EHJ January 2005, pages 16-18

The contaminated land regime has its limits, but it has also produced success stories. Marie-Claire Kidd visited the former Mirvale tar distillery in West Yorkshire to find out what happened when this particular site became "special"

Back in 2001, four closed landfills and two chemical works were the first six "special sites" designated under the new contaminated land regime. Among them was the former Mirvale tar distillery on the banks of the river Calder in West Yorkshire, where a cocktail of chemicals had been flowing from groundwater into the river, and which became the first "orphan site" to be remediated under part IIA of the Environmental Protection Act 1990.

The river bank at the site had been seeping with what looked like oil for decades, producing a film on the river surface extending bank-to-bank and for hundreds of metres, and eventually turning into a grey-black sludge that would stick to vegetation and boats.

There was a large plume of volatile organic compounds, phenols, pesticides and herbicides originating from the site, clearly visible on a calm day and producing a distinctive odour. Only when the slick hit a weir 800m downstream did it disappear, unlike the smell.

The culprit was highly toxic creosote, both neat and in various states of decomposition, which is present in large quantities in the land at the former works. It leaches into soil and groundwater, as creosote easily does, and was flowing freely into the river.

It was the police, on a helicopter flight above the site, who first realised the extent of the pollution. They recorded the slick on video and notified the Environment Agency, sparking off an investigation, which led to the eventual determination of the site as contaminated under part IIA, and its successful remediation.

"I've seen the video and it looked quite horrendous," says John Barber, contaminated land officer at the agency's Leeds office. "The police had the perfect viewpoint to see the extent of the pollution. It must have been a day when the river was very flat. They thought there'd been a catastrophic incident and flagged it up to the agency, in fact it had been going on for decades."

Kirklees MDC determined the site as contaminated under part IIA in April 2001, just a year after the new regime was enforced. The contaminant was named as creosote, the pathway was the made ground and alluvial sands and gravels beneath the site and the receptor was groundwater and ultimately the river. Because the contamination affected controlled waters the area was designated a "special site" and immediately fell into the hands of the Environment Agency, which had by then been investigating the site for years.

There is anecdotal evidence that there were works on the site as early as 1890 but the first detailed records show Mirvale was taken over for munitions manufacture during the First World War.

In the 1950s it became a chemical plant, and the scene of a massive fire in the 1970s. The works became known as Mitchell Cotts in 1976, and went through two changes of ownership until Dow Chemical acquired it as part of Ascot plc in June 2000, just two months after the site was determined.

Mitchell Cotts still operates on part of the site, where it manufactures pyrethroids, active pharmaceutical ingredients and intermediates. These include cypermethrin, an insecticide used by farmers to protect crops such as cotton, cereals, orchard fruits and vegetables, permethrin, used in pet collars to kill fleas, permethrin BPC, used in headlice shampoos and scabies creams, and travaprost/prostaglandin, for use in the prescribed drug travatan, which is sold for the treatment of glaucoma. The remainder, which also belongs to Mitchell Cotts and which is thought to be the most contaminated section, is derelict.

But it was during the intervening period, when the site was a tar distillery that the most serious contamination seems to have occurred. Large storage tanks, of about 40,000 gallons capacity, appear to have been emptied into the ground, probably towards the end of the tar distillery's life or on its conversion to a chemical works.

"The original polluter was no longer in existence. The land had been bought from them by Mitchell Cotts," says Mr Barber. "There's nobody on site that was there when the contamination occurred.

"It was in the fifties or even earlier when these tanks were last used," he continues. "In the fifties one was partially demolished. It's probable that was when the contamination occurred. It's assumed they contained creosote."

Mr Barber says organic chemicals are difficult to identify. "They're not just a single simple chemical, they're a mixture of several hundred different chemicals. It could take several years to migrate into the river, and when it migrates its character changes, some parts of the mixture stick to soil, some dissolve, some evaporate into the air, but it still smelt like creosote."

On designation of the area as a special site, the agency undertook further investigations, into both "appropriate people" and options as to the site's remediation. Under the regime, it could seek to recover some but not all of its remediation costs from the landowner, Mitchell Cotts, which as the occupier was deemed a "class B" company and thus liable for part of the clean up operation.

After negotiations between the agency and the company it was agreed that the costs would be split 50-50. The agency would shoulder the £500,000 capital works, thanks to a Defra supplementary credit approval grant, and the company would pay ongoing operational, maintenance and monitoring costs for approximately 20 years - also expected to reach the value of about £500,000.

Following year-long negotiations with Railtrack, which owns the mainline viaduct under which works had to be undertaken, the agency was able to install a cut-off wall and equipment to pump and store the creosote. The works were completed on 9 July 2002.

The interception system, designed to reduce the amount of non-aqueous phased liquid or NAPL entering the river, involves collecting the creosote from a horizontal pipe behind a sheet pile dam of some 150m in length located next to the river upstream of the slick.

NAPL is lighter than water and floats on the surface of groundwater, allowing it to be skimmed off the surface. The trench is equipped with underground skimmer pumps, maintainable via a series of manholes along the banks of the Calder, which pump the creosote via a pipeline to a holding tank on the Mitchell Cotts site. The creosote collected in the tank is disposed off site.

The scheme does not deal with the dissolved part of the contamination plume, which is addressed by dilution in the river - a method that would not be considered ideal by the agency - or the volatile organic compounds.

The results have been remarkable. The residents of Mirfield used to presume that Mitchell Cotts was responsible for the unpleasant smell that occasionally wafted over the town. The clean-up operation has proved them wrong - the chemical works is still there but the smell isn't.

"The amount of creosote that now enters the river has dramatically reduced," says Phil Waine, Environment Agency technical advisor. "Although you get the occasional blob coming up to the surface, you don't get the huge outpourings any more and the smell has gone. Since working on the site, we've regraded the soil and put down grass seed. A lot of self-seeding flowers have grown there and it now looks quite pretty."

Mitchell Cotts is continuing to monitor the scheme in case any more treatment is needed, and is required to submit an annual report to the Environment Agency. Its second annual report is expected in February.

"The main thing was that there was no creosote or creosote products visible on the river and it's achieved that," says Adrienne Goodwin, Dow Chemicals' UK regulatory affairs leader. "There hasn't been since about three weeks after the scheme was installed, which you'd expect because the scheme's got to settle."

While the Environment Agency monitors the river for levels of creosote, the firm checks the equipment on a weekly basis and it employs consultants TES Bretby to sample groundwater across the site every quarter.

"The contamination could divert and we want to make sure that it doesn't. It doesn't look like it is, it's been superbly engineered. It's almost like the advert - it does what it says on the tin," says Ms Goodwin.