SGVs: simply not good enough
Hayley Moore and Geoff Card call for a review of the guidance for blended natural soils and recycled green waste, to prevent the over-remediation of sitesThe failure of imported topsoil to meet nationally set guidelines for hydrocarbon contamination on a major development has led engineering consultancy Card Geotechnics to review the standards.
Holmethorpe Quarry in Redhill, Surrey, is the site of a 40ha residential development called Watercolour. The development consists of apartments, houses and shops. There will also be social and leisure facilities. A canal, water courses, reed beds and two lagoons on the southern edge of the site have been retained as a wildlife sanctuary.
The site was formerly Holmethorpe Sand Quarry, which was backfilled with up to 16m of inert and commercial waste materials. Card has been working for the local council and advising the developer's project team on ground works and landfill gas assessment, together with specifications for imported materials, including topsoil.
Remediation standards are based on soil guideline values (SGVs). These guideline values have also been used for imported topsoil, together with additional criteria published by the Chartered Institute of Environmental Health. SGVs testing provides criteria for soils contaminated with petroleum-based, chemically produced hydrocarbons - such as coal or mineral-oil derivatives. But the analyses are unable to distinguish hydrocarbons derived from natural organic degradation products.
Failing the test
For this reason, many sources of topsoil - particularly material derived from recycled green waste rich in organic material - can fail SGVs for imported topsoil. Initially, the majority of proposed topsoil for the Watercolour development was rejected due to elevated concentrations of aromatic hydrocarbons. This was unexpected, given the nature of the topsoil - natural loamy clay with composted organic matter from recycled green waste.
In its research into the soil chemistry of natural materials, Card found current SGVs are not appropriate for topsoil and other materials with green waste-derived organic matter typically greater than 5%. Where the organic content exceeds 5%, the concentration of hydrocarbons can exceed the SGVs. This results in the SGVs exceeding naturally occurring concentrations in many organic-rich soils and compost materials used for gardens and landscaping. Commercially available topsoil can have an organic content of 25%. Card's studies indicate the strict application of SGVs to compost soils comprising recycled green waste breaks down, particularly for hydrocarbons.
Topsoil produced by blending various soils and green wastes can include many organic substances that contain naturally occurring hydrocarbons similar to petroleum hydrocarbons in their molecular formula.
Taking account of the chemistry
Natural fibres such as wood, grass, and vegetable matter contain humus material, which comprises polysaccharides, cellulose, amino acids, terpenes, ketones and degraded lignin. These can be falsely accounted in the laboratory. Card's discussions with chemists have indicated that, while some natural hydrocarbons will be removed from the material during the testing process, humic acids and associated derivatives can remain.
This means topsoil rich in humus could feasibly be reported as containing elevated concentrations of petroleum hydrocarbons. Chemical analysis of commercially available bagged topsoil and compost resulted in the same hydrocarbon fingerprint as imported topsoil delivered to Watercolour. Strict interpretation of the SGVs would result in such materials being banned from sale for garden and landscaping use.
The issue was raised during a question-and-answer session at the end of a conference on the subject of British Standards specification for topsoil. It was agreed separate values for topsoil, and other organic-rich material, is required. But it was also noted there were many issues surrounding topsoil to be resolved. This did not appear to be a high priority due to a number of pressing issues, such as removal of topsoil from site under waste codes.
The issues faced by the Watercolour development are well publicised at present, with industry calls for a single set of SGVs that will not lead to the over-remediation of sites. The approach being used on this site is just another example that illustrates current SGVs are unworkable in many situations.
Card has devised in-house guidance values for topsoil and organic-rich soils. This criteria was adopted at Watercolour to enable completion of private gardens and
landscape areas using topsoil derived from blended natural soils and recycled green waste.
Hayley Moore is an engineer and Geoff Card is chairman at Card Geotechnics
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