The capacity of drinking water wells, i.e. the yield for a given drawdown, is often decreasing after a certain time of operation. This effect is called well ageing and is due to different processes related to the geology and hydrochemistry at any given well site and to the construction and operation of these wells. The Hydrogeology workgroup and partners investigate wells in Berlin and France in terms of their ageing behaviour with the aim to determine suitable measures helping to slow down well ageing processes and optimise strategies for well operation and maintenance. A precondition for well clogging by iron incrustations is the mixing of different groundwaters with incompatible chemical properties in the well and/or within aquifer and is induced by combined hydrochemical and microbiological processes. The assessment of (i) formation of reduced/oxidized groundwater layering in the aquifer, (ii) localization of mixing zones and (iii) mixing ratios within the well was done by field and laboratory studies. The research reveals that redox condition in the well and the surrounding aquifer are subject to short to long-termed variations. These variations are caused by operation intervals of the wells and by seasonal effects. The results permit a characterization of oxygen enrichment and transport dependent on well operation, location and design and further on an input-output balancing and a modeling of incrustation rates.