The Thai steel company SSI was unable to continue steel making on the banks of the River Tees without financial support, as the UK government opted not to step in to keep the steel making going. Subsequently the South Tees Site Company (STSC) was set up to maintain the assets abandoned by SSI. The broader South Tees site was defined as the 4,500acres of what had been largely the different steel works, including British Steel (Chinese owned), PD Ports' Teesport, NWL's Bran Sand water treatment plant, Hanson Cement, Redcar Bulk Terminal, BOC, landfill sites and a number of other businesses within it.

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STSC was essential to safeguard the dangers the site presented as it still had 1000s of tonnes of chemicals left over from the previous site operations and as such the site is classified as an Upper Tier COMAH (Control Of Major Accident Hazards). Most of the hazardous chemicals were remnants of the coke oven byproduct plants where the coal tar, which is distilled from coal during the making of coke, is processed. The site COMAH status highlights what makes the site so dangerous. These abstracts show the dangers present with information about relevant dangerous substances which could cause a major accident:

and an example of the way in which the hazards could occur:

In fact a considerable amount of pyrophobic (catches fire in presence of oxygen) materials were present and a 7km coke oven gas main had to be kept under nitrogen to avoid fire and explosion. The COMAH status refers to one specific incident and it does not cover the effect of less catastrophic events which could be expected with decaying industrial plants, such as simple leaks or the incorrect disposal of cleaning water. The South Tees is highly complex, but its development is happening a breakneck speed, normally with the cheapest tender by many different firms, so knowledge of the site is limited. This is likely to make simple mistakes more likely due to unfamiliarity of the people working on the site, not unlike a caretaker flushing the sediment from a tank into the River Yealm which contained large amounts of tri-butyl tin so resulted in massive death of marine life (Banned chemical spilled into conservation area).

The danger of the work required to decontaminate parts of the site was highlighted by WSG, the contractor, which announced in December 2022 that the 7km coke oven gas main had finally been successfully decontaminated (WSG completes major waste project).

Bearing in mind the aquatic toxicity of the materials on the site, all users of the site and the River Tees should have been made aware of the risks posed.

In October 2021 the entire crab population up and down the coast from the mouth of the Tees was wiped out. A huge number of dead and dying crabs washed up very rapidly onto beaches and fisherman returned from normally plentiful crab areas with no catch. So this was not only a natural disaster, but also a livelihood disaster for local fishermen.

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The picture was taken 5th October 2021 on South Gare Beach showing the thick covering of dead crabs that appeared overnight

A combination of government sponsored agencies looked for the cause of the die-off and in the spring of 2022, they announced that the cause had been a perfectly natural algal bloom - Joint agency investigation into Teesside and Yorkshire Coast Crab and Lobster mortalities. Many local people were surprised as they had not noticed any bloom and some of the evidence was photographs that had been supplied by a fisherman in a previous incident report about contamination by rust. The report also commented on the high level of pyridine found in the dead crabs.

The North East Fishing Collective crowd funded for their own investigation supported by many local people also concerned about the environment. A pre-print of the NEFC sponsored Northern Academics work was produced which demonstrated not only pyridine was toxic to crabs, but also that the dispersion of material dumped off the Tees Estuary from an unusually large maintenance dredging operation corresponded to the areas where crabs died off - Determining the toxicity and potential for environmental transport of pyridine using the brown crab Cancer pagurus (L.).

In October 2022 the Parliamentary Environmental Food and Rural Affairs Committee (EFRA) held a public session to gather evidence about the crab die-off, as well as the government sponsored agencies, they also heard from the North East Fishing Collective about the scale of the disaster and Dr Gary Caldwell from the University of Newcastle on the sponsored academic research carried out at the universities of Newcastle, Durham, Hull and York (transcript of session). EFRA concluded that the previous report was not supported by the evidence they were given and requested that DEFRA ask the government chief scientist, Professor Patrick Valance, to pull together an independent panel of experts to look into the cause of the crab die off and carry out an extensive survey of the River Tees to determine what contamination exists(EFRA Chair letter to DEFRA 1st November 2022, DEFRA response to EFRA 15th November 2022).

In late 2022 the DEFRA chief scientist, Professor Gideon Henderson, put together a panel of experts to look into all the evidence. This panel reported in early 2023 and concluded that it was as likely as not that the crab die-off was caused by an unknown novel pathogen - Independent expert assessment of unusual crustacean mortality in the North-east of England in 2021 and 2022. The report also specifically ruled out capital dredging carried out as part of the Freeport development, maintenance dredging of the Tees, an algal bloom, pyridine or any other chemical cause, worryingly the report did not make any comment about the possibility that a combination of chemicals could have caused the die off.

DEFRA did not respond to the request to survey the River Tees for contamination.

