The City of Stockholm is situated on fourteen islands and on the banks to the archipelago where Lake Mälaren meets the Baltic Sea. The city centre is virtually situated on the water. The area of Stockholm is one of several places in Sweden with a joint valley terrain. In these landscapes erosion along geological joints has split the flattish upper surfaces into low-lying plateaus. In the case of Stockholm the plateau surfaces are remnants of the Sub-Cambrian peneplain. Islands and islets Extant islands and islets Historical islands and islets References: Lakes and watercourses The access to fresh water is excellent in Stockholm today. Historically, lakes and watercourses were used as refuse dumps and latrines, causing epidemic cholera and many other diseases. By the 1860s water was being drawn from Årstaviken, the waters south of Södermalm, and was treated in the first water-purifying plant at Skanstull and from there distributed through water mains. In modern times the city's water supply is from Lake Mälaren purified by plants at Norsborg and Lovön, together producing 350,000 m³ per day, which means Stockholm residents are consuming 200 litres per day on average. Water is purified at three plants at Bromma, Henriksdal and Loudden, together filtering some 400,000 m³ sewage per day from pollution, including nitrogen and phosphorus, before discharging it into the Baltic Sea. Levels of several pollutants in lakes in the central parts of the city, especially on the western side, are far above average, including substances such as cadmium, copper, mercury, and lead. Decreasing usage of several of these substances has reduced these levels in the upper sediments of the lakes. The Stockholm area used to contain many more lakes and watercourses than it does today, much due to post-glacial rebound, but also because of lake reclamation for settlements and health. Historical lakes, such as Fatburssjön on Södermalm and Träsket on Norrmalm, were putrid and associated with a high mortality rate in Stockholm until the late 19th century. Other historical lakes, like Packartorgsviken and its interior part Katthavet, were filled with mud and equally repulsive. Other lakes still present today were once much larger – such as Magelungen, Drevviken, Judarn, and Råstasjön – while some bays of today once were proper lakes – Brunnsviken and Hammarby sjö. As in other urban areas, the lakes of Stockholm are directly affected by the city's sewer system and pollution from settlements, traffic, and industry. Sewers