Tarmac proves perfect tonic for remediation

A new land remediation product is working wonders for cleaning up acidic colliery spoil, offering a sustainable alternative to traditional 'dig and dump' methods

Contaminated land at the former Fryston and Wheldale collieries in Castleford, West Yorkshire has been undergoing environmental treatment thanks to a novel sustainable remediation programme.

The 74 hectare site, which will provide a mix of housing and public open space, contained around 33 hectares of land which was severely affected by acidic colliery spoil. This is a common problem found on former industrial sites where pyrite reacts with water and oxygen, to generate sulphuric acid. These conditions can lead to the death of vegetation and an increased risk of pollution to water courses.

"In the past, the immediate solution would have been to cap the sterile colliery spoil with imported topsoil at a cost of around £11 per tonne placed, with a total cost to the scheme in excess of £1.5M," explains Dr Jamie Cutting, environmental geochemist at Scott Wilson, who is leading the remediation project in conjunction with contractor Birse Civils.

He adds: "The application of limestone to neutralise acidity is another technique commonly used in colliery reclamation schemes. However, its inability to provide long-term protection against acid regression is viewed as problematic, requiring expensive maintenance and repeated applications."

Strategic treatment
At Fryston and Wheldale, a multi-component amelioration strategy was designed to treat areas affected by acidic colliery spoil using Tarmac Tonic, a new land remediation product which includes under-utilised steel slag fines to treat acidic soils below 3.5pH, and provide long-term protection against latent acidity.

The product was successfully combined with other imported by-product waste materials such as paper crumb and treated composted sewage sludge to deliver a cost-effective and technologically superior alternative to previously available methods.

Dr Cutting says that as part of its on-site geochemical testing facility, set up to test metals and polycyclic aromatic hydrocarbons, accelerated weathering tests were conducted to provide rapid turnaround data for dynamic control of amelioration works.

This approach provided information on the absolute acid generating potential of colliery shale on a plot-by-plot basis. This enabled the application rates for Tarmac Tonic to be modified and target the most severe areas of potential acidification.

According to Dr Nizar Ghazireh, head of R&D at Tarmac - which helped develop the Tarmac Tonic solution - the steel slag fines produce a powerful and slow release liming effect that can help to correct the acidity from 3.5pH to an alkaline 6pH.

"Materials which release the lime immediately are only effective for a limited duration. At the Fryston and Wheldale sites our modelling work has demonstrated that the treatment will last up to 15 years, and by that time the vegetation will be established again with no further treatment required," he explains.

The development of Tarmac Tonic follows a five-year research project led by Tarmac with its partners, the University of Birmingham and Harper Adams University to test the product's performance at two research trial sites in Cumbria as well as accelerated growth trials in the laboratory to simulate 10 to 20 years of acidity regulation and grass growth. The trials were overseen and supported by the Environment Agency with funding from Defra.

Blending for the better
The composition and the application rate of Tarmac Tonic can be specifically blended to counter the contamination levels found on a particular site and will in future, utilise more organic by-product materials.

Dr Howard Robinson, head of product development at Tarmac, says: "On our trial sites we have successfully combined steel slag fines with limestone dust from our quarries together with certified green compost, a material produced from green-segregated household waste from PAS 100 compliant facilities.

"Tests have revealed that these three materials have consistently helped to increase and maintain pH values from high acidity to neutralised levels and accelerated testing has shown it can provide a barrier to future contamination for around 20 years.

"With around 20,000 hectares affected by colliery spoil across the UK, it is a major step forward to find a durable solution to this problem and which provides contractors with an alternative to financially costly 'capping' or 'dig and dump' approaches."

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