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Date: 14 October 2008
Dear Mr Card,
Freedom of Information Request
The Environment Agency has been passed a copy of your letter to Thanet District Council, dated 28th August 2008. The Environment Agency received this request on 1st October 2008 from Thanet District Council. The letter was passed to the
Environment Agency Groundwater and Contaminated Land team to enable us to provide you with additional details on the investigation and technical assessment relating to the contamination of the Chalk aquifer and the groundwater beneath the former Sericol site. This response seeks to provide responses to the questions you have raised (particularly questions 1 and 2) where the Environment Agency are the lead authority and hold the relevant documents rather than Thanet District Council.
I have noted that you have requested “all the information” held on the Sericol site but I am hoping that a summary of the main facts and details of the investigation will help answer your query, as we have several boxes of information on the detailed site investigation and subsequent remediation. In order to answer the questions fully I will, firstly, explain the Environment Agency's involvement, outline the remediation and the investigation in a summary form and, secondly, relate the individual questions raised in your letter to the work carried out.
The Environment Agency (predecessor National Rivers Authority) first became aware of a potential solvent spill at the Sericol site in Poorhole Lane in the early 1990s. Leon King, the Managing Director of Sericol, called a meeting with water quality and groundwater quality staff from the Environment Agency and legal representatives for both parties. At the meeting it was explained that Sericol were investigating the potential loss of a particular solvent, Cyclohexanone, to ground. At that stage all potential routes for the loss were still being investigated including short delivery, stealing and a loss to ground.
It is our understanding that loss of solvent from the system had been identified when a new stock management system had been installed at Sericol. Investigations were in progress. A pressure test had been carried out on the solvent storage tank and the below ground pipelines to assess the integrity of the system. No leaks had been detected. As a result Sericol management had reviewed all other possibilities including short deliveries, spillage and stealing. They also continued to investigate whether there was a leak to ground. They had carried out some limited intrusive investigations to assess whether there was any sign of solvent in the chalk. It was at this stage that they started discussions with the Environment Agency.
As part of a more thorough investigation of the pipes and tanks Sericol then carried out additional positive pressure tests and vacuum tests on the pipework (to test the system using different methods). While the positive pressure test resulted in a further “pass” the pipe did not hold vacuum during the vacuum test, suggesting that there was a small hole in the below ground pipe leading from the tank to the process area. Sericol then employed specialist consultants to carry out a targeted investigation in the immediate vicinity of the below ground pipeline and solvent tank. As solvents tend to move straight down through the Chalk the investigation had to be targeted right on the point of loss.
As a minimum the Environment Agency required the following:
• Details of the solvent, an assessment of the likely volume lost to ground and the likely duration of the loss
• Full investigation of the depth, extent and the magnitude of the plume of solvent in the Chalk rock and in the groundwater beneath the site (and, longer-term, the details and justification of the remediation scheme to be installed to clean up the groundwater)
• Immediate replacement of the leaking below ground solvent storage facilities to an above ground system with appropriate bunding, stock-control facilities etc along with identification and rectification of other poorly-contained areas of chemical use on the site
Sericol responded on all aspects of the Environment Agency's requests. There were regular meetings attended by Environment Agency, Sericol and their parent company, Water Research Centre (specialist hydrogeological consultants with expertise in Chalk, solvents and water industry issues) and Southern Water. Thanet District Council Environmental Health team also attended some of the key meetings but not all of the technical meetings.
• Sericol immediately replaced the solvent storage tank and pipework with an above ground system with the appropriate level of control to prevent further contamination of the ground and groundwater. This also enabled better access for full investigation of the area.
• The solvent, Cyclohexanone, is commonly used in the inks and dyes trade.
o is a clear fluid - so does not lead to staining of the rock
o has a density of 0.9478 at 200 / 40 - so is slightly lighter than water and moves down through rocks and then floats on the groundwater.
o has a solubility of 8.7g/100 ml at 200 C - so is partially soluble in water
o has a dielectric constant of 18.2 at 200C - so could be classed as a weakly polar solvent
o has a strong odour and can be picked up at a very low taste threshold in water used for drinking
• Sericol had several attempts at determining the potential volume of solvent lost to ground. It was soon apparent that this would not be possible to predict, with any accuracy, as there had been no proper stock control system in prior to the problem being identified (good knowledge of deliveries and production but incomplete details of solvent use in the process) and the tank with the below ground pipeline had been in use for a long time potentially with a small leak throughout its lifetime.
