Physical models such as surface infiltration experiments in the lab and field are an approach to understand processes in the unsaturated soil zone. In the case of mapping processes influencing the operation of real-world managed aquifer recharge schemes they are helpful tools to determine interactions between processes in the unsaturated soil zone, and site-specific as well as operational parameters. However, the multitude of assumptions and scale-related limitations of downscale investigations often lead to over- or underestimations, rendering their results useless when translated to field-like conditions. Various real-world managed aquifer recharge operational scenarios were simulated in three physical models, a 1D-lab column, a rectangular shaped stainless steel 3D-lab infiltration tank and a rectangular shaped 3D-field unit, to understand the impact of the experimental set-up on the assessment of processes and to identify the experimental set-up which is most-suitable to describe these processes. Results indicate that water flow velocity, water saturation and oxygen consumption are often overestimated in 1D-column experiments due to sidewall effects and no existing lateral flow. For precise analysis of infiltration processes in general as well as during operation of managed aquifer recharge, 3D experiments are recommended due to their more realistic representation of flow processes.

Soil aquifer treatment (SAT) is a nature-inspired solution for improving the water quality through soil percolation. The biodegradation of organic matter typically occurs in the shallowest soil layer and it depends on the contaminant’s characteristics (water solubility, molecular structure) and specific soil properties (pore size distribution). The present study aims at identifying which grain size fraction of typically used sandy soils in the shallowest layer of SAT systems can provide the optimal conditions for microbiological growth that can be reached by a trade-off between soil moisture as well as nutrients and oxygen supply. For this, soil columns were used at a laboratory scale to determine the relationship between the pore size distribution of four different grain size fractions and biodegradation rates of organic matter from synthetic wastewater. The results obtained from this experimental setup indicate that bacterial colonies reached optimum growth when about 60% of the available pore space was filled with water. For the selected soil, this was achieved by the fraction with grain sizes in the range of 630 µm to 1000 µm, having pore diameters between 87 µm and 320 µm and a mean pore diameter of 230 µm.

Decentralized water management requires innovative technical solutions due to restricted operational and economic resources. In this study, a combined, decentralized infiltration system in the form of closely-spaced sub-systems for precipitation water and treated wastewater has been numerically analyzed. Flow and transport simulation shows that a closely-spaced system, by arranging the infiltration pipes closely in a longitudinal manner, is feasible under the consideration of German national guidelines for both infiltration methods. Precipitation events up to a recurrence interval of five years can be infiltrated alongside with treated wastewater of a one-family house without significant reduction of the wastewaters’ residence time. Scenario analyses highlight that harmless wastewater infiltration remains mainly undisturbed for a broad bandwidth of hydrological, subsurface, and technical conditions.

As an alternative to the use of tracers, easy-to-measure soil moisture dynamic parameters (e.g., water content) provide in situ estimates of the infiltration rate reduction state of a soil medium. For instance, managed aquifer recharge operations control the hydraulic state of their infiltration basins by measuring the infiltrated volume under constant head conditions. Instantaneous profile measurement systems can be used to manage the basins by determining the reduction of the infiltration rate over time. This study combines the empirical methods of Libardi, the self-developed water content and root mean square difference-based procedures and the trigger time method, to get a similar reproduction of tracer results from river water spreading basins. The methods based on water content showed a good fitting in comparison to the results obtained with the tracer experiment and represent a promising source for detecting changes in the flow impedance during infiltration events.