Finland’s capital Helsinki usually gets its tap water from Lake Paijanne via the world’s longest water supply tunnel. But for the past eight months, the capital has been drinking water from the murky Vantaa River as workers have renovated the Paijanne Water Tunnel.
By Christmas, however, city officials will have the 120-kilometre bedrock tunnel back in operation according to the Helsinki Times newspaper. The incredibly long tunnel was built in 1982 to access the pure drinking water from Lake Paijanne, an enormous body of uncontaminated water.
Renovation work focused on a 56-kilometre stretch of the tunnel that needed concrete reinforcement. Even though the Vantaa River is much muddier than the lake, the quality of Helsinki’s tap water is still far better than most cities. Ari Nevalainen of Helsinki Water explains: “We have had to use three-four times more chemicals to treat the river water since its quality is poorer and more variable than water from Lake Paijanne.”
The water in Lake Paijanne is pure enough to drink without any chemical treatment. It is Finland’s deepest lake, and easily meets EU requirements for water safety. Now that the repair work is finished, Helsinki Water thinks the tunnel won’t need any maintenance for another 30 years.