Päijänne Water Tunnel

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Map of the Päijänne Water Tunnel.
1. Asikkalanselkä
2. Kalliomäki water distribution station
3. Korpimäki water pump station
4. Artificial lake of Silvola

The Päijänne Water Tunnel (Finnish: Päijännetunneli) is the world's longest continuous rock tunnel located in the Southern Finland. It is 120 kilometers (75 miles) long and runs 30–100 meters under the surface in bedrock. The purpose of the tunnel is to provide a fresh water supply for more than a million people in the Southern Finland cities of Helsinki, Espoo, Vantaa, Hyvinkää, Järvenpää, Kerava, Kauniainen, Kirkkonummi, Sipoo and Tuusula. The former Porvoo Rural Muncipality, now merged with the municipality of Porvoo, also took part in the building of the scheme but has never drawn water from it for domestic use.

The tunnel starts at Asikkalanselkä in Lake Päijänne, which is the second largest lake in Finland, with an area of about 1080 km². The tunnel slopes slightly downhill so that water flows naturally. Water from the southern portion of Lake Päijänne is of rather good quality at the water tunnel intake and is usually drinkable without processing. The tunnel ends at the artificial lake of Silvola in Vantaa in the Greater Helsinki area. From the artificial lake, water is pumped to water treatment plants in Pitkäkoski and Vanhakaupunki, but the constant temperature in the deep tunnel ensures high quality during transport, so only minimal processing is required before use.

Tunnel construction started in 1972 and was completed in 1982 at a cost of 190 million euros. In 2001, portions of the tunnel required repair due to rock falls. In 2008, the tunnel is undergoing an extensive renovation. The southern part of the tunnel will be reinforced during the renovation in order to avoid cave-ins. During renovation work, from April 15 to December 31, 2008, the raw water for the metropolitan area will be obtained from the River Vantaa. The tunnel is wide enough for a truck and allows 10 m³/s water flow. At current water usage rates, treatment plants take water at a rate of about 3.1 m³/s for drinking water processing. The tunnel can be used as an emergency water reserve during water supply disruption.