The slate roofs of the houses shine silvery black in the early morning light. The air is cold as we depart from Lehesten (1). The mist is dissipating between the undulating slate hills of Thuringia. We enter the forest and, leaving the car, walk along the dirt road to a viaduct that lies directly below the meeting point of two small streams. The viaduct recalls the old slate-railway, which was closed down definitely in 1951 as part of the measures taken by the former GDR to secure the border to West Germany. Sunshine filtering through the leafy canopy invites one to forget old times and enhances the great natural beauty of this relatively untouched strip of green forest and landscape along the border that divided the nation in two. There are indications that the orchid known as "the red forest bird" (Cephalanthera rubra) and the Carthusian truffle can be found here. Fauna also finds its habitat: the common grass snake, the "northern moss maid" dragon fly and the owl.

By white streams

Engrossed in the atmosphere, we gaze at the stream from the viaduct. But something jarrs on the senses. What is it? The reason for the irritation suddenly emerges as from a picture puzzle; the water isn't clear – or even muddy brown – but milky white. Glimpses of the bed of the stream reveal a white coating. Gerhard Furrer is impressed. Here in this slate mining region, the geochemist from ETH observes for the first time an experiment, the basis of which he analysed his laboratory. Together a TV camera team and his research colleague, Kai-Uwe Ulrich from the Rossendorf Research Centre in Dresden, who brought the white streams to his attention and sent him samples from the stream to analyse, Furrer is now standing on the bank of the softly babbling stream. Ulrich explains what we are looking at. One of he stream is acidic and permeated with heavy metals and aluminium that have dissolved in the water. If the latter meets with water from another source, the acidity changes. Within a certain pH range an aluminium-13-nanocluster results. These are anorganic complexes that contain thirteen aluminium atoms. Heavy metals can adhere to the peripheries of the complexes. These are the "building blocks" of the white flocs in the stream.

Steinbach an der Haide, a typical village in the region with slate-roofed houses. large

Lively exchange

A change of scene, a few kilometres away: we are in the Rehbachhalde. Over a stretch of two kilometres slate waste heaps up to 60 metres high are piled up. The region has a 500 years' tradition of mining slate; today only one place remains where it is still mined. A few isolated silver birches on the stony slopes present an alluring contrast to the dark slate. Here on the column road we are standing directly on the former border fence between East and West Germany. The camera team films the scientists as they pass the one-time transit passage for spies, a gate in the fence, and point out the dangers lurking in the area. The danger becomes evident when we come face to face with another white stream. Ulrich explains that oxygen causes the iron pyrites in the wet stone to oxidises and forms sulphuric acid. In its turn, this acid eats away at the rock and releases poisonous heavy metals and lethal aluminium ions. The ensuing aluminium-13-complexes block the gills of any fish and eventually dissolve all forms of life, plant as well as animal, in the stream.

IIdyllic landscape in the slate hills of Thuringia, but the white stream, Loquitz, is as good as dead. large

Restoration efforts run aground

Dead streams are nothing new in this region's long history of slate mining. Already in at the turn of the 19th to the 20th century communities took a big mine owner to court a number of times after pollution caused the death of a lot of fish. The result of these processes was the construction of a lime pool. A long time afterwards, in 1997, a modern trial plant was built near Lehesten to clarify the acid mining water from the stream. A report on the results, published in 2000, states that the nature friendly plant succeeds in neutralising the water and that heavy metals and aluminium are separated out.

We stand on the edge of the clarification basin. There is not much water to be seen, the stilting continues. Restoration efforts have run aground. Lack of funding means that the plant cannot be kept running. According to the report, the treatment of a cubic metre of water costs € 0.75. Furrer is cast down. He explains that the aluminium-nanocluster and heavy metals can be drawn along and spread for hundreds of kilometres. The geochemist reminds us that globally, a surface area of 240,000 square kilometres is touched by pollution problems that have arisen from mining. In the small world in southern Thuringia time seems to have stood still.

Danger and opportunity

Early next morning Gerhard Furrer and I begin our journey back to Switzerland. The slate hills with their picturesque villages seem like ghost communties. Many people have left, equalisation payments, I remember Ulrich telling me, no longer exist. Furrer ponders what we have seen. Even though he and Ulrich already published a paper on the subject last year and he had scientifically "digested" the situation, the direct confrontation with the aluminium flocs has upset him (2)(3). Slate, also called blue gold in Thuringia, is scintillating in many respects. This is also true of the aluminium complexes. When Furrer looks to the future he does not only see disastrous tractor reagents from heavy metals polluting natural waterways; he also sees the binding agent, which, properly employed, can clear the ground of dangerous elements. Thuringia is behind us. Day has broken.

Taking a water sample from a polluted stream: ETH geochemist Gerhard Furrer. large