• Boreholes were drilled in to the Chalk to determine the shape (depth and extent) of the plume of Cyclohexanone contamination in the ground and to determine the concentration (amount at any point) of the solvent within the plume. While it had not extended far laterally it had made its way down through the Chalk rock all the way to the groundwater. The solvent had preferentially moved down through fractures and fissures (natural cracks) in the Chalk but had also soaked through the main matrix (big blocks) of Chalk. This column of contaminated Chalk was “mapped” out from the information derived from the investigation boreholes, detailing the depth, extent and magnitude of contamination in the rock above the groundwater. The contamination in the Chalk by the solvent means that the block of Chalk will continue to act as an on-going source of contamination to the groundwater for many years.
• Deeper boreholes were drilled in to the groundwater, initially near the area where the tank and pipeline had been and, subsequently, further out in order to establish the full extent of the plume of contaminated groundwater. Particular investigation was carried out to see if the contamination had migrated towards the public water supply abstraction system (which had been switched off). Again, the solvent had found preferential flow paths through the fractures in the Chalk.
• With immediate effect a system was put in to prevent the movement of the plume of contaminated groundwater and to draw back the plume and treat the water where possible. This was achieved by operating a system of three deep pumps that controlled groundwater levels and brought the contaminated water up for treatment.
• Additionally the on-going source of solvents leaching out of the column of contaminated Chalk rock was treated by using a system known as Soil Vapour Extraction (SVE). This effectively pulled the solvent from the rock by applying a vacuum to a series of several boreholes. The solvent rich vapour was then treated in a treatment plant that condensed the vapours and retrieved the solvents. This generated heat which was fed back into the ground to help enhance the movement of solvents from the ground in to the vacuumed vapour by heating. The treatment plant included a full vapour recovery system so that solvents were not discharged to the air.
• Other methods were used to help speed up the removal of Cyclohexanone from the column of Chalk, including controlled discharge of water through the column. This method was successful for a time.
• By measuring the “burn-rate” of the reaction in the treatment plant it has been possible to calculate the total amount of solvent retrieved from the Chalk and the groundwater.
A pilot remediation scheme was put in to start with and soon developed to a full
scale system. Remediation has been underway at full scale since late 1996. The
remediation scheme has worked very well with a present total of nearly 470000 kg
(470 tonnes) of solvent having been removed from the Chalk aquifer.
There have been very few interruptions to the remediation in the last few years. There have been a few planned down-times for each pump in order to remove the build-up of biofouling, which is a natural occurrence resulting from an accumulation of micro-organisms. In areas of groundwater that have a large plume of contamination the natural micro-organisms in the ground start to slowly break down the solvent. As part of the process they use up the oxygen and produce iron as a by-product. It is, therefore, not uncommon for this increased microbiological activity to cause a build-up on the pumps and infrastructure within a plume and to require regular cleaning. It gives rise to a red/orange coloured solid material. There have also been a few unplanned stops caused by issues during the redevelopment of the site by new contractors, during work to replace the road and pavement in Poorhole Lane and when groundwater levels suddenly rose at the site due to the water main being severed. During the events the alarm systems and action taken resulted in adequate containment of the plume of groundwater contamination.
The control of the plume of contaminated groundwater by pumping and the removal of solvents from the column of Chalk above the groundwater has been on-going since the initial installation of the remediation system. At present this is being reviewed by Sericol, their present consultants and the Environment Agency. As with all remediation technologies there comes a point the amount of contamination being removed from the ground diminishes and eventually there is no benefit in continuing with the technology. The remediation phase is then assessed and future remediation options (including further technology and monitoring) are implemented. In time, when all available clean-up options have been exhausted or cost-benefit-analysis and environmental impact assessment shows there is no benefit in continuing the remediation scheme will be signed off as complete.
