It is a relatively rare occurrence that a self builder’s dream plot will be a greenfield plot that has not been previously developed.
Often, plots come in different guises: they can be home to run down properties that are ripe for renovation or conversion or brownfield sites that may have been used for industrial purposes in the past.
Those looking to build in more urban areas are likely to build on brownfield sites but need to watch out for ground contamination from its previous uses.
How Do I Know if my Site is Contaminated?
You’d expect to find contaminated soil on brownfield sites where, perhaps, an industrial building, petrol station or coal yard stood.
However, its presence is not always so obvious. A site history check of your plot may reveal a lighter industrial past or agricultural use resulting in contamination. For example:
- An old timber yard site may be rich in copper-chrome-arsenic (CCA) from timber preservative spilled over years of wood treatment
- A garage plot may be rinsed with hydrocarbons from spilt fuel
- Even agricultural land can be polluted by insecticides from years of crop spraying.
Even when nature has recovered the land, the legacy of its past can remain in the soil after the source of pollution has been forgotten.
Identifying Site Contamination
Start with a desktop study. This is an in-depth historical check of the plot, and the first of up to four stages to work through. It’s also often made a condition of planning permission.
Even where it isn’t a planning condition, requirement C1 of the Building Regulations requires precautions to protect health and safety from contamination in the ground.
What if Site Contamination Levels are Low?
Even when there are many contaminants on a site, often the levels are extremely low and below the action level set for each contaminant at which they can be a risk to our health.
Some chemicals and metals present may not be directly harmful but they could be damaging to eco systems or the groundwater and so they do affect us indirectly.
This means that the controls allow for a surprisingly wide range of circumstances, for example:
- residents growing their own food
- aquifer areas where groundwater is collected for our water supplies
- soil digestion by young children.
Also bear in mind that small traces of some contaminants are still a threat.
Assessing and Dealing with Contaminated Land
Environmental or geo-technical consultants carry out contaminated land assessments in four stages:
- Desktop study (including conceptual site model)
- Site investigation report (including site walkover survey and soil analysis)
- Remediation strategy
- Validation and certification.
These look at the site history, geology and location and provide the baseline about whether the site is likely to be contaminated. The CSM (conceptual site model) included is a risk assessment that identifies the receptor pathway from the source to the recipient.
What is a Receptor Path?
A receptor path is the link between contamination and human harm, such as via a watercourse, underground aquifer or growing vegetables in the garden.
Where found in a presence exceeding the set action level for each contaminant set out in the Building Regulations, they may well have to be dealt with, but only when there is a defined receptor path through which they can cause us harm.
In the case of ‘mobile’ contaminants, some will have definite paths such as radon gas or methane, where the vapour will head to the surface. Other chemicals can sink to pollute the groundwater.
Site investigation report
If the desktop study recommends it, this report will involve a site walkover survey and soil sampling to identify:
- any chemicals present
- their concentration levels
- whether they are inert or mobile
It could even involve drilling boreholes to test groundwater for contamination or to monitor rising gas levels.
Soil and water samples are usually sent to a laboratory for analysis but soil screening for volatile organic compounds (VOCs) and petroleum hydrocarbons (TPH) can also be carried out on site using a photo-ionisation detector.
Your garden surrounding the home is the main problem. Soil will be valuable at the surface as a growing medium for vegetables and it will be exposed for children to come into contact with. For these reasons alone, contaminated land has to be dealt with effectively.
This is usually a specialist contractor’s job. The report needs to conclude with any remedial treatment necessary and most importantly, an analysis of the cost.
If there are particular hotspots on the plot then options may include:
- remove the hotspot by bulk excavation and refill with topsoil
- cover over by a hard-surface driveway and parking area
- dilute or bio-treat the affected area
It is also possible that continual long-term monitoring of the soil is needed, particularly where contamination of groundwater is a risk.
Given the ‘perceived risk’ generated by such monitoring exercises and the threat of devaluing or blighting your new home, you might want to think long and hard about building on a plot where long-term monitoring is required.
Common Contamination Solutions
- Removing contaminated soil. A licence for transporting it to a landfill site will be needed
- Biochemical or fungal treatment to stabilise the contaminants. Metals, like lead and iron, can’t be treated in this way
- Flushing contaminants out with water or other chemicals as a method of washing the soil can also be done with some contaminants. Even vacuum suction treatment can deal with petrol hydrocarbons and other volatile pollutants
- Containing the contamination within it by an impervious layer that prevents it from surfacing. Clay is a natural containing material that is often used to line landfill sites
Validation and certification
The re-testing and certifying of the soil after treatment is the final part of the process, since it will prove that protection is effective or the ground has been effectively cleansed.
Your building control officer and mostly likely your home warranty surveyor will require copies of this certification before your new home is deemed safe for occupation.