Part 2A of the Environmental Protection Act 1990 was introduced by the Environment Act 1995. It came into force on 1 April 2000. Contaminated land is defined in Part 2A as follows:
“any land which appears to the local authority in whose area it is situated to be in such a condition, by reason of substances in, on, or under the land that:
(a) significant harm is being caused or there is a significant possibility of such harm being caused; or
(b) pollution of controlled waters is being, or is likely to be caused”.
Local Authorities have a duty under the act to identify land within their area that is likely to fall under this definition. If following investigation, the land is formally determined as being contaminated, the Local Authority must ensure that it is remediated to an acceptable standard. Formal determination that land is contaminated is based on identifying what are known as significant pollutant linkages between sources of contamination and receptors via an existing migration pathway (i.e. the pollution can reach someone or something and cause them or it harm). It should be noted that Part 2A only considers current land use and existing migration pathways. The authority must demonstrate that there is a significant possibility of significant harm (SPOSH) being caused to a receptor, or that actual harm is occurring.
The presence of gas in the ground at high concentrations alone does not constitute SPOSH. There must be a credible pathway for it to reach a receptor and sufficient volumes of gas must be generated to allow it to migrate in sufficient quantities to pose a hazard. The gas screening values (GSV’s) in CIRIA C665 cannot be used to determine whether a site is considered to be a significant risk of significant harm (although they can be used in a preliminary screening assessment to identify where further risk assessment may be required).
The guidance on SPOSH does not really address the issue of landfill or ground gas. The following framework is proposed by EPG for the definition of SPOSH on gassing sites:
1. The site investigation must have identified with reasonable confidence that there are credible and realistic pathways for gas to reach a receptor. Examples of things that need to be considered are provided below.
Figure 1: Site with credible pollutant linkages in one area and none in another
Figure 2: Site with potential gas migration in rock below a site but no credible pollutant linkage
The assessment of credible pathways should consider geology, topography, nature of soils and rocks and distance to potential receptors. It should always consider preferential pathways via more permeable layers in the ground, services, etc. Usually it is gas migration via these pathways that causes problems rather than bulk migration through the whole ground mass. A Part IIA investigation should always include collection of buried service plans.
2. Once credible pathways have been identified an analysis and/or measurement of methane or other gas concentrations inside the occupied parts of buildings (or unoccupied parts where there may be an ignition source) will be necessary. The results can be used with the table below to assist in defining the level of risk. The flow of gas into the building for Part IIA assessments should be based on analysis of gas migration through the ground and into the building by advection or diffusion. Simple assessments using the gas screening value and Pecksen’s correlation of borehole flow to surface emission rate should be avoided. The assessment should be based on driving pressures and the permeability and effective diffusion coefficients for the soils/rocks below a site. The assessment should consider internal pressure inside a house and, if appropriate, atmospheric pressure variations.
The table below considers the estimated risk of adverse health effects occurring and the confidence in the available data. The shading of each cell in the table denotes whether a site is considered to be contaminated land, possibly contaminated land or not contaminated land. Within each cell is a statement on whether further assessment is required. Further assessment may comprise risk analysis and/or collection of further data. This should be targeted to remove identified uncertainty in the risk assessment and gas flow modelling. Quite often the most crucial data is NOT gas monitoring but details of floor slab construction and the ground conditions below buildings.
Examples of good, reasonable and poor confidence
Examples of what is required for good, reasonable and poor confidence are provided below. The final decision should be agreed jointly by all those involved in the assessment. The requirements should not be seen as prescriptive, but rather an aid to judgement.
Good confidence
Nature of source material is well characterised, potential pathways well characterised and understood, likely volume of gas generation is characterised, impact of ground gas relationships/variations is understood, building foundation and floor slab construction known. Multiple lines of evidence available.
Reasonable confidence
Nature of source material is well characterised, potential pathways are understood and reasonable parameters can be assigned, likely volume of gas generation can be estimated, building foundation and floor slab construction is known with reasonable certainty. Multiple lines of evidence available.
Poor confidence
Limited information/understanding of source material, pathways, volume of gas generation, building foundations and floor slab construction. Many assumptions and limited lines of evidence.
Note: the base table, definitions of risk and requirements for further assessment were developed by Rob Ivens and Judith Lowe. EPG has only added the specific values for landfill gas.
Multiple lines of evidence (MLOE) is using different information to determine the likely risk associated with ground gas. The lines of evidence have to use different information or assessment methods. For ground gas the following are considered separate lines of evidence:
- Desk study identifying age and nature of gas source – whether it is likely to be producing large volumes of gas.
- Desk study to identify how long receptors have been exposed to source and whether there have been any reported issues (odours, drowsiness, etc).
- Site investigation to give visual assessment of gas source together with gas generation modelling.
- Flow modelling of gas migration through the ground and into buildings.
- Confirmation that gas migration is occurring by gas monitoring in the ground.
- Flux chamber testing of surface emissions
- Confirmation of gas ingress into buildings by internal gas monitoring.
Note
This is a living document and will be subject to updating based on experience and comments. We would be grateful for any comments on the framework, especially from local authority contaminated land officers.