Any construction project built on greenfield land is not sustainable. No matter how laudable the rest of the project may be, the use of undeveloped land for construction is indefensible. As our cities grow ever outward, leaving derelict urban gap sites behind them and devouring more and more "green belt" land every day, there is no way that a project which aims to be sustainable can be built on anything other than re-used land.
The issues involved here are as complex as other aspects of Sustainable Construction. The destruction of agricultural land becomes ever more serious as we begin to realise that intensive farming methods have serious drawbacks: development of more environmentally responsible agricultural practices will almost certainly require a larger area of good quality farmland. Also, the development of dormitory towns and out-of-town shopping centres is rarely matched by appropriate sustainable development of communication links, causing knock-on effects such as pollution, congestion, traffic-related illnesses and deaths.
The terms "contaminated land" and "brownfield site" convey a perception of risk to developers that is no longer warranted. The assessment and remediation of these sites is now a major international business. Guarantees and indemnities are available on remediated sites and the economics of development of these sites is attractive. This is demonstrated by the fact that over half of all residential development nation-wide is on re-used sites. Residential development is the most sensitive end use to which these sites can be put, yet it is now more common than not to build houses on "contaminated" land.
A contaminated land investigation typically consists of discrete stages. The first is a desk study, using historical information to establish past uses of the site and its surroundings to give a general feel for the likelihood of contaminative risks.
In addition, information from statutory bodies and local authorities about the site and surrounding areas is reviewed, including data regarding the location of landfill sites, water quality classifications and site investigation reports. A review of the geology and hydrology of the site is usually undertaken as part of a geotechnical investigation, and this information is used as a means of identifying the potential residence and migration of contamination to and from the site.The potential is considered for contamination and risks to future users of the site and to areas outwith the site. Site conditions are assessed to determine:
Many water authorities and companies now require that all new services must be laid in uncontaminated ground. Developers are required to demonstrate that service runs are uncontaminated by providing chemical data on ground conditions, as well as the presence of groundwater and soil gases. As of July 1999, local authorities are required to build up a database of contaminated land within their area. To assist them in this considerable task, many are asking that their planning departments require an environmental site investigation for any proposed change of use, regardless of site history.
The second stage is an investigation of the nature and extent of contamination actually found on the site. This will typically involve intrusive investigations using trial pits, boreholes and gas bar probes. Soil and groundwater samples are taken and analysed for a range of chemical parameters. The chemical parameters will depend to some extent on the past uses of the site, as certain uses imply certain types of contamination. Some examples are given below:
The laboratory chemical test results are usually compared with the guidelines given by the Inter-departmental Committee on the Redevelopment of Contaminated Land ICRCL (Guidance Note 59/83).
Analysis of Chemical Data
The ICRCL publication introduced the concept of Trigger concentrations (Threshold and Action) to assist in determining the extent to which a site is contaminated. Assigned Trigger concentrations vary depending on the proposed end use of the land. For domestic developments where risks exist of direct ingestion of soil or consumption of contaminated food grown on the site, the acceptable levels are lower than for commercial or industrial development where these risks are less.
The significance of concentrations of contaminants in relation to the ICRCL Guidelines is discussed below.
If the remediation options for a particular end use are unacceptable then a re-assessment may be necessary in order to examine alternative use of the site.
ICRCL distinguish between Group A contaminants, which may pose hazards to human health, and Group B contaminants which are phytotoxic (poisonous to plants) but are not normally hazardous to human health.
It should be noted that not all potential contaminants are covered by the ICRCL Guidance Note 59/83. Where other contaminants are present reference must be made to other published criteria such as the Greater London Council (GLC) Guidelines and Dutch Guidelines.
New, updated guidance is expected to be enforced around the middle of 1999, as a result of legislation introduced in the Environment Act 1995.Soil Gas
The site will usually also be investigated for soil gases. Gas can diffuse up to near-surface layers from underlying coal and shale seams. It can also be generated by micro-organisms in the soil through biodegradation of organic materials. In either event, harmful concentrations of gas can build up in the soil and particularly in voids beneath and within buildings.
In the majority of cases, the main gases produced are methane and carbon dioxide. These gases are produced in differing ratios depending on the amount of oxygen present within the ground.
When flammable gases are mixed with air at certain concentration known as the flammable or explosive range, the mixtures may ignite to produce fires and explosions. The investigation analysed gas from a number of trial pits and boreholes for these gases.
If present in sufficient concentrations, carbon dioxide can cause a risk of asphyxiation, especially when the oxygen levels are less than 18% by volume. Guidelines specified in the Health & Safety Guidance Note EH40, Occupational Exposure Limits, state a short-term carbon dioxide exposure limit of 1.5% by volume over 10 minutes, with an occupational exposure level standard of 0.5% by volume over 8 hours. Building Regulations Addendum C suggests specific design measures where carbon dioxide concentrations exceed 5% by volume.
Source-Pathway Receptor Analysis
The risk to a proposed development from contaminated land is usually assessed using Source-Pathway-Receptor analysis. This technique establishes the presence of a chain of events required to present a risk.
First, a source of risk is required. In this context, this means the discovery of one or more chemical parameters in excess of guidance limits or an excess of soil gas. Once a source is discovered on site, the investigation will analyse the presence of any Pathways by which this source can be transmitted to a sensitive receptor. Such pathways include direct ingestion, aerosol exposure, contact etc.
Once a Source and a Pathway have been established, the final link in the chain is a Receptor. A Receptor is essentially anything or anyone that can be adversely affected by the contamination. This includes a buildings occupants, the fabric of the building itself, landscaping plants, local watercourses etc. The Receptor (and indeed the Source) need not be within the boundaries of the site itself. Particularly in areas where the underlying drift geology is relatively porous, the potential exists for contamination to spread to the site from surrounding areas and to surrounding areas from the site.
This means, for example, that a clean site near a landfill site may be affected by soil gas migrating onto the site, even when the site itself has never been put to any potentially contaminative use in the past. Similarly, a contaminated site may adversely affect the quality of a nearby river by runoff or contamination of groundwater.
All these considerations and more must be borne in mind when drawing up a remediation strategy. The nature of this strategy can vary enormously, depending on the size and location of the site, the extent and nature of contamination and the design and use of the proposed development, among other factors. One common theme, however is that it is possible and practical to remediate the vast majority of brownfield sites economically and effectively.
There is no need for brownfield development to carry any additional risk or liability. Brownfield sites often offer excellent locations at good prices and remediation of such land can be counted as a net benefit to the environment, which is rare indeed.
More detailed information is available from Alistair Jamieson.