 In the end of the 1930s sewers were built in the area and the waste water fed into the lake during the 1920s and 1930s is today brought to the wastewater treatment works at Henriksdal, but old sewers still leak into the lake and the water was considered unfit for bathing at several occasions during the 1990s.Långsjön is one of few lakes in Stockholm where spillway overflow adds appreciable amounts of phosphorus.Surface runoff adds some 70 kg of phosphorus and 1,000 kg of nitrogen annually and more than half of phosphorus added through surface water is produced by the surrounding one-family houses and their gardens.