Conceptual site model: how pollutant linkages work
Break the source, the pathway or the receptor and there is no linkage to manage. How the conceptual site model drives the sampling strategy and how to report it.
A pollutant linkage needs three things present and connected: a source, a pathway and a receptor. Break any one of them and there is no linkage, and without a linkage there is no unacceptable risk to manage. That single proposition is what the conceptual site model exists to test, and it is why a good model is worth more to a scheme than another twenty boreholes.
Reviewers see two failure modes. Models that list every conceivable contaminant without connecting any of them to a receptor, and models that quietly drop a linkage after the investigation without saying which evidence closed it.
Source, pathway and receptor as a linkage
A source is a substance in, on or under the ground with the potential to cause harm. Made ground with a coal ash content, a former gasworks retort area, an in-ground fuel tank, imported fill of unknown origin, asbestos fragments in demolition rubble, or a natural elevation such as arsenic in a mineralised area.
A receptor is what could be harmed. Human health, split by user group. Controlled waters, meaning groundwater in a designated aquifer and any surface water body. Property, which covers buildings, services, crops and livestock. And ecological receptors, which in practice means designated sites rather than general habitat.
A pathway is the route between them. Ingestion of soil, dermal contact, inhalation of dust or vapour, uptake into home-grown produce, lateral or vertical migration to groundwater, permeation through water supply pipework, and ingress of ground gas into a confined space.
The discipline is to write each linkage as one sentence naming all three. Hydrocarbons in shallow made ground, by dermal contact and ingestion, to future residents in private gardens. If the sentence cannot be completed, there is no linkage to assess.
How the model is built in a Phase 1 desk study
The preliminary model is assembled from documentary evidence and a walkover, before anything is drilled.
The sources that actually change a model:
- historic Ordnance Survey editions across the full sequence, read for uses that no map legend names, such as unlabelled tanks, ponds that appear and disappear, and rail sidings
- geological and hydrogeological data, including superficial deposits, aquifer designation and any source protection zone
- regulatory records: landfill sites, environmental permits, pollution incidents and previous investigations
- the site walkover, which finds the surface evidence that records miss, including stained ground, odours, drums, fly-tipping and unexplained level changes
- interviews with people who worked on the site, which frequently produce the single most useful fact in the report
The output is a preliminary conceptual site model, a plan and a cross-section, and a schedule of potential pollutant linkages with a qualitative risk rating for each. That schedule is the specification for the intrusive investigation, and a Phase 2 designed without one is a fishing expedition.
The contaminated land assessment covering Phase 1 and Phase 2 is a report type where a suitably qualified and experienced person is required. An AI-assisted draft can structure the reporting, and the professional supplies the judgement, the site work and the sign-off.
Refining the model after intrusive investigation
The model is a living document. It should be revised at the end of every stage, and the revision should be visible.
What a reviewer looks for in a revised model:
- which linkages were confirmed by the data, with reference to specific sample locations and results
- which were closed, and by what evidence, since absence of detection in three samples is not the same as absence of a source
- which were added, because intrusive work regularly finds sources the desk study missed
- how the ground model changed, particularly the depth and lateral extent of made ground and the water table
- whether the proposed development layout has moved since the model was drawn, which changes the receptors
Risk assessment then proceeds in tiers. A qualitative preliminary assessment, then a generic quantitative assessment comparing concentrations against criteria derived for a defined land use scenario, then a detailed quantitative assessment with site-specific parameters where the generic comparison is too conservative to be useful.
Do not skip to the detailed tier because the generic one gave an inconvenient answer. Explain why the generic criteria are inappropriate for this site, with the parameter that drives it.
Where the model drives the sampling strategy
Sampling design follows the linkage schedule, not a template.
Depth is the clearest example. A direct contact linkage to garden users is tested in the top half metre, because that is where exposure happens. A controlled waters linkage is tested at the base of made ground and in the underlying deposits, with groundwater monitoring wells. A vapour linkage needs shallow soil vapour points and a building footprint, not a bulk soil result.
