You will learn
From a list of findings to a picture of the site
By now you have a long list: old tanks, a filled pond, soft clay, groundwater, a river, knotweed, an old drain. A list is not yet an answer. This lesson shows you how to turn it into a clear picture of what could go wrong, for whom, and how badly.
- Explain sources, pathways and receptors, and what makes a contaminant linkage.
- Build a conceptual site model, covering both contamination and the ground.
- Rank the risks, and suggest zones that keep the investigation team safe.
Why it matters
No link, no risk
Contamination in the ground is not, on its own, a risk. It only becomes a risk if it can reach someone or something that it can harm.
Oily soil buried under a car park, with no way to reach anyone, may be a low risk. The same soil in the garden of a new home, where children play, is a very different matter.
The point
The conceptual site model is how you show which findings really matter. It is the heart of the desk study, and the part the reader will look at first.
The idea, in plain words
Source, pathway, receptor
Sources
Fuel from old tanks, metals from industry, ash, asbestos in rubble, gas from landfill or filled ground.
Pathways
Touching or swallowing soil, breathing dust or vapour, gas moving into buildings, contamination carried by groundwater, an old drain to a river.
Receptors
People (future users, workers, neighbours), water (groundwater, rivers), wildlife and habitats, and property such as buildings and water pipes.
Break one link, and the risk goes
This is also how contamination is dealt with: remove the source, block the pathway, or protect the receptor. A clean cover layer in gardens, for example, blocks the pathway between buried soil and the people using the garden.
The idea, in plain words
Two models in one
A desk study for a building project needs two pictures of the site:
The contamination model
The sources, pathways and receptors, and which linkages are possible. This is what protects people and the environment.
The ground model
The layers you expect, how thick and how strong they might be, where the groundwater is, and the features that affect design and construction: soft ground, filled hollows, obstructions and flooding.
Both are preliminary. They are your best picture from existing information, and the ground investigation will test them. Write them so they can be updated, not defended.
Key words
Conceptual site model (CSM): a written, tabled or drawn picture of the site, its possible contamination linkages and its ground conditions.
Contaminant linkage: a source, a pathway and a receptor that connect.
Preliminary: based on existing information only, to be checked by investigation.
The idea, in plain words
Ranking the risks
Not every linkage matters equally. For each one, ask two questions:
- How likely is it? Is the source probably there, and is the pathway open?
- How bad would it be? A minor nuisance, or serious harm to people or a river?
Put the two together to rank the risk. Here is a simple version:
| Mild | Medium | Severe | |
|---|---|---|---|
| Likely | Moderate | High | High |
| Possible | Low | Moderate | High |
| Unlikely | Low | Low | Moderate |
Likelihood down the side, consequence across the top. Many UK practitioners use a more detailed matrix based on CIRIA report C552, with four levels of each and five levels of risk. Use the version your organisation uses, and say which in the report.
Ranking is a judgement, so show your reasons
Two engineers may rank the same linkage differently. That is fine, as long as each says why: “Likely, because the tank is shown for 70 years and staining was seen on the walkover.”
The idea, in plain words
Keeping the investigation team safe
The people who drill and dig on the site will be the first to touch any contamination. UK drilling industry guidance, from the British Drilling Association, sorts sites or parts of sites into three categories, and says the category should be decided from the desk study:
No significant contamination expected. Normal good hygiene.
Some contamination possible. Extra precautions and protective equipment.
Significant contamination likely. Stricter controls, and specialist advice.
This is a broad summary. The guidance itself sets out the controls for each category.
Not knowing is not “Green”
A common mistake in reviews is to call a site Green because nothing specific has been found, when it sits on made ground or on a working industrial site. Green should be earned by the evidence, not assumed. If in doubt, it is Yellow.
Contamination can change the pipes
Some contaminants, such as fuels and solvents, can pass through plastic water pipes. Where the desk study finds them likely, the water company may require a protective pipe material through that ground. Flagging it early avoids a redesign late in the project.
On our site
The Brindlemere Yard model
Everything from Lessons 3 to 6 comes together here. Look for findings from each lesson.
Brindlemere Yard: Desk Study Report (fictional example)
7. Preliminary conceptual site model
Contaminant linkages
| Source | Pathway | Receptor | Risk (likelihood × consequence) |
|---|---|---|---|
| Fuel from former tanks (north) and garage | Contact with soil in gardens; vapour into homes | Future residents | High Likely × Medium |
| Fuel from former tanks | Groundwater in the sand and gravel | River Brindle | Moderate Possible × Medium |
| Metals from the iron works and plating shop (north-east) | Contact with soil and dust in gardens | Future residents | Moderate Possible × Medium |
| Contaminated water in the old drain | The drain itself, straight to the river | River Brindle | Moderate Possible × Medium |
| Ground gas from the filled pond (south-east) | Migration into homes | Future residents | High Possible × Severe |
| Ground gas from the former tip (150 m south) | Migration through the ground | Future residents | Moderate Unlikely × Severe |
| Possible asbestos in demolition rubble | Breathing dust | Workers, residents | High Possible × Severe |
| Fuel in the ground | Through plastic water pipes | Drinking water | Moderate Possible × Medium |
Preliminary ground model. Made ground, about 1 to 2.5 m thick, with buried slabs and foundations, over soft clay near the river, over sand and gravel, over mudstone. Groundwater at about 2 to 3 m. Soft filled ground in the south-east hollow. Flood Zone 3 strip along the eastern boundary.
Zones for the investigation.
No part of the site is Green, because made ground is expected everywhere.
The highest risks are fuel near the former tanks, ground gas from the filled pond, and possible asbestos in the rubble. These need the most attention in the investigation.
Notice the drain and the plating shop
Two of these linkages came from the walkover, not from any record. The model is only as good as all the evidence behind it.
Your turn
Build your site’s model
Wherever you are
Source, pathway and receptor is used around the world, and so is ranking risk by likelihood and consequence. The names of the risk levels and the safety categories vary, so use your own country’s guidance where it exists, and say which you used.
Task (25 minutes)
- List the possible sources, pathways and receptors for your site, from your answers to Lessons 3 to 6.
- Make a linkage table like ours: Source, Pathway, Receptor, Risk, with a one-line reason for each rank.
- Write a short preliminary ground model: layers, groundwater and features that affect construction.
- Sketch the site and mark the zones you would suggest for the investigation team.
Quick check
Five quick questions
This is not a test. It simply shows you whether you are ready for Lesson 8. Choose an answer to see the explanation.
Key points
What to take away
- A risk needs a source, a pathway and a receptor. No link, no risk.
- Break one link, and the risk goes. That is how contamination is dealt with.
- A desk study needs a contamination model and a ground model, both preliminary.
- Rank each linkage by likelihood and consequence, and give your reasons.
- Suggest zones for the investigation team from the evidence. Not knowing is not Green.
Where to read more (free)
- LCRM Stage 1: risk assessment (Environment Agency)
- Part 2A Contaminated Land Statutory Guidance (Defra, PDF)
Up next: Lesson 8
Writing the report. Putting it all together into a clear, honest desk study, with recommendations a client can act on.