Last edited 20 Aug 2018

Contaminated land for construction

Approved Document C, Site preparation and resistance to contaminants and moisture, defines a ‘contaminant’ as: ‘Any substance which is or may become harmful to persons or buildings, including substances which are corrosive, explosive, flammable, radioactive or toxic’.

Contaminated land is land that presents a hazard in the form of material that has the potential for harm. For example, a landfill site may contain contaminated land.

Assessment of the risk of harm is based on the likelihood, frequency and seriousness of adverse consequences, which might include:

Threats to human health.
Damage to flora and fauna.
Contamination of ground water.
Damage to foundations and structures.
Settlement
Subsidence.
Migration of contaminants to adjacent land.

The introduction of the Environmental Protection Act has been the driving force behind the treatment of contaminated land. The main types of contaminant identified are:

Toxic or carcinogenic chemicals such as cyanide, arsenic, mercury and benzene.
Toxic or phytotoxic metals such as lead, chromium, nickel, copper, cadmium and zinc.
Organic contaminants such as oils, solvents and phenols.
Corrosive substances such as acids and sulphates.
Flammable, toxic or asphyxiating gases such as methane, hydrogen sulphide, and carbon dioxide.
Combustible material.
Asbestos.
Radioactive substances.

Some likely sources of contamination are:

Animal and animal products processing works.
Asbestos works.
Cable burning and bonfire sites.
Ceramics, cement and asphalt manufacturing works.
Chemical works.
Dockyards and dockland.
Engineering works.
Garden soils, especially in the capital can contain cadmium.
Gasworks can result in the presence of cyanide and phenol.
Industries making or using wood preservatives.
Landfill and other waste disposal sites.
Metal treatment and finishing.
Mining and extraction industries can result in arsenic contamination.
Munitions production and testing sites.
Oil storage and distribution sites.
Paint and dyestuff industries.
Paper and printing works.
Petrol stations and refineries.
Power stations.
Railway land, especially large sidings and depots.
Road vehicle fuelling, service and repair: garages and filling stations
Scrap yards.
Sewage farms and works can result in zinc and copper contamination .
Tanneries.
Textile works and dye works.

If contamination is suspected, desktop studies of site history should be carried out to establish whether there is a need for further investigation. Sources for a desktop study might include:

Maps: Ordinance Survey (current and historical), geological survey maps and town plans.
Statutory authority and utilities data, such as local authorities, river purification boards, the Health and Safety Executive and the National River Agency.
Trade information from directories and trade associations.
Photographic records, particularly aerial shots.
Technical data from public literature.
Knowledge gained from adjacent development.
Anecdotal information from libraries, local residents and local newspapers.
Meteorological, mining and hydro-geological records.

If a desktop study raises concerns, or if the history of a site is not fully known, then trial pits and borehole investigations should be undertaken, concentrating on the areas of greatest suspicion. Tests may be based on soil samples taken at a variety of depths and locations in order to determine the nature and level of contamination as well as its extent. Initially, this is likely to be at least nine samples per hectare, but more testing may be necessary depending on what is found.

There are three main clean-up techniques:

Excavation and removal of contaminated soil followed by either disposal or off-site treatment.
Limiting the spread of the contamination.
Using a treatment to destroy, remove or detoxify containments.

Remediation can be in-situ (on site on undisturbed soil) or ex-situ (applied to excavated soil either on or off site).

TECHNIQUE	DETAILS
Soil removal	Followed by: Disposal. Off-site treatment.
Containment	For example, 'pathway interdiction' using high density polythene as a membrane both horizontally and vertically as a capping, encapsulating the contaminants and separating them from new construction. This option is chosen if other techniques result in unrealistic costs or create potential hazards.
Soil treatment	Contamination can be: Destroyed. Removed. Detoxified. This can be in-situ or ex-situ.

The main types of soil treatment are:

Biological treatment.
Thermal treatment / desorption (using heat to increase the volatility of contaminants so they can be removed).
Chemical immobilisation / stabilisation / solidification.
Washing (injecting clean water and extracting contaminated water).
Soil vapour extraction using vacuum extraction (this is particularly effective with volatile chemicals, such as petrol and chlorinated solvents).

Biological treatment, also known as bioremediation, is the most common technique. It utilises microorganisms and plants and is particularly suitable for fuel-based contaminants. Microbes 'eat' the chemicals found in oil spills, digesting them to produce water and carbon dioxide. For the bacteria to grow, the right temperature, nutrients and amount of oxygen must be provided. This can be achieved by pumping in air and other substances such as molasses.

In some countries, the cold weather conditions means that the soil has to be excavated and cleaned above ground with the help of heaters, and an oxygen supply. Bioremediation allows cleaning on site, generally it does not require much labour or equipment and so is usually cheaper than other methods.

Although some solutions are cheaper than others, the cost of site investigation and soil treatment is still significant. Developers suggest that it should be up to the government to cover the cost of cleaning up contaminated land, otherwise, the need to pass on costs to purchasers means that it will not always be possible to provide affordable housing on such sites.

NB: Government grants for cleaning up contaminated land ended in 2017.