Quantification of the influence of a Rhine flood on the microbiology of groundwater

Bank filtration is a widely used method for the pre-treatment of drinking water. During a flood situation, surface water can penetrate the groundwater to a greater extent due to flooding or erosion of the colmation layer. This can affect the microbial quality of the groundwater and is relevant for drinking water protection, as the introduction of microorganisms must be prevented and monitored. This study determined whether and how microorganisms are introduced from river Rhine into the groundwater during floods. The selected groundwater wells represented a gradient in distance from the Rhine (17 to 78 m) and were sampled at three different depths (18 to 27 m). In January 2024, surface and groundwater samples were taken five days after a flood situation from the Rhine in Düsseldorf and from neighboring groundwater wells. In April, the procedure was repeated at normal water level. Community composition was analyzed using 16S rRNA gene sequencing, cell density was measured using flow cytometry and internal ATP was determined using BacTiter-Glo Microbial Cell Viability Assay. A principal component analysis (PCA) revealed that the microbial composition of the groundwater in the samples near the Rhine was more similar to that of the Rhine water in January than in April. During floods, up to 65.5 % of the identified amplicon sequence variants (ASVs) were transported from the Rhine into the groundwater. In April this value dropped to 10.8 %. In general, the more distant the wells were from the Rhine, the fewer ASVs from the Rhine are found in the groundwater. The cell count in the wells closest to the Rhine was higher in January than in April. This also applies to the internal ATP. We conclude that the Rhine water transports microorganisms into the groundwater to a considerable extent and changes its composition. The effect is increased during floods. The effect of bank filtration can be measured based on the number of cells and the microbial composition in the groundwater. The groundwater community appears to be resilient to this influence and can return to its original state.