Birse cleans up in Castleford

Modern science is being called upon to remedy the environmental impact of our past industrial achievements. As our industrial heritage continues to be celebrated, the physical legacies that lie beneath our scarred landscapes require an altogether more scientific solution.

Contaminated land at the former Fryston and Wheldale collieries in Castleford, West Yorkshire has been undergoing environmental treatment thanks to a unique, sustainable remediation programme led by main contractor Birse Civils and designer Scott Wilson.

The 74ha site, which will ultimately provide a mix of housing and public open space, contained around 33ha of land that 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 lead to the death of vegetation and an increased risk of pollution to water courses.

Dr Jamie Cutting, environmental geochemist at Scott Wilson, explains: "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.

"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."

Land remediation

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 pH3.5, and provide 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 an alternative to previously available methods.

Cutting adds: "As part of our onsite geochemical testing facility [set up to test metals and polycyclic aromatic hydrocarbons], we conducted accelerated weathering tests 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.It 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, the steel slag fines produce a powerful and slow release liming effect, which can help to correct the acidity from 3.5pH to an alkaline 6pH. "Materials thatrelease the lime immediately are only effective for a limited duration," he adds. 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."

The development of Tarmac Tonic follows a five-year research project. The University of Birmingham and Harper Adams University tested the product's performance at two 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.

Application rate

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, explains: "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."

The tests revealed that these three materials have helped to increase and maintain pH values from high acidity to neutral levels, andaccelerated testing demonstrated that it can provide a barrier to future contamination for around 20 years.

"With around 20,000ha 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 unsustainable and financially costly capping or dig-and-dump approaches," Robinson says. "Critically, this is a sustainable approach, which re-uses under-utilised by-product materials to tackle the legacy of our industrial past."