During routine operation, drinking water production in Berlin does not involve any disinfection measures. The stated goal of Berlin’s water utility BWB is to maintain natural water treatment without disinfection in the future, too. In this regard, the drinking water protection zones in the proximity of drinking water wells are of particular importance. Zone II is supposed to ensure protection against microbial contamination and is defined as the start line, from which groundwater has a travel time of 50-day from any point below the water table to the abstraction source. This 50-day line also applies to the residence time of the infiltrate in the underground during bank filtration and managed aquifer recharge (MAR). Previous studies, for example within the project NASRI, have however shown, that depending on climatic and operating conditions, the 50-day residence time may temporarily be undercut in the proximity of certain abstraction wells. Within NASRI, the travel times at individual field sites were determined by means of monthly measurements of stable isotopes and other tracers along transects. The residence times relating to drinking water wells have however not been determined so far or only in individual cases respectively.

For this reason, the T-MON project will examine the following issues:

1. How to monitor the residence times during managed aquifer recharge by a simple and inexpensive method during operation of the abstraction wells?

2. What control measures are applicable in routine operation in order to prevent the breech of the 50-day rule?

Therefore, it will be investigated to what extent the seasonal temperature fluctuations in the surface waters and infiltration basins and the delayed and attenuated temperature signals in the wells situated nearest to the banks and basins can be used for a quick and easy determination of residence times. Since May 2014, preliminary studies have been carried out by Berliner Wasserbetriebe (BWB) including the equipment of selected measuring points with pressure/temperature loggers or the activation of the temperature record in existing water level loggers. In addition, KWB will sample selected wells and measuring points regularly over a period of 15 months with regard to parameters which can be used as conservative tracers. The parameters and wells are selected on the basis of existing data yielded by previous research projects and the routine monitoring carried out by BWB. The measurement campaign will allow the determination of thermal retardation factors for the two sites investigated through calibration against the conservative tracers. In addition, it will be assessed to what extent the continuous temperature measurement is suitable for determining the residence time of the enriched subsurface water in a reliable way, so that a critical undercut of the 50-day line can be detected in the future.