Use of organic trace substances for water management at a bank filtration site at the Rhine, Germany

The increasing demand for water in industry, agriculture, and private households as well as climate change are leading to more dynamic river and groundwater levels. This demands a new and improved monitoring approach to groundwater resources. The BMBF-funded research project iMolch (project number: 02WGW1667D) aims to develop sustainable water management concepts for Germany using innovative monitoring strategies. The general purpose of the investigations is to gain a complex understanding of hydrodynamic and hydrochemical processes in order to enable a more sustainable use of water resources on the basis of the indicator concept. This study investigates the spatial and temporal variation of different substances using as one example the urban bank filtration site in Düsseldorf, Germany. This site is used to draw conclusions on groundwater quality and dynamics as well as redox processes using the transport and retention of organic trace substances.

Hydraulic and hydrochemical measurements were carried out fortnightly over a period of 1,5 years (31/01/2018–08/05/2019) across a study transect on the Rhine riverbanks with 15 measuring points. The analyses focus on changes in the concentrations of organic trace substances over time and the relationship to flow distance, flow duration and climatic conditions. Based on land use and occurrence, various organic compounds, such as fertilisers and pesticides, but also pharmaceuticals and detergents are monitored. The dependence between substance concentrations and Rhine river level decreases with increasing distance to the Rhine. Following the extreme drought in the summer of 2018, during which only low concentrations of trace substances were detected in the entire study area, there was an abrupt increase in substances entering the Rhine. This coincides with a significant Rhine high stand in the winter of 2018/2019. It is noticeable here that an increased concentration of substances discharged from the Rhine can also be detected at measuring points beyond the well gallery, on the land side of the measuring point transect.

In dry periods, such as the summer of 2018, the proportion of bank filtrate in the raw water is significantly lower. In contrast, the Rhine floods the pumping well gallery during high water level periods, such as the winter of 2018/2019. During this time there is no landward flow to the pumping wells. The occurrence of organic trace substances even shows that the flood of the river Rhine pushes the water in the bank filtrate up to the well gallery and far beyond into the hinterland despite ongoing water pumping. These observations are crucial for the subsequent water management during different water levels of the river Rhine. Prediction models will be built up to help as water management tools to improve monitoring systems and for transfer of these results to other sites.