Geological Survey mapping and the borehole record archive.
• Local authority records, including any entry on the contaminated land register and previous planning history.
• Mining and natural cavity records where relevant.
• Trade directories, aerial photography and site specific records such as tank registers or previous investigation reports.
• A walkover with photographs, recording surface evidence, odours, staining, vegetation and remaining structures.

Dates matter. Record the edition and date of every map and record, because a reviewer checking whether a feature was missed will go to the same sources.

How the model drives the investigation scope

Once the linkages are listed, the investigation designs itself. Each plausible linkage needs either evidence that it does not exist or data to assess it.

A suspected underground tank needs holes at and around its recorded position, to a depth below its invert, with a hydrocarbon suite and vapour sampling. Made ground across part of the site needs enough coverage to characterise it, with a broad suite including metals, polycyclic aromatic hydrocarbons and asbestos screening. A gas source on adjoining land needs monitoring wells on the boundary, installed and monitored over several rounds including falling atmospheric pressure. Controlled waters as a receptor needs groundwater sampling and a depth to water measurement rather than soil data alone.

That logic is what turns a cost into a justified scope. It is also what allows you to argue down a validation requirement for a full site grid where the history shows one small former workshop and nothing else.

Updating it after intrusive work

The revised model is the deliverable that closes a planning condition, and it is the version most often missing.

After the investigation, each linkage should be marked as confirmed, discounted or still uncertain, with the data that supports the conclusion. Where a linkage is confirmed, the assessment moves to a comparison against generic assessment criteria and, if necessary, a site specific quantitative assessment. Where it is discounted, say why in a sentence that refers to the data.

Then the remediation strategy addresses the confirmed linkages only, and the verification report demonstrates that the works broke them. A verification report that does not refer back to the model leaves the officer unable to confirm the condition has been met, which is the commonest reason discharge applications are refused on contamination grounds.

An illustrative model for a former garage site

Take a corner plot occupied until recently by a vehicle repair garage with a forecourt, proposed for six flats with a small communal garden. The model would record two underground tanks shown on a 1974 plan, an interceptor at the forecourt low point, a workshop with an inspection pit, made ground of unknown depth across the yard, and asbestos cement roof sheets.

Receptors are the future residents, including children using the communal garden, construction workers during demolition, the proposed buildings and services, and a minor aquifer beneath the site. Pathways are direct contact and ingestion in the garden area, vapour ingress into ground floor flats from hydrocarbons in soil and groundwater, inhalation of dust during construction, and leaching to the aquifer.

The investigation that follows is targeted: holes at both tank positions and the interceptor, deeper holes to characterise groundwater, vapour pins at the proposed building footprint, a broad suite across the made ground and asbestos screening throughout. Figures and features here are illustrative rather than taken from a real site.

What a contaminated land officer checks

Six things, and they are the same at almost every authority.

1. That the model is specific to this site, with a plan and a diagram rather than a page of generic text.
2. That the history has been worked through across the full map sequence, with dates recorded.
3. That every plausible linkage appears, including gas, vapour and controlled waters as well as soil contact.
4. That the analytical suite is justified by the sources identified.
5. That the model has been revised after the intrusive work, with linkages confirmed or discounted against data.
6. That the verification report refers back to the model and the remediation strategy.

A contaminated land assessment has to be prepared by a suitably qualified and experienced person in accordance with the relevant British Standard, and national policy places responsibility for securing a safe development on the developer. A PlanScope draft gives the structure and the policy framing and is a starting point for that professional rather than a substitute for them. Where ground stability is also in issue, the geotechnical assessment should be commissioned in the same investigation so the rig mobilises once.

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.