(1) Did the investigation obtain a figure for the volume of missing solvent? (i.e. From the Sericol stock and production records)
The investigation did obtain a range of figures for the volume of Cyclohexanone that was estimated to be missing. From the outset the estimate, however, was not regarded as accurate as there were too many unknowns in the assessment, including how long the leak had been occurring for and what the actual throughput of solvent was. The most accurate figure available from the investigation is the volume of Cyclohexanone removed from the ground by the remediation, which is presently 470 tonnes.
(2) Did the extent and depth of chalk staining accord with the volume of
solvent missing or was there reason to corroborate Sericol's alleged first suspicion that there had been theft of solvent and that spillage occurred during the process of thefts?
Given the solvent losses detected by Sericol and the initial facts (apparently a secure tank and pipeline) the company procedure to look at all possibilities including theft, spillage and losses to ground is standard for most companies.
The subsequent findings, including a small hole in the below ground pipeline and the presence of solvent in the ground, confirmed that one of the ways that the solvent had gone missing was a leak to ground from the solvent storage at Sericol.
Cyclohexanone is a clear liquid so did not lead to staining. Assessment of the extent, depth and magnitude (amount) of solvent in the Chalk was made by detailed testing of the Chalk rock when boreholes were drilled in the spill area. Estimates of the likely volume of solvent in the Chalk were attempted. As the Chalk is a fractured limestone the solvents have moved through the fractures (cracks) in the rock to the groundwater and have also soaked in to the main matrix (big blocks) of the Chalk. Calculations of the volume of solvent present were complex and only provided an estimated figure for the solvent lost.
It was not possible to accurately determine the volume of solvents lost from Sericol's storage tanks and lines. It has been possible to determine how much solvent has been removed from the Chalk to date. It was not possible to determine how much very dilute solvent has flowed away in the groundwater. It is not easy to estimate the volume of solvent still held up in the Chalk. As a result it is not possible to determine whether the volume of missing solvent matches with the volume of solvent accounted for in the Chalk, in the groundwater and that removed from the Chalk.
It is clear, however, that the volume of solvent retrieved from the ground far exceeds the volume of solvent thought to be lost originally. As the investigation has progressed the onus was on solvent loss to ground rather than any more attention being given to determine whether losses had been via stealing or spillage during stealing.
The Environment Agency has been involved with the technical investigation of the groundwater environment and, other than being aware that stealing was being investigated as an option at the beginning, has not been involved in any part of any wider investigation.
(6) What was the name of the solvent involved? Was it polar and water absorbent and would it produce conductive flame if ignited?
Answered in summary report and in Thanet District Council response.
(7) Was the incident reported because red staining of Chalk was a visual giveaway to the eventual whistleblower? And what risk assessments have TDC made for detection and prevention of incidents which may give no visual evidence of occurrence? Surely we should not tolerate having chemical storage where spillage can go to rainwater drain-off to soakaway?
The Environment Agency can confirm that Sericol Senior Management contacted the Environment Agency (then National Rivers Authority) directly to report the loss of solvents to ground. This was because they recognised the potential risk posed to the groundwater and the drinking water supply for the area if the solvent had been lost to the ground. The incident has impacted a large area volume of Chalk and groundwater but Sericol's action, subsequent investigation and clean-up programme prevented the solvent plume spreading and the problem being much worse.
Red staining of the Chalk has never been mentioned. There is some natural red staining of the Chalk due to the small areas of Clay cover above the Chalk. The groundwater pumps can sometimes become covered in a red/orange deposit due to a build up of natural micro-organisms, a process known as biofouling, in significant plumes of contamination. Cyclohexanone itself is clear.
The Thanet District council response covers precautions taken, risk assessments and pollution prevention measures required to safeguard the water environment. In addition the Environment Agency would like to draw your attention to the current guidance document used by planning authorities, industry and regulators to protect groundwater from solvents: “Groundwater Protection Code: Solvent Use and Storage” published by DEFRA in 2004. Copies of this document are available on the DEFRA website. The Environment Agency seeks to prevent pollution of the groundwater via the Planning process, the use of Codes of Practice, awareness campaigns, site visits and, in the unfortunate event of an incident, enforcement action, investigation and remediation.
We hope that this response gives you a full understanding of the solvent and groundwater investigation at the Sericol site and answers your questions fully.
Senior Technical Specialist - Groundwater and Contaminated Land
Kent and East Sussex Area