Pyridine has been present extensively on the banks of the Tees, both because it was used as chemical ingredient in a number of chemical plants and as a minor component of the coal tar produced as a by-product of the multiple coke works present that have operated on the South Tees since 1840 (Possible Major Point Sources of Pyridine Discharge to Tees). However, pyridine is both volatile and liable to biological breakdown, so it will not persist in its free state in the river for more than approximately 8 days. This was one of the reasons why the DEFRA independent panel dismissed pyridine as a cause. The report also quoted a paper (The Search for a Volatile Human Specific Marker in the Decomposition Process) which stated pyridine was formed in tissue after death, but this paper did not look at crab tissue and in fact said that pyridine could be used to show that tissue samples were of human origin. The fact that pyridine had never been seen in high concentration in the Tees was also stated, but as sampling for pyridine has never been more frequent than monthly this is not relevant as it is known that pyridine disappears rapidly, so would only be seen if coincidentally dumped at the time of the sampling.

However, the NEFC sponsored Northern Academics had detected, seven months after the die-off, low levels of pyridine in the sediment both in the Tees estuary and at the dredging disposal site. This implies that the concentration of pyridine had been far higher when the dredged material was dumped, as it is known that pyridine is rapidly removed from the aquatic environment.

Bizarrely, the DEFRA chief scientist used the pyridine concentration from seven months after dredging to argue that this showed that there was only a very small level of pyridine present. So asserting that pyridine would disappear rapidly, but any found was the total that had been there seven months previously is inconsistent.

So it is felt that the independent report could no more rule out chemical contamination as the cause of the crab mortality than it could rule in the novel pathogen. Frustratingly this left the ultimate cause of the die off unknown. Bearing in mind the massive ecological damage that had occurred and the impact on livelihoods, the likelihood that a novel pathogen could be spread around the world in the bilge water of cargo ships, then the logical action of the agencies should surely have been to continue to search for the pathogen they believed was the cause. Instead the agencies declared the matter closed, stating that any future incidents would be investigated.

The academics have continued their research looking into whether pyridine is a natural product in tissue when crabs die, multi-chemical contaminant toxicity based on the chemicals that are present in the River Tees sediments and further sampling of the River Tees.

Sadly die offs have continued to be reported (Chronology of Marine Die-offs), but as the Environment Agency Log shows these have been classified as either natural occurrences or insignificant meaning no further investigation has been happening.

The River Tees has been used for over 140 years as a chemical sewer and as such it is known that the sediment is highly contaminated. So fishermen suspected that an accelerated dredge in September 2021 had released large amounts of chemical contamination and was the cause of the die-off. PD Ports, the statutory harbour authority for the River Tees, carry out the regular maintenance dredging, argued that maintenance dredging happens every year and no die-offs have previously occurred, this could not be the cause. It was only in the EFRA sessions that they submitted that the specific dredge had been carried out rapidly as a hired dredger was being used and an underwater landslip had occurred which necessitated the channel being cleared(transcript of session|https://committees.parliament.uk/oralevidence/11451/pdf/]).

So did this maintenance dredging accidentally deposit highly contaminated sediment at sea rather than the mixture of washed in sand and washed down soil / peat that is normally dredged?

The South Tees site was rebranded Teesworks, and Teesworks UK has been adopted for the company responsible for developing the site.

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Since SSI collapsed in 2015, the contamination of the South Tees site has been highlighted on many occasions, for example to underline the need for compulsory purchase order to bring it into public ownership, in the South Tees Regeneration Mater Plan to show the need to remediate the site. So when the crab die-off happened it was surprising that people didn't immediately question whether some of the stated large amounts (1000s tonnes) of chemicals highly toxic to the aquatic environment weren't the cause. Instead the accelerated dredging that happened in late September 2021 was assumed by many local people to be the cause. Once people started to think about Teesworks, this was conflated with the dredging, allowing Teesworks to state without fear of correction that they had not carried out any dredging prior to the crab die-off.

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However, Teesworks had been carrying out extensive land based operations prior to the die-off(Teesworks - Timeline of groundworks including prior to October 2021). These included removing heavy oil tanks, driving piles into the ground, demolishing structures and blowing up structures as part of the demolition operations. Teesworks in 2021 held 1000s of tonnes of highly aquatically toxic liquids in the decaying remnant SSI structures, specifically coke oven by-product plants, tanks, and pipework across the site.

The ground under Teesworks was and is still highly contaminated due to the multiple industrial uses of the land. The coal tar produced by the coke oven by-product plants will be present across large areas of the site and is the reason why coke oven, coal tar and gas-works sites normally take decades to clean up (The Avenue Coke Works Remediation Project).

