Knowledge of the amount of bank filtrate in raw water is crucial for drinking water suppliers using water wells in close proximity to rivers, in particular if those rivers are strongly affected by anthropogenic activities, e.g. receiving effluents from waste water treatment plants. Analysis of organic micropollutants as tracer compounds is costly and time-consuming, and results may be biased by processes such as sorption, degradation or even by other input pathways such as land use activities. In this study, the use of gadolinium (Gd) as an alternative conservative tracer to indicate the amount of bank filtrate in raw water from drinking water wells close to rivers was investigated. In two case studies in Germany and Luxembourg, river water and water from drinking water wells at several distances from the rivers was sampled and analysed for anthropogenic Gd as well as for some of the organic trace pollutants conventionally used as tracer compounds. The amount of bank filtrate as calculated from Gd was compared with the estimates derived by conventional tracers and by hydrological flux modelling. The results indicated that the measurement of Gd may provide a promising alternative to monitor infiltration of river water in ground water used for production of drinking water.

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