Shallow weathering patterns and linked hydrostratigraphy in topographic recharge areas are rarely explored even for used groundwater flow systems, as (scientific core) drilling was usually carried out in downstream zones of productive resources. In the Hainich Critical Zone Exploratory (NW Thuringia, Germany), we constructed a well network encompassing the recharge area (hilltop, midslope) in discharge direction (footslope) for long-term monitoring of the links between surface and subsurface biogeosphere (Küsel et al., 2016). In total, ~730 m of rock cores from the limestone-mudstone alternations of the Upper Muschelkalk, including drillings in the thick hillslope aeration zone, were used for the exploration of hydrogeological functions, flow paths and endolithic habitats (Kohlhepp et al. 2017). Based on the core and monitoring data, an advanced conceptual model on multidirectional flow patterns and subsurface ecosystem compartmentalization was developed (Lehmann and Totsche, 2020). Novel and surprising findings revealed unseen patterns in the Upper Muschelkalk flow system: confined weathering phenomena in fissures and pores indicate localized, oxic conditions in sections that were previously encountered elsewhere as anoxic and unweathered. This indicates a higher complexity of flow and transport pathways (e.g., oxygen supply) than commonly expected, despite simple layer-cake geometry of the bedrocks. Besides relief position, it points to a strong influence of fractures on flow in the aeration zone.
References: Kohlhepp et al. (2017): https://doi.org/10.5194/hess-21-6091-2017; Küsel et al. (2016): https://doi.org/10.3389/feart.2016.00032; Lehmann and Totsche (2020): https://doi.org/10.1016/j.jhydrol.2019.124291