Location follows the same logic. Targeted sampling at identified source areas answers whether the source exists. Systematic sampling across the site, on a grid or herringbone pattern, answers whether contamination is widespread. Most sites need both, and the report should say which points served which purpose.
The analytical suite should match the source. A blanket standard suite run on every sample wastes money on a former agricultural site and misses the determinands that matter on a former dry cleaner or a plating works. Where asbestos is credible, it needs specific sampling and analysis rather than a note that none was observed.
Illustrative example: a former commercial garage with an in-ground tank and workshop area. Twelve targeted trial pits at the tank, pump islands and workshop, six systematic locations across the remainder of the site, shallow samples in the proposed garden areas, two monitoring wells screened in the underlying sand, and a hydrocarbon-focused suite with speciated analysis. The scope is illustrative.
Why a broken linkage means no unacceptable risk
This is the part that saves schemes, and it is under-used.
Contamination at depth beneath a proposed building with a ground bearing slab, no basement, no excavation into the affected horizon and no gas or vapour pathway, presents no human health linkage. It may still present a controlled waters linkage, and those are assessed separately.
Remediation works by breaking linkages rather than by achieving purity. A capping layer with a marker geotextile breaks direct contact. A gas membrane and ventilation layer break the ingress pathway. Barrier pipe breaks the permeation route to drinking water. Removal of a hotspot removes the source. Each is a legitimate answer, and each has to be verified.
Where a linkage is broken by construction rather than by ground treatment, the verification and the long-term management have to be secured. A capping layer is only a barrier while it stays in place, so a materials management plan and, on some sites, a note for future occupiers about not excavating below a stated depth are part of the answer.
Reporting the model so the council can follow it
Contaminated land officers read the linkage table first. Make it the centrepiece.
A workable table gives, per row: the source with its location, the pathway, the receptor, the status as potential, confirmed or closed, the evidence, and the proposed action. Number the rows and carry the numbering through every subsequent report, so the verification report can be read against the Phase 2 without a translation exercise.
Support it with a plan showing sample locations against the sources, and at least one annotated cross-section showing made ground, the water table, the proposed building level and the pathway in question. A cross-section communicates a model in a way that four pages of text does not.
Then check who else will read it. Where the site drains to a watercourse or sits over a principal aquifer, the Environment Agency is likely to be consulted, and on coalfield sites the Coal Authority is. Knowing which statutory consultees will see the report tells you which linkage rows need the most support. Where the same investigation informs foundation design, run the geotechnical assessment from the same ground model rather than commissioning a second set of holes.
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. Enter your postcode on PlanScope to see which ground condition documents your site is likely to need.
Frequently asked questions
What is a pollutant linkage?
A connected combination of a source of contamination, a pathway by which it can reach something, and a receptor that could be harmed. All three must be present and connected. Write each one as a single sentence naming all three elements, and if the sentence cannot be completed then there is no linkage to assess for that combination.
Who counts as a receptor?
Human health, split by user group such as future residents, site workers and neighbours. Controlled waters, meaning groundwater in a designated aquifer and any surface water body. Property, which covers buildings, buried services, crops and livestock. And ecological receptors, which in practice usually means statutorily designated sites rather than general habitat.
Does remediation have to remove all contamination?
No. Remediation works by breaking linkages rather than by achieving purity. A capping layer with a marker geotextile breaks direct contact, a gas membrane and ventilation layer break the ingress pathway, and barrier pipework breaks permeation into drinking water. Each solution has to be verified, and construction-based barriers need long-term management arrangements.
How should sampling depth be decided?
From the pathway being tested. A direct contact linkage to garden users is tested in the top half metre where exposure occurs. A controlled waters linkage is tested at the base of made ground and in the underlying deposits with monitoring wells. A vapour linkage needs soil vapour points rather than bulk soil results.
When should the conceptual site model be updated?
At the end of every stage of work, and visibly. The revision should state which linkages were confirmed and by which results, which were closed and on what evidence, which were added because intrusive work found a new source, and how the ground model or the proposed layout has changed since the previous version.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.