In line with UK planning policy, standards of remediation have been based on Soil Guideline Values (SGVs) published by the Department for Environment, Food and Rural Affairs (Defra), and supplemented by additional SGV data from the Chartered Institute of Environmental Health.
The SGV testing methodology offers acceptable criteria for soils contaminated with chemically produced hydrocarbons, such as coal or mineral oil derivatives. However, analyses are unable to distinguish hydrocarbons derived from natural organic degradation products.
For this reason many sources of topsoil, particularly material derived from recycled green waste that is rich in organic material, can fail SGV criteria for imported topsoil, despite being available to members of the public at retail outlets.
Our research shows that current SGVs are not appropriate for topsoil and other materials with high green waste-derived organic matter, typically greater than 5%.
Where the organic content exceeds 5% the concentration of hydrocarbons can exceed the SGVs because of naturally occurring concentrations of hydrocarbons. Commercially available topsoil can have an organic content of 25%.
Our studies indicate that the strict application of the SGV criteria and test methodology to soils comprising recycled green waste breaks down, particularly for high carbon molecule hydrocarbons.
Topsoil produced by blending various soils and green wastes can include organic substances that contain naturally occurring hydrocarbons that are similar to petroleum hydrocarbons in their molecular formula.
Natural fibres such as wood, grass, leaves and vegetable matter contain humus material, which itself comprises polysaccharides, cellulose, amino acids, terpenes, ketones and degraded lignin that can be falsely accounted in the laboratory.
Discussions with analytical chemists indicate that while some natural hydrocarbons will be removed during the testing process, humic acids and associated derivatives can remain. Thus topsoil that is high in humus could be reported as containing elevated concentrations of petroleum hydrocarbons.
Analyses of commercially available topsoil and compost resulted in the same hydrocarbon fingerprint as imported topsoil delivered to one of our developments.
The project, known as the Watercolour development, is at the former Holmethorpe Sand Quarry in Redhill, Surrey (south east England), a site backfilled with up to 16m of inert and commercial waste materials.
Initially the majority of proposed topsoil for this development was rejected due to elevated concentrations of aromatic hydrocarbons. This was surprising given the nature of the topsoil (natural, loamy clay with composted organic matter from recycled green waste).
It shows that strict interpretation of SGVs would result in commercially available materials being banned from sale for garden and landscaping use.
The issue was raised at the Launching BS 3882:2007 specification for topsoil and requirements for end use conference hosted by BSi, SCI Environment Group and RSC Water Science Forum in November. It was agreed that separate values for topsoil, and therefore other organic rich material, is required. The Environmental Industries Commission is calling for a single set of SGVs that will not lead to over-remediation on sites. However, Defra shows no inclination to revise these values.
The failure of the Watercolour imported topsoil to meet nationally set guidelines led our engineers to review and revise these standards.
Given the uncertainty and lack of government guidance we have devised in-house acceptance values for topsoil and organic-rich soils with typically greater that 10% soil organic matter. We will use these to overcome the difficulties confronting developers seeking approval from regulators for ground remediation.
Geoff Card is chairman and Hayley Moore an engineer at Card Geotechnics.