Collstrop Site near Hillerød is fenced in with chains and locks on gates. In addition, signs indicating no access are posted. Photo: Google Streetview.

Arsenic Contamination at Collstrop Site in Hillerød Exceeds Previous Estimates

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New investigations on the Collstrop site in Hillerød reveal that the arsenic contamination is much larger than previously thought.

The contamination on the Collstrop site originates from the heavy metals that the Collstrop company used for impregnating telephone poles and utility poles on the site from 1936 to 1976. The site, which is fenced off and inaccessible to the public, has been designated as one of Denmark’s 10 legacy pollution sites.

To map the extent of the contamination, over 400 boreholes have been made on the site, and more than 4,000 soil samples have been analyzed. The investigations have cost approximately 6 million Danish kroner and are much more comprehensive than previous ones. They reveal that there is closer to 105 tons of arsenic on the Collstrop site, rather than the 35 tons that was previously estimated.

The investigations also show that the contamination is moving very slowly through the soil layers. Despite being over 50 years old, it has not spread to an underlying groundwater reservoir, from which there is otherwise a risk that it could move further towards Esrum Lake.

-“The contamination on the Collstrop site has proven to be extremely extensive and spread out over the entire six-hectare fenced area. The good news is that the contamination has not spread very far down into the ground. Therefore, we are sticking to the plan we have for managing the contamination,” says Charlotte Schleiter, the environmental chief of the Capital Region.

Containing the contamination

The Capital Region’s short-term plan is to contain the contamination on the site. A minor spread of arsenic has been found in the nearby ditches outside the fenced Collstrop site. Next year, the region plans to clean up the ditches and establish a barrier to prevent further spreading.

In the long term, the Capital Region expects that the most sustainable and cost-effective solution will be to leave the majority of the contamination on the site and manage it there. According to the region, removing the contamination would be both environmentally burdensome and expensive since it involves moving significant amounts of soil, which would then need to be transported away in trucks and treated elsewhere.

Instead, the region is working with Danish and international researchers to develop a method to bind the contamination to naturally occurring minerals in the soil.

-“There is currently no solution for this, so the region is collaborating with Danish and foreign researchers to develop one. The fact that there is so much arsenic on the Collstrop site may mean that handling it could be very costly and challenging, but it is still too early to say for sure,” says Charlotte Schleiter, the environmental chief.

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