With the application of advanced treated wastewater to the ancient wastewater irrigation field Karolinenhöhe, the Berlin Water Company (Berliner Wasserbetriebe) started in 1990 an ecological passable and water economical reasonable cultivation of an old wastewater irrigation field. After a period of 10 years of operation the functional capability and the efficiency is assed. In the first aquifer a hydrodynamic impact by the trickled water could be proved in the measuring facilities and certified by modelling. Altogether the application of the advanced treated wastewater stabilized the water balance and therefore the basis flow of the river Havel. Especially admissions during the low water periods (summer and autumn) counteract the low water level in the surface water by a raised basis discharge. Since 1990 the ecological status of the first aquifer of the wastewater irrigation field is clearly improved. The concentration of the relevant parameters of eutrophication NO3, NH4 und PO4 regressed significantly. In the second aquifer the parameter boron and phosphate improved from 1990 to April 2002. For the other parameters a diffuse basis load remained. The admission of the advanced treated water has a positive effect to the quality of the groundwater compared to the initial situation. The concentration of most of the parameters regressed. Just the concentration of potassium (in the first and second aquifer) and of AOX (in the first aquifer) stayed almost constant because of the input. There are no risks by continuing applications. There is a degradation of the nitrogen compounds due of the soil passage of the water and a dilution by the natural groundwater recharge. Therefore the receiving water bodies are less polluted compared to a direct discharge (exception potassium). A clear improvement is reached for the river Havel. The retention time of the water in the underground passage is very long because of the great depth of the groundwater level. Therefore a degradation of germs is assured. More detailed studies and analyses must follow this primal estimation.