Infiltration refers to the process of precipitation water (from rain, hail, snow) entering the ground via technical infiltration systems. In this case, the water may be subject to pollution. An alternative term for the discharge of precipitation water into the ground is irrigation. The infiltration of polluted water that requires treatment (usually pre-treated polluted water) into the ground takes place after appropriate cleaning along an infiltration run (irrigation run). Infiltration is mainly used for the discharge of precipitation water from built-up areas and traffic areas where no suitable canal system or water courses are available. This system is also used for the disposal of water in order to counteract the lowering of the groundwater level owing to the sealing of surfaces, and to retain the natural water cycle as far as possible.