Roughly 80 per cent of drinking water in Switzerland stems from groundwater. A quarter of this water is fed from river water which infiltrates via river banks. Water filtration through the bank area is frequently the only barrier separating the drinking water pumping station from the river. As a result, the processes in the rivers and infiltration zones are very important in terms of providing clean drinking water. Just how climate change affects the bank filtration is still largely unknown. Can the quality of the drinking water be increased through improved waste water treatment? Various biological and chemical processes play a role in this respect. These processes are influenced by factors such as the quality of untreated water, infiltration rate and water temperature, which in turn are exposed to climatic changes. Bank filtrate can contain a significant share of waste water which varies depending on the water level and degree of dilution. Additionally, the process configuration of waste water treatment plants may change greatly in future if advanced waste water treatment is implemented for the purpose of removing trace materials. Despite the relevance of these processes, there are no extensive studies to date on the influence of climate scenarios on bank filtration.
This research project examined interrelations between the anthropogenic water cycle and bank filtration. In the first part of the project, laboratory experiments were carried out on the photochemical breakdown of trace materials in order to examine the influence of the water quality and the light on the reduction in the concentration of trace materials. In the second part, a model was built in the laboratory with sand columns in order to examine the influence of factors such as untreated water composition, the infiltration rate and temperature on the redox medium in the infiltration zone. The sand for the column experiments was taken from the bank area at the field site. In a third part of the project, samples were taken over several days and long-term monitoring was carried out at the field site in order to examine the influence of the temperature and run-off on the redox-sensitive parameters of oxygen and nitrate. The long-term observations on the oxygen concentrations in the infiltration area were also used in a 3D groundwater flow and transport model.
The findings gained from the photochemical experiments enable an assessment of the breakdown of certain antibiotics and anti-corrosion substances in surface water. The impact of climate change on the redox processes in the infiltration zone could be examined through experiments in laboratory columns and in the field. These experiments have shown that the temperature and the organic material attached to – not detached from - the sand exert the greatest influence on the redox medium. The dependence of the oxygen uptake on the temperature was also simulated; this revealed that the oxygen in the infiltration zone is used up during periods of heat. However, the nitrate present in the water acted as a redox buffer, thus preventing the release of iron and manganese.
In practice, this means that there will be no need in the near future to start removing iron or manganese in water supply systems with river bank filtration. Nevertheless, the concentrations of oxygen and nitrate in bank filtration zones during hot spells should be monitored in future.
The interpolation methods developed for estimating non-linear water levels were successfully implemented and can be applied profitably in future studies on bank filtration. The successful approach for the simulation of reactive transport processes in bank filtration and the resulting estimation of the oxygen concentration in the groundwater under varying climatic and hydrological conditions can also be transferred to other systems with similar characteristics.
Riverbank filtration under climate change scenarios (RIBACLIM)
