Neglected but pivotal: Complex matter dynamics in the aeration zone contribute to groundwater quality evolution

Fluid and matter dynamics in thick aeration zones of topographic highs are insufficiently understood, particularly in their role for groundwater quality evolution and subsurface ecosystem functioning. We apply novel drainage collectors, installed in sub-horizontal boreholes, to fill the observational gap of bedrock percolates. In the Hainich Critical Zone Exploratory, 20 spatially distributed collectors act as a super-collector for percolates from the highly geodiverse aeration zone, complementing a lysimeter and well network. For over 3 years, we investigated percolate volume, quality, and controlling factors. 65% of the annual percolation flux occurred during winter. Of this, extreme rainfall (33%) and snowmelt events (25%) together accounted for 58%, depending on the recent past of subsurface moisture conditions. Collectors captured 13% of topsoil seepage, with soil thickness, sub-season, slope, and fracture properties as major factors. The mobile percolate inventory showed strong seasonality, driven by weather conditions. The solute signature differed markedly from that of soil seepage, more closely resembling groundwater. Winter high-flows translocated most of the organic carbon and a broad spectrum of mineral particles, up to 160 µm large mineral-organic aggregates, and bioparticles. Bacterial diversity suggests a distinct aeration-zone microbiome adapted to fluctuating habitat conditions, serving as a dispersal source for phreatic communities. Our findings showcase complex dynamics between matter sources, transformations, and sinks. Weather extremes affect the formation and transport of matter through regolith and bedrock. Increasing above- and belowground effects of climate change could thus impair subsurface services, including the provision of high-quality groundwater. We advocate considering thick recharge-area aeration zones as key compartments for subsurface life and for the evolution of groundwater quality.