Complex dynamics of fluid and matter sinks, transformations, and sources in thick recharge-area aeration zones contribute to groundwater quality evolution

Thick aeration zones beneath topographic highs, being neither “vadose” (Latin vadosus = shallow) nor “water-unsaturated”, play a critical yet poorly understood role in groundwater quality and subsurface ecosystem functioning (Lehmann et al., 2026). Using a network of twenty spatially distributed sub-horizontal drainage collectors in the groundwater recharge area of the Hainich Critical Zone Exploratory, we quantified bedrock percolation volumes, solute and particle transport, and their controlling factors over three years, complementing existing lysimeter and well networks. The newly developed drainage collectors fill an observational gap in subsurface water research. Approximately 65% of annual percolation occurred in winter, with extreme rainfall and snowmelt events accounting for 58% of this flux, depending on antecedent moisture conditions. Collectors captured 13% of topsoil seepage, controlled by soil thickness, seasonality, slope, and fracture properties. Analysis of multiple factors linked mobile inventory dynamics to deterministic chaos. Percolate composition showed strong seasonal variability, differed markedly from soil seepage, and resembled groundwater signatures. Winter high-flow events dominated the transport of organic carbon, mineral particles, mineral–organic aggregates up to 160 µm, and bioparticles. Notably, highly geodiverse aeration zones (Lehmann and Totsche, 2020; Aehnelt and Totsche, 2025) not only transform and retain but also generate mobile matter, including microorganisms. Our results highlight complex interactions between weather extremes, regolith–bedrock structure, and matter transport. Thick aeration zones in recharge areas, being foremost exposed to (belowground) climate change, should be recognized as key compartments in subsurface ecosystem functioning and in groundwater quality evolution and thus incorporated into monitoring, modelling, and water resources management.