In 2019, the Association of Geotechnical & Geoenvironmental Specialists (AGS) published formal guidance on the classification of waste soils as either hazardous or non-hazardous. The aim of this was to provide simple and clear advice on best practice - at least when it comes to more straightforward sites - in an area that has often involved confusion.

The disposal of waste soils is often one of the most significant in-ground costs for brownfield developments. It's also a cost that's often not accounted for fully in the early stages of development planning.

Waste versus reuse

In England and Wales, any soil that the holder discards, intends to discard or is required to discard is considered to be waste and thus is subject to the Waste Framework Directive. Note that in Scotland, soil does not become waste until it leaves the site.

Uncontaminated natural soils that are excavated in the course of construction and are to be reused for the purposes of construction are not considered to be waste, and can be reused freely on the same site. If these natural uncontaminated soils are to be reused as a fill material on a different site, you will need one of the following:

• A U1 exemption from the Environment Agency (free of charge but generally limited to 1000 tonnes)
• A Materials Management Plan (MMP) prepared in accordance with the CL:AIRE DoWCoP (Definition of Waste Code of Practice), under which all activity must be carried out

The DoWCoP also permits the reuse of made ground, as distinct from natural soils, on the site of origin, provided that:

• The material is suitable for use without posing a risk of environmental pollution or harm to human health
• No more material is used than is required for that use
• There is certainty that the material will be used and not just stockpiled

Classifying and exporting waste

If waste soil is to be exported from the site, it must be classified in accordance with the guidance provided by the Environment Agency's publication Technical Guidance WM3: Waste Classification - Guidance on the classification and assessment of waste. The waste will end up classified as either:

• Hazardous waste with the waste code 17-05-03 (soil and stones containing hazardous substances)
• Non-hazardous waste with code 17-05-04 (soil and stones other than those mentioned in 17-05-03)

It is an offence to dispose of waste at an inappropriate disposal site, so it isn't legal to just assume, conservatively, that unassessed waste is hazardous. The difficulty is when the specific chemical species of the substances present in the soil are not known and cannot be determined economically, meaning that hazardous properties cannot easily be assigned to the waste. Bear in mind that all potentially hazardous properties of the soil need to be considered, including parameters that are additive between contaminants. This means it is not sufficient to simply compare the results of chemical analyses to a table of threshold values.

The AGS guidance on the application of WM3 to soils addresses many of the challenges in this process, and encourages the adoption of a correct and consistent approach to the classification of waste soils across the industry.

The cost of waste disposal

The difference in disposal cost between hazardous and non-hazardous waste is currently around £25 per tonne. Industry quotes can involve error arising from confusion over the applicable Landfill Tax rate. Landfill Tax is charged as follows:

• Standard rate: £98.60 per tonne (rising to £102.10 in April 2023)
• Reduced rate: £3.15 per tonne (rising to £3.25 in April 2023)

A key point is that only non-hazardous materials that are listed in the 2011 (Qualifying Materials) order can be taxed at the lower rate. For soils this is limited to naturally occurring rocks and soils. As made ground is by its nature not naturally occurring, it will generally not qualify for the reduced rate irrespective of contamination and waste classification.

Landfill Tax is only chargeable on waste sent to landfill. No tax is charged on natural soils reused on site, nor on any materials that are reused under a U1 exemption or the DoWCoP, which can result in considerable savings.

Where to send waste

There are three types of landfill where soils can be sent for disposal:

• Hazardous Landfills that can receive hazardous waste
• Non-Hazardous Landfills that can receive non-hazardous waste
• Inert Landfills, which are for a sub-category within non-hazardous waste

For material to be suitable for disposal at an Inert Landfill, it must not:

• Undergo significant physical, chemical or biological transformation
• Dissolve or leach or produce an ecotoxic leachate
• Combust
• Physically or chemically react
• Degrade
•Adversely affect any matter that it comes in to contact with in a way that is likely to give rise to environmental pollution or harm to human health

Inert wastes are typically from a consistent source that meets all of the above requirements. Made ground commonly will not meet the requirements for inert waste.

Waste Acceptance Criteria testing

WAC testing does not provide a measure of the total hazardous content of the soil (only the leachable content), so WAC data cannot and must not be used for waste classification.

For non-hazardous waste no WAC leaching thresholds have been set, so no WAC testing is required.

Soils being sent to landfill as a hazardous waste or an inert waste will usually require Waste Acceptance Criteria (WAC) testing, to confirm that the material will not produce a hazardous leachate and that it does not contain a significant organic content or sulphate concentration, such that it would breach the licensing requirements of the tip.

Waste classification and ground investigation

Waste classification should be based on analyses that are representative of the waste load, so it is normally not possible to classify the soils at the investigation stage, since the materials that will become waste have generally not been defined yet. Ground investigation data can, however, provide a useful guide to the likely waste classification and can indicate the scale of the classification analyses that will be required after the waste has been defined and before it is generated.