The presence of coal tar and related coal derived hydrocarbons is what makes decontaminating so difficult. Coal tar is in the class of ground contaminants called dense non-aqueous liquids (DNAPLs), as it is only slowly soluble in water and being heavier than water sinks to the first impermeable layer it finds. In 2003 the Environment Agency produced a handbook (An illustrated handbook of DNAPL transport and fate in the subsurface) to explain the likely ways in which DNAPLs contaminate ground and explain how such complex materials should be decontaminated from any brownfield sites - this schematic shows the complex way in which coal tar is able to sink in the ground, but can travel into a river.

{html}<img src=“./Supporting/Release of coal tar DNAPL into Triasic Sandstone.jpg” alt=“Schematic from Environment Agency - 2003 An illustrated handbook of DNAPL transport and fate in the subsurface” width=“100%” />{/html}

The handbook first states that all sources of DNAPLs must be located then it gives two decontamination options, either literally remove all the sources of DNAPLs from a site or entomb the DNAPLs so they are unable to contaminate surrounding ground combined with long term monitoring to ensure leakage is not happening. The Avenue Coke Works Remediation Project took the first option, as the land was going to used for housing and nature, as it was considered that a clean site was required. Sites such as St Anthony's Tar Works in Newcastle after two failed decontamination attempts when contamination was still being found on adjoining properties after decontamination, took the second option, by placing an impermeable buried wall underground and capping the surface to ensure no coal tar can escape.

Teesworks has not followed either of these approaches, instead the ground is simply being capped with clean soil to protect workers and any contaminated excavations are being cleaned before reuse on site. This is because it has been assumed, that as limited site sampling has not found coal tar, coal tar does not exist and so it is not necessary to deal with DNAPLs. This makes Teesworks unique as all other coal tar related sites assume that the ground will be contaminated with coal tar as the chances of finding all of it are very low due to its dispersed localised nature - so either like the Avenue actively cleaned all ground (over 2 million tonnes) or like St Anthony entomb and measure.

The River Tees is a highly turbid river, so it is impossible to see if coal tar is seeping up onto the riverbed, however the image below shows a river next to a coke oven with a puddle of coal tar with house bricks for scale.

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Teesworks has taken the gamble that coal tar will not travel into any of the water on or surrounding the site.

MPs requested that the independent panel look into Teesworks' on land activities (Teesworks - Timeline of groundworks including prior to October 2021) which could have contributed to the crab die-off. The official response was that the land activities did not contribute as nothing was happening within 20m of the River Tees. This answer ignores the extensive streams, pipes, conduits, culverts and other structures that crisscross the site and provide routes for material to enter the Tees from anywhere on the site. It also ignores the possibility that, so close to the water, the works had disturbed coal tar which had sunk through the ground, causing the coal tar to be mobilised and travel into the River Tees. Coal tar is a complex mixture of toxic hydrocarbons which contains pyridine and pyridine derivatives.

Of course carrying out explosive demolition near to decaying structures containing 1000s of tonnes of aquatically toxic material was always going to be a gamble, doesn't the crab die off show that this gamble didn't pay off?

That Teesworks is the most obvious / simplest cause of the die offs appears completely reasonable as little needs to be assumed, but we don't have all the evidence largely because the agencies with the ability to find it haven't actually looked. Teesworks' COMAH status states that the site presents a major risk to the aquatic environment and the Environment Agency said that “These decontamination operations should be completed prior to any demolition or longer term restoration of the site.” (Teesworks - Was this when it all went wrong), in fact there was no decontamination before work started to demolish structures on Teesworks and then there was a mass aquatic die-off event. Isn't it obvious that this was the cause? COMAH said it was hazardous and the Environment Agency said don't do it the way that they did.

When all the information about Teesworks is taken into account, it is as likely as not that Teesworks was the cause of the crab die off, either through direct dispersion of material from Teesworks that had entered the River Tees or indirectly through settling onto the Tees riverbed and then being picked up inadvertently by the accelerated maintenance dredging for disposal at sea.

You will find a lot of further details around what has gone on on this site:

• Overview of the Teesside Freeport
• Teesworks - Planning Officer Reports by Geography - these reports give you an idea of what is in the full planning applications - up to over 100 documents with over 2000 pages in a few documents
• Teesworks - South Tees Planning Applications - for definitive application documents go to Redcar and Cleveland Borough Council Planning Portal where you will need to search for the specific Teesworks applications
• River Tees - Marine Licenses - for definitive application documents go to MMO' Marine Case Management System where you will need to search for specific Teesworks applications
• River Tees - Water Quality Archive - links to relevant Environment Agency water sampling
• Teesworks - Environmental - Conceptual Site Models
• Teesworks - Some Risks
• and lots more

I presented my understand of the environmental risks that Teesworks presents to the Joint Councils' Crustacean Death Working Group on 1st September, you can find a re-recording of the presentation I gave here - JCCDCWG 1st September 2023.