Dynamics of aeration zone mobile inventory preshape groundwater quality evolution - results from multi-year sampling of regolith seepage

The aeration zone (AZ) below the soils sensu stricto is still neglected compartment regarding its structure, diversity of life and habitats, and role for the provision of ecosystem services. Especially in thick AZ of topographic recharge areas, fluid flow dynamics and the exchange of the total mobile inventory (Lehmann et al. 2021) and their roles for the quality-evolution of groundwater are largely unknown. In the low-mountain topographic recharge area of the Hainich Critical Zone Exploratory (central Germany), we study spatiotemporal dynamics of the fluid fluxes and mobile inventory within the shallow (upper) AZ (regolith) and compare their signature with soil seepage and perched groundwater (deeper AZ). Percolates from 20 drainage collectors (DC) covering a diversity of Triassic mixed carbonate-siliciclastic (sedimentary) bedrock, soil types, and installation depths were sampled for more than 3 years on regular (monthly) and event-based basis and analyzed by various physico-/hydrochemical and spectro-microscopic techniques.

On average, the DC captured ~13% of the percolate from the forest topsoil seepage and 2.4% of precipitation. Seepage volume was mainly influenced by the factors soil thickness and sampling month, followed by scarp slope gradient and seasonal differences. In the upper AZ, the mobile inventory exhibited strong seasonality (e.g. EC, pH, nitrate, sulphate, K, Si, Mn, Al, Fe, particle concentration) and were more dependend on seasonal weather conditions and single (extreme) events (e.g., snow melt, rain events) than on lithology, followed by site-specific structural factors (location, slope), or pedological settings (e.g. overburden soil type, soil thickness). Generally, our results show fluid-rock interactions within the upper AZ with a more similar hydrochemical water signature to perched groundwater. Contrastingly, particulate mobile inventory showed a strong connection to soil seepage signature, comprising a diverse spectrum of mineral particles (mainly clay minerals) and mineral- and mineral-organic associations up to 160 µm, including aggregates and microorganisms. The different flow regimes that prevail during different seasons and weather conditions mainly influenced the amount and spectrum of percolate mobile inventory. During summer, dry periods in conjunction with extreme precipitation events favored translocation of small-sized particles. In winter, fast-flow regimes during normal precipitation as well as during snowmelts contributed strongly to the translocation of organic/inorganic carbon and mineral particle through the AZ and to groundwater. We conclude that the AZ is a complex biogeochemical reactor, that severely alters the percolate composition and properties, already preshaping the biogeochemical groundwater quality as well as due to its functions and services (e.g. water-purification and storage). As such, the aeration zone hast to be considered as a crucial compartment for groundwater quality evolution, especially in topographic recharge areas.

 

Lehmann, K., Lehmann, R., Totsche, K. U. (2021) Event-driven dynamics of the total mobile inventory in undisturbed soil account for significant fluxes of particulate organic carbon. Sci. Total Environ. 756, 143774, doi: https://doi.org/10.1016/j.scitotenv.2020.143774