In Berlin, 70 % of the drinkinq water is derived from bank filtrate or artificially recharged water. Because the surface water system contains elevated proportions of secondary treated municipal sewage, a number of sewage indicators from various sources can be detected in the bank filtrate. An artificial recharge site and a bank filtration site in Berlin Tegel are introduced and compared in terms of their hydrogeological and hydrochemical properties. Because of a permanent clogging layer and the geological properties, travel times are slower at the BF site and the hydrochemical conditions are more reducing. First estimates for the reaction rate constants of oxygen and nitrate are obtained with exponential data fitting. Some of the effects of the different redox conditions on minor substances such as drug residues are highlighted.

Induced by well abstraction, surface water infiltrates into Berlin aquifers and is used for drinking water production. A major advantage of bank filtration is the capability of the subsurface to remove contaminants and save natural groundwater resources. Since a large proportion of the surface water in Berlin originates from treated effluents released by wastewater treatment plants, certain wastewater residues can be traced into the groundwater. A powerful tool to characterise bank filtration systems is the use of wastewater indicators and additional environmental tracers to estimate flow velocities and proportions of bank filtrate in the abstraction wells prior to reactive transport evaluations. Examples for tracer applications at the Berlin system are introduced in this paper. In addition, an overview on results of various studies conducted on contaminant transport and removal during underground passage of the bank filtrate in Berlin is given.

The city of Berlin is using bank filtered surface water and artificially recharged water for drinking water production. As far as some hydrological trends and development of anthropogenic pollutants may threat the future of the ground water resource in Berlin, it is important to measure the capacity of ground filtration to answer to such developments, and to secure the use of this systems through the development of the most appropriate practices and the related technologies. This was an obvious reason to initiate a multidisciplinary cooperation project at the Berlin Centre of Competence with the topic "bank filtration and artificial recharge" named Natural and Artificial Systems for Recharge and Infiltration (NASRI). It will focus, for example on questions of the emergence and removal of pharmaceutical residues during bank filtration. The fate and the destination of other specific trace substances as well as of bacteria and viruses are other objectives of the research programme (KWB 2002).

The T-He age dating method uses the ratio of the concentration of radioactive tritium (3H) derived from atmospheric nuclear bomb testing and its decay product Helium (3He) in the groundwater to determine a groundwater age, i.e. the time passed since the water had its last contact with the atmosphere. At the Free University of Berlin, hydraulic and hydrochemical processes accompanying bank-filtration are currently examined at two very different locations: In metropolitan Berlin and the rural Oderbruch polder region. The city of Berlin enhances bank-filtration through well galleries located adjacent to the surface water system. The spatial and temporal development of the bank filtrate is studied in cooperation with the Berlin Waterworks and the Berlin Centre of Competence for Water at several exemplary piezometer transects. The system generally behaves highly transient due to continuously changing pumping regimes. At the gallery Lake Wannsee, the well filter screens are pumping water from 3 different glacial sand layers separated by aquitards. The well water is a mixture of very old deeper groundwater, medium old water from the middle layer and very young bank-filtered water. The Oderbruch is located north-east of Berlin aside the river Oder. Intensive melioration activities in the past 250 years converted the former swamp into a fertile, agricultural region and lead to the permanent infiltration of river water into the shallow, confined aquifer. Compared to Berlin, the infiltration is a long-term, very stable process. The groundwater is getting older with increasing distance and travel-time from the river. The concentration of “stable” tritium (sum of 3H and tritiogenic 3He) increase from the river inland reflecting the decrease of 3He in the atmosphere from the early 60’s onwards. Peak concentrations are encountered in 2.1 km river distance whereas further inland (3.4 km river distance) old water which infiltrated prior to the nuclear bombing peak is encountered. In addition, the groundwater has a high radiogenic 4Heterr concentration which also indicates that the groundwater is more than a few decades old. Even further inland, in the central polder areas, the groundwater is unconfined and continuously recharged to some extend by percolating water infiltrating through shrinkage fissures in the overlying dried alluvial loam. The water is a mixture of young seepage water and very old bank filtrate, the resulting “mixed” T-He age is getting younger again. The T-He method was successfully applied to support estimated groundwater ages derived from tracer analysis (e.g. 2H, 18O, EDTA, Gd) at both locations. In the Oderbruch, the T-He ages were used to calibrate a flow model. The method also proved to be a very good indicator for the identification of mixing processes.

The present report characterizes the field sites Lake Tegel and Lake Wannsee as well as the artificial recharge site GWA Tegel in terms of their clogging layer, sedimentary, hydraulic and hydrochemical properties. As a result, a solid basis for the interpretation of specific compounds evaluated within NASRI and for subsequent modeling and quantification of the data is given. Major problems or difficulties where identified, in order to focus investigations on aspects not fully understood to date in the next project phase. The combination of different tracers enables the interpretation of the flow regime. With the help of T/He analysis, ages of different water bodies can be estimated. The analysis of tracer showing distinct seasonal variations is used to estimate travel times while water constituents which are either mainly present in the bank filtrate or the background water are used for mixing calculations. The proportions of treated wastewater in the surface water were estimated in front of the transects. The surface water composition varies largely both in time and space, which is a problem at Wannsee, where the surface water sampling point is not representative for the bank filtration input. Estimates for travel times of the bank filtrate to individual observation and production wells are given and vary between days and several months. The production wells are a mixture of bank filtrate and water from inland of the wells and deeper aquifers, proportions of bank filtrate are given where possible to differentiate between contaminant removal and dilution. They vary between < 20 and > 80 %. The new observation wells enable a vertical differentiation of the infiltrate. It becomes clear that at Tegel and Wannsee, there is a strong vertical succession towards larger proportions of considerably older bank filtrate with depth. At the Wannsee transect, the observation wells deeper than the lake do not reflect the surface water signal at all. It will be important to combine the new information with hydraulic information of existing flow models (mainly of the IGB “model” group). The evaluation of the redox conditions shows that redox successions proceed with depth rather than (only) in flow direction. In addition, the redox zoning (as characterised by the appearance or disappearance of redox sensitive species) is very transient. The zones are much wider in winter than in summer, in particular at the artificial recharge site GWA Tegel, probably due to temperature effects. This poses a challenge for the desired modelling and the interpretation of data from redoxsensitive substances.