First insight of the “aeration zone” in the Hainich Critical Zone Exploratory (CZE) in central Germany.

The Aeration Zone (AZ), as part of the Critical Zone, connects the soils sensu stricto with the permanently water-saturated (phreatic) zone, including the groundwater aquifers, and comprises variably water-saturated regolith and perched groundwater bodies as well as the phreatic groundwater fluctuation zone. Because of the inaccessibility of fluids in the AZ, hydrogeological properties (i.e., quality, fluid migration, and matter transport) are poorly understood in CZ research so far. To address this research gap, we developed sub-horizontal novel drain collectors and installed within four different aquifer storeys in the fractured carbonate AZ of the Hainich CZE (central Germany). For 2.5 years, physio-chemical properties, chemical composition, particle size, and spectrum of the drainage were analyzed on regular and event-basis by a range of spectroscopic, chromatographic, and microscopic techniques.

Besides showing a seasonal variation, the amount of drainage volumes and quality varied extremely between the locations. We attribute this to the different flow regimes, fracture intensity, depth, and the spatial extents of the “capture zones” that recharge the drain collectors. However, the gravity-driven fast flow regime is predominant, especially during and after extreme events. During percolation, the upper part of the AZ supplies ~74% of the water used by the trees. Therefore, AZ is an important contributor to the plant water supply system.

Furthermore, we confirmed that the physico-/hydrochemistry of the drainage in the AZ is significantly different from soil seepage and shallow groundwater. The mobile inventory in the AZ is mainly encompassed of dispersed calcite, clay, quartz, and bio-particles, including diatoms. So, we suggest that soilborne colloids and soilborne substances can be tracked in the fluids within the AZ. Due to the significantly higher particle concentration in winter, percolating water can collect the mobilizable mineral particles, which are made “susceptible” to release by weathering and alteration, freezing and thawing within the AZ, by gravity-driven fast flow and subsurface aeration due to rapid and frequent groundwater fluctuation; thus, essential for the mobile inventory within the AZ and at the end nutritional supply of groundwater ecosystems and, groundwater quality.