The Maardu area is among the most polluted regions in Estonia. Past pollution here comes mainly from the phosphorite mining and processing. Waste hills at Maardu contain some 73 million tonnes of alum shale comprising over two million kg of uranium, which leaches into the surface and ground waters and enters the Gulf of Finland. High radon concentrations up to 10 000 Bq/m3 have been recorded on the outcrops of alum shale and they are dangerous to human health.
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