Redox processes during bank filtration were evaluated in Berlin, where bank filtered water is abstracted for drinking water production. The investigations included the mapping of the infiltration zone, a column study and hydrochemical analyses of the groundwater sampled between lake and production well. The organic carbon content increased and the permeability of the lake sediments decreased with distance from the shoreline. The most important changes with regard to the redox state of the infiltrate occurred within the first metre of flow. Infiltration was mostly anoxic, as oxygen was rapidly consumed within the organic rich sediments. The infiltration zone revealed a vertical redox stratification with hydrochemical conditions becoming more reducing with depth rather than with distance from the lake. The redox zones were found to be very narrow below the lake and wider towards the production wells, suggesting that other than differing flow paths, reaeration after infiltration may also occur and possible mechanisms are presented. Redox conditions were influenced by strong annual temperature variations of the surface water affecting the microbial activity. Aerobic infiltration only took place close to the shore in winter.

Berlin relies on induced bank filtration from a broad-scale, lake-type surface water system. Because the surface water contains treated sewage, wastewater residues are present in surface water and groundwater. Multiple environmental tracers, including tritium and helium isotopes (3H, 3He, 4He), stable isotopes (d18O and d2H) and a number of persistent sewage indicators, such as chloride, boron and a selection of pharmaceutical residues (phenazone-type analgesics and their metabolites, carbamazepine and anthropogenic gadolinium, Gdexcess), were used to estimate travel times from the surface water to individual production and observation wells at two sites. The study revealed a strong vertical age stratification throughout the upper aquifer, with travel times varying from a few months to several decades in greater depth. Whereas the shallow bank filtrate is characterized by the reflection of the time-variant tracer input concentrations and young 3H/3He ages, the deeper, older bank filtrate displays no tracer seasonality, 3H/3He ages of a few years to decades and strongly deviating concentrations of several pharmaceutical residues, reflecting concentrations of the source surface water over time. The phenazone-type pharmaceuticals persist in the aquatic environments for decades. Bank filtration in Berlin is only possible at the sandy lakeshores. In greater water depth, impermeable lacustrine sapropels inhibit infiltration. The young bank filtrate originates from the nearest shore, whereas the older bank filtrate infiltrates at more distant shores. This paper illustrates the importance of using multiple tracer methods, capable of resolving a broad range of residence times, to gain a comprehensive understanding of time-scales and infiltration characteristics in a bank filtration system.

The behaviour of residues of phenazone-type pharmaceuticals during bank filtration was investigated at a field site in Berlin, Germany, where bankfiltered water is used for drinking water production. The concentrations of the pharmaceutical residues in the shallow, young bank filtrate (travel times < 1 month) were correlated to the prevailing hydrochemical conditions at the field site. In addition, their behaviour in an undisturbed sediment core from the lake base at the site (clogging layer) was evaluated in the laboratory. Phenazone, 4-acetylaminoantipyrin (AAA), 4formylaminoantipyrin (FAA) and 1,5-dimethyl-1,2- dehydro-3pyrazolone (DP) were eliminated more efficiently under oxic conditions, while 1-acetyl-1-methyl-2-dimethyloxamoyl-2phenylhydrazide (AMDOPH) was not eliminated at all. The redox conditions and the elimination of the respective pharmaceutical residues displayed strong seasonal variations. Oxic conditions were only encountered close to the shore in winter, when temperatures were low. The column study showed that the elimination is restricted to the uppermost decimetres of the lake base, where oxygen is present. While phenazone elimination is almost complete during aerobic rapid sand filtration in the waterworks, the compounds were found to be more persistent under anoxic field conditions.

The transect of the bank filtration site at Lake Tegel is characterized with regards to their redox conditions using a Cluster analysis. Four different groups of observation wells could be found, enabling the derivation of a redox zoning with horizontal boundaries, which are moving downward during winter time. At the same site, Regression analysis served to examine influencing variables on the reduction of the pharmaceutical Carbamazepin during bank filtration. Two different regression models for summer and winter time were found, with each of them including the standardized temperature and the travel time as influencing variables. Whereas during winter time the redox conditions seem to have a significant influence on the reduction of Carbamazepin, the same influence could not be found for the reduction of Carbamazepin during summer time.

Hydrochemical conditions were evaluated at both bank filtration and artificial recharge sites in Berlin. All bank filtration sites show a strong vertical age stratification. Rather than showing a typical redox zoning with more reducing conditions in greater distance from the surface water, the redox zones are horizontally layered, with more reducing conditions in greater depth. This is believed to be an effect of the strongly alternating groundwaterlevels and by the age stratification. The redox conditions are generally more reducing at the bank filtration sites, mainly as a result of the longer travel times and operational differences. Redox conditions at all sites vary seasonally in particular at the artificial recharge site, which is mainly caused by temperature changes.