The lovely countryside of the Kempen region in the provinces of North Brabant and Limburg conceals a nasty secret. For more than half a century there has been a layer of toxic zinc slag under as much as 750 kilometres of the roads. In places where the concentrations are high, or where it can easily end up in the environment, it will have to be removed.

The problem is that it is not easily visible where the zinc slag is most dangerous. AbdK (Actief Bodembeheer De Kempen/Active Ground Management the Kempen) has therefore developed - in cooperation with a number of international engineering agencies and research institutions - a special "zinc slag monitoring bus". Tom Kamsma of the AbdK explains what exactly zinc slag is:

"At the end of the 19th century, they used smelting furnaces in the process of zinc extraction. The material remaining in the furnaces was known as slag. It is a cinder-like substance which can be used as filler in road building or as loose powder for hardening ground."

Free
As the zinc industry in the Kempen region produced huge amounts of zinc slag, until it was piling up around their ears, companies and private individuals were encouraged to come by and pick up the cinders for a song, or even for free. By the 1970s, when it became clear that the heavy metals in the slag - zinc, lead and calcium - were toxic, the damage had already been done. The cinders were everywhere under and on the roads and were releasing heavy metals into the groundwater due to leaching with rainwater.

Around 750 kilometres of the roads in the Kempen region are now contaminated with zinc slag. But it is not known where most of it is or where concentrations are actually low enough so it can be left alone. This is why the AbdK built their monitoring bus. The bus can detect zinc slag without having to dig up the road, which would be completely unaffordable.

Around the world
Zinc slag is not the only cinder-like material which has been used for road building and not the only one which is toxic. Around the world there are hundreds of countries and uncountable roads where the road surface essentially consists of toxic waste. Zinc slag itself is no longer produced because zinc extraction no longer takes place in furnaces but by using the much cheaper and more efficient electrolysis process. Nevertheless, the AbdK's monitoring bus will still prove useful elsewhere in the world; it looks at the ground condition and it searches for metal. What kind of metal is just a matter of adjusting the equipment.

Tom Kamsma describes the bus' three different search methods with some pride:

"...This first thing is the Electro-Magnetor which measures magnetism in the ground, because the heavy metals in the zinc slag can influence the natural magnetism...this is the Gamma Spectrometer which can "feel" if there is any slight deviation in the natural radioactivity of the ground...and this is the Ground Penetrating Radar, just like ordinary radar, really, but pointed downwards, because we need to know whether the ground under the road is water-permeable or not."

Decontamination
The bus crawls along the Kempen roads at a speed of 10 kilometres per hour. This autumn it should become clear what the situation is. Tom Kamsma says the "open slag" roads will be the first up for decontamination. These are cart tracks and woodland trails where the zinc slag is open and bare on the tracks. These roads are the most dangerous since rainwater can just wash through and zinc slag can blow away. The end date for the project is 2015.

Photos courtesy of Actief Bodembeheer De Kempen