An important factor in judging the environmental relevance of an aquifer thermal energy storage plant is whether it requires treatment of the water and by what method. Problems that may call for water treatment are the precipitation of carbonate compounds when the water heats up, the formation of iron oxide hydrates and manganese oxide hydrates, and, possibly, heavier corrosion. The paper describes different water treatment methods: Decarbonization through addition of lye of mineral acid, softening by Ca-Na ion exchange, stabilization of water hardness through lowering of the pH, fluidized-bed heat exchangers, stabilization of hardness through CO2. (BWI)