Infiltrating flood waves into aquifers: Column experiments on the suitability of filter materials

With global warming, a rising amount of people will be affected by water scarcity and droughts. At the same time, precipitation intensities and thus flood risks are projected to increase. Since these extremes often occur in the same location, co-management strategies of floods and droughts may offer a promising solution. The project Smart-SWS, funded by the BMBF as part of the initiative WaX – Hydrological Extreme Events, links drought prevention and flood protection with a concept for infiltrating flood waves into riverine aquifers as decentralized, technically supported underground storage. The two main challenges of the project are ensuring water quality with respect to health and environmental risks and controlling clogging of the system. The temporal asymmetry between very rapid infiltration of a flood wave and long-term storage in the aquifer, coupled with relatively long periods of no infiltration, results in stringent requirements for infiltration system design and materials.

The goal of the infiltration process is to improve retention of undesirable materials while maintaining high infiltration rates. As part of this work, potential materials will be evaluated for their suitability for infiltration of flood waters into the aquifer. For this purpose, river water quality needs to be seasonally monitored. Parameterization and characterization of clogging can be performed in column experiments using different potential materials, at different hydrodynamic and hydrochemical conditions, and with defined infiltration and dry phases. Established concepts for the transport of (bio)colloids as well as the substitution of contaminants by fluorescent tracers with similar sorption properties can be used to demonstrate the efficiency of retention and the local formation of clogging in time and space.

To test potential materials for the infiltration ditches and their contaminant retention behavior during wetting and drying cycles, a transparent column setup with temperature, pressure, electrical conductivity, redox potential, pH, and turbidity probes, as well as visual monitoring was established. This allows to record spatially resolved breakthrough curves, depositions, and reactions. We expect an increase of contaminant retention with an increase of filtered fines from the infiltrated water. Both, inorganic and organic colloids, are tested for this purpose and supplemented by experimental data from field sites. The shear forces in the porous materials are matched to the expected shear forces in the infiltration ditch. The hydrochemical stress due to a reduction in ionic strength during infiltration is also simulated in the experiments.

With this work, the behavior of contaminants and particles in infiltration systems can be predicted and optimized in order to fulfil environmental and legal requirements for the water quality.