Large-scale dispersion was studied in an unconsolidated, sandy, glaciofluvial, river-recharged, and confined aquifer in Germany. Groundwater observation wells from a 3.5-km-long transect located in flow direction from the river Oder into a large lowland area (Oderbruch polder) were sampled for noble gases in order to date the groundwater with the tritium and helium (3H-3He) technique. The apparent 3H-3He ages of the groundwater increased from only a few months to >40 years along the flow path. Highest values for initial 3H (sum of 3H and its decay product tritiogenic helium, 3Hetri) were encountered in 2.6-km river distance. Concentrations of 4He in the water increased to 1.1 × 10-7 cm3 STP/g with distance from the river. The initial 3H data enabled an estimation of the longitudinal dispersivity with a simplified one-dimensional transport model. The best fit of modeled and measured initial H data was obtained using a dispersivity of 120 m. Deviations of modeled hydraulic ages and measured apparent 3H- 3He ages for older samples can be explained by dispersive mixing.

Managed aquifer recharge is gaining importance as a practice to bank and treat surface water for drinking water production. Neon (Ne) concentrations were analysed at four different recharge sites in and near Berlin, where groundwater is recharged directly from surface water courses, either by near-natural bank filtration, induced bank filtration or engineered basin recharge. Neon concentrations in excess of saturation (DNe) were used to identify excess air in the infiltrates. Excess air concentrations were around saturation at the near-natural bank filtration site, where river water infiltrates through a permeable river bed into a confined aquifer under completely saturated conditions. At two induced unconfined bank filtration sites, samples generally contained excess air (up to 60% DNe). Highest excess air concentrations (up to 81% DNe) were encountered at the engineered basin recharge site. The degree of water table fluctuations, the water saturation of the sediments in the infiltration zone and the presence of a confining layer affect the formation of excess air. Excess air can only be used to trace bank filtrate or artificially recharged water in a setting where the ambient groundwater in the near vicinity of production wells is not affected by large water-table fluctuations. Nevertheless, excess air concentrations provide valuable additional information on the type of recharge (saturated or unsaturated, degree of water table fluctuations).

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.

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.

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.