​​​HSY has one groundwater plant in use, Kuninkaanlähde in Tuusula. The Kuninkaanlähde plant is located in Tuusula, in the area of Maantiekylä. The distribution area of Kuninkaanlähde comprises mainly the urban districts of Korso, Vallinoja and Vierumäki in Vantaa. 

The plant went through a complete modernisation during the past few years. The new treatment process of the plant removes carbon dioxide, typical for groundwater, through aeration. For improving the technical quality of water, the water runs through organic limestone filtration. Water disinfection through ultraviolet light minimises the usage of chemical disinfectants. Before conveying domestic water, i.e. potable water, into the water supply network, a small amount of chloramines, a bound chlorine compound, is added to it in order to keep the microbiological quality of the water good.