The Trust is carrying out a trial using willows (Salix) to rectify pollution of acidic tars in a lagoon and restore the site so that it can become a diverse habitat. The location is Cinderhill in Derbyshire. This trial is the largest yet anywhere.
Answers to the questions asked regarding Cinderhill.
Why bother?
The reason we wish to remediate the site is that it is at present a risk to human health as well as being a blight to the surrounding area, the rainwater that fell onto its surface prior to the start of the project became acidic and destroyed all life when it ran off the surface of the tar. This run off is a perceived threat to local streams and watercourses in the area. It is a danger to all vertebrate and invertebrate fauna that might come in contact with the surface.
What are the problems?
The tar within the pit is the remains of recycled oils from the automotive industry. The practise began during World War II when oil was in limited supply and in huge demand. Specialist companies, usually in remote locations, away from the centres of engine or armament manufacture took all waste oils and rerefined them. The refining required high strength sulphuric acids to break contaminants away from the waste oil. This waste was the disposed of in any hole that was available to be a depository. The recycling of oil in this way continued to the 1980 when a more environmentally method was developed.
The acidic nature of the cleaning process leaves the sludge wastes highly acidic, with a pH of 1. This is hazardous and can cause severe burns to skin. The oil waste itself is recognised as a carcinogen that is known to contain Polyaromatic Hydrocarbons PaHs) if ingested.
What are the main contaminants?
PaHs are carcinogenic and need to be treated when in large volumes, they are though an everyday contaminant of many areas including tourist spots such as the Forest of Dean, where ground is exposed or in streams across the area. They come from the residues of coal and oil refining, wood distilling and many other historic industrial processes.
Sulphuric acid is distilled from the breakdown of sulphurous metal ores, coal and metals. It is easy to manufacture and as such is cheap to make: thus, it is too costly to recycle.
What are PaHs?
These are common materials that as already stated above are deleterious to health. They are still common in air, soil and water around the globe. A residue of hydrocarbons. They are found along roads in varying quantities along roads emanating from the internal combustion of petrol and diesel. They may also be a by-product of some electric vehicles due to oils and grease being heated by the engines drive mechanisms.
Why willows and not oak or birch?
Willows, are considered most suitable for the decontamination, growing on acid tars because they have unique properties that other trees do not possess. They have both shallow and deep roots so are both anchored and searching downwards constantly looking for moisture. This, one they are established can be taken by breaking down acids. Trials have demonstrated that willows are able to grow well in acidic (pH <2 to 3) mine water in Cornwall. (Severn Trent Ltd / Clean Rivers Trust 1995). The birch is shallow rooted and takes the majority of its nutrients and moisture from the surface layers of any growing area. They like willow are swift growing trees but are, over time susceptible to lack of rain and acidity, pH <4.5. Oaks are deep rooted and are tolerant of acidic conditions, they are though much slower growing.
The roots of the willow or any fast-growing tree can transport the naturally occurring bacteria that is on the surface of the tar, and transport them, slowly, initially into the tar itself. This action transforms the heavy sludges into a dark friable soil, the acidity and hydrocarbon elements such as PaHs being transformed to CO2 and Oxygen and Esters that are vented from various nodes in its structure, branch, twig and leaf. The acidic materials are also transformed into non sulphurous material.
In certain instances, one might consider poplar as an alternative tree option, but their acid tolerance is not so high, and the number of node exhalation points are not as great in number.
What are bacteria?
Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically, a few micrometres in length, bacteria have several shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth and are present in most of its habitats.
Bacteria are the workhorses of the whole scheme. Some types have the ability to break down any material. They are naturally occurring and may be expected to be found in or on sites where there are issues of contamination after several years. The natural processes of breaking down acidic tars by bacteria is often noticeable at the edges of such sites.
What do bacteria do to tar?
Bacteria can do many things to tars, degrade to a soil like substance, leaving behind a material able to mix with decaying vegetation. They will, in a flooded environment demonstrate the breakdown of oils in tars by producing oil emulsions, usually a brown excretion which is an available food source for other bacteria. Both signs have been observed on site and at other sites close by.
