Cinderhill Acid Tar Pit, Cinderhill, Denby, Derbyshire by Clean Rivers Trust
Shortlisted for Category 2 - Best Scientific/Technical/Digital Advance and Category 4 - Best Application of Remediation Technologies
‘This trial is internationally significant as a method for managing acid tar pits, with the potential for remediation in situ, whilst protecting people from hazardous materials and improving the ecological condition of the affected sites.’
University of Nottingham, Independent Report on the Cinderhill Project.
In January 2018 Clean Rivers Trust were asked by Harworth Group plc to research the situation of Schedule IIA contaminated land near Belper in Derbyshire. This land was polluted by the legal disposal of recycled sump oil residues that had been deposited in old clay pits in 1976. The Trust carried out the research required but also suggested that the tar pits might be remediated using natural methods rather than those originally envisaged, which were either encapsulation using a concrete cap over each filled pit or the removal and reinterring elsewhere.
The Trust was asked for a proposal in April 2018: Harworth Group gave the go ahead later that month for a full-sized trial on the most unattractive of five available tar depositories on site. The chosen site was rectangular in form and surrounded by woodland. The surface was made up of black tar with no growth or deposits on the surface and gave off a strong hydrocarbon odour. Each time rain fell on the site, runoff was acidic, pH 1. The tar was soft, quaking, anything placed on the surface sank slowly below its surface.
The tars have been analysed and reported on in 1994, 2002 and 2007, this later report for the local authority who took it to the Minister for it to be determined as contaminated land. It was accepted that the site was judged detrimental (carcinogenic) to children and young people. The oils were judged high in PaHs and a source of acid burn.
The design brief for the site required the method to:
deny persons easy entry of the site,
remove the acidity,
create a biodiverse surface suitable for ecological recovery,
degrade the tar and remediate over time.
The Trust had previously (1993-6) carried out research on acidic tolerant plants and trees, it had also worked on stabilisation of dredgings and other quaking materials. The proposal took account of these pieces of research and a very short greenhouse trial of plant growth response to the tar was carried out at our premises in Birmingham.
The use of naturally occurring bacteria which was believed to be on the surface of the tar pit was to be a vital element in the project. There were signs of crumbing on the surface of parts of the tar surface which was considered to demonstrate the presence of such. Another tar pit close by which was covered by a surface of standing water demonstrated this was likely as there were algal blooms developing.
The proposal included the use of spent mushroom compost and the planting of a selection of cloned biotypes of willows. These were to be planted in either constructed mats or single laterals on or just beneath the compost layer. Some other cuttings were to be directly driven into the tar.
The planting took place in the last week of May 2018 and the site was left to settle due to the intense rainfall at the time. This was the last rain till September. Over the next three months the site required irrigation twice a week and even then, this did not allow the developing roots to survive across 50% of the site beyond their first flush of sprouting. Surface temperatures on the tarpit surface reached >40o C in full sun and ‘cooked’ many of the young plants.
By the end of the 2018 drought it became apparent that a proportion of the site would need to be replanted at the time of the planned addition of compost across the site. This was carried out in January/February 2019. With the spring came the growth that should have occurred in the first year across the site. Throughout the summer 2019 the willows grew, and roots began to enter the tars beneath the compost. Some damage to willow growth was unavoidable when a bore hole was put down at one edge of the site. This action vented a visible gas from within the tar which instantly defoliated those trees close by and downwind of the operation. Some trees did regreen, but several died. This though was only on a small fraction of the site around the monitoring point used by the University of Nottingham who had been commissioned by Harworth Group plc to independently monitor and access the trial. (A one-page summary of their findings is attached.)
Throughout the period ecological monitoring was carried out to record changes and see the effects that work was having on the habitat value: from a bare acid surface where no creature could survive, the site was ringed by rotting snail shells and body parts of amphibians touched by the acid run off from rainfall onto the site. Within the first months frogs were noted to be traveling across the site, water puddled across the area was no longer acid being pH 7. In the spring a Great Crested newt was found. Birds were feeding amongst the willows off aphids, badger and fox were passing through the latter hunting rabbits that were eating some of the young willows. Deer began to visit the willows and began to damage the plantings at the edges necessitating the erection of a stock fence around the outside of the site.
By the end of the 2019 growing season much of the willow cover was between 2 and 3 metres in height and beginning to branch at low levels. Throughout the winter the site was monitored and by February 2020 the willows were flowering and leaf buds opening.
The conclusion of the project demonstrated that the acid tar was being broken down by the willow roots, which took naturally occurring bacteria on the surface of the site into the tar thus breaking it down as it penetrated. The process continues as the roots continue to develop downwards. The gaseous elements being transferred via the willow roots to the body of the plant is discharged via the various nodes of the tree, including branch and leaf. The residual tars will be broken down into a sand-like structure.
The project has demonstrated that this method of remediation:
removes hydrocarbon odour,
stabilises a quaking surface,
stops acidification of run off,
denies easy access to acid tars,
degrades the tar,
allows the establishment of ecologically rich habitat,
some of the growing willows can be used for cuttings on similar sites for further remediation.
The site has been visited by the Environment Agency, Natural England, the local authority, Wildlife Trust and several other organisations who have an interest in this high-profile site. Industry has visited and expressed interest as there are other similar polluted sites locally as well as nationally and across Europe.
At the end of February 2020, the Independent Final Report on the project by the University of Nottingham was delivered to Harworth Group plc who allowed Clean Rivers Trust sight of it. The conclusions of the report prompted this award entry.
The work outlined above has allowed for small trials of other pollutions, polluted materials that have proved satisfactory. Phosphorus compounds, glaze waste, rubber waste and PCBs.
Stuart Ashton and Harworth Group plc, Matt Johnson and University of Nottingham, Nathan Coop, Sophie Annable and Eddie Peat.