The processes are by their nature slow, but by making the environment richer in nutrients and aiding bacteria gain access to the tar below the surface faster the process of remediation can be accelerated.
Where did you get the idea?
We have over the last 30 years looked at many plants and their abilities to cope in acidic environments and at the same time engage with bacteria. At Wheal Jane and Tregembo Marsh in Cornwall we identified willows as able to take up and hold metals in acidic environments with pH<3. We further discovered that certain bacteria though not present at all times are drawn rapidly to sites that have benefit to them, food or energy source. The wetland trial at Wheal Jane demonstrated this remarkably with bacteria flourishing within the site that had only been known to be present on the Rio Tinto in Spain and in the acidic springs in Yellowstone Park in the US.
The Cinderhill site had been abandoned for forty years and the edges of the tar surface showed signs of developing crumb, the nearby flooded tar deposit demonstrated the presence of a wide range of bacteria and algae with both emulsions and flowering present.
A quick greenhouse trial with a sample of tar surface and willow cuttings demonstrated the view that bacteria such as were required were needed as tar pH dropped rapidly thus allowing that a full scale trial was likely to prove itself efficacious to the site.
Is it working?
Yes. The first year of growth was severely hampered by a drought that carried on throughout the growing season. Many willows died due to the heat, some of the cuttings were cooked in surface temperatures >40oC. The surviving saplings demonstrated their tolerance of the acidic conditions and the surface of the tar was stabilised where before it quaked.
These findings allowed us to replant in the early spring of 2019 with more than 85% take of plants. Analysis of the tar and plants over the late winter of 2019/20 showed acidy neutralised to the depth of the roots into the tar, tar breakdown and a granular deposit replacing it. Carcinogenic material had been broken down and neutralised.
How long will it take?
The process is not a speedy fix but should with some nurture of the willows allow for the full depth of the tar to be decontaminated over 20 or so years. There is no set time, the project is the first of its type to be carried out globally, and as such monitoring and sampling will need to be carried out over a prolonged period. The results though so far demonstrate that the contamination is unlikely to damage human health as it was previously.
Is it hurting wildlife?
No quite the contrary, amphibians, birds and mammals, from mice to deer visit the site to feed. The site has also developed a diverse flora that was not present prior to the trial.
Is it a cop out?
It is not, it is a low-cost method of bringing a site back to being ecologically useful.
By being low cost does not mean second rate: the price is not cheap and the results are to take longer to demonstrate than those that might have been deployed such as soil washing and encapsulation or burning in a cement kiln. Reburying elsewhere has also been a favoured solution by some.
None of these could be achieved without the despoliation of the uncontaminated site, destroying ecologically significant habitat. The other negatives being the high costs which would be around tens of millions of pounds. Alongside the cost health and safety issues may well preclude many methods and options.
Wouldn’t it be better to dig it up and put it elsewhere?
Why should any other area have to have this blight placed in it. It is far better to treat it onsite and remove the dangers progressively over time.
Why not just put a concrete cap over the site?
It would be a simple option but not one that could be seen as sustainable. To leave the tar beneath a concrete cover is only delaying the inevitable need to treat it.
Could it be burnt in an incinerator?
The tars and acids could be separated out and the tars made suitable for burning at a concrete kiln. The digging out of the waste, separating the fractions, and transporting from site let alone the incineration cost preclude this. The processes would be expensive, dangerous and could be seen as unallowable.
Could it not be dug up and dumped at sea?
This was done when the rebuilt A38 was put through part of the area requiring some smaller tar lagoons to be removed. Problems were several, damage to the lorries used due to acid eating the containers, damage of acid on the motorways and other roads. These are minor compared to the damage of
How do willows work?
Is there a secret ingredient?
No secret ingredients, all that is needed is an understanding of the problem and a knowledge of natural processes.
Harvey Wood
26th May 2020