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Abstract:
The quality of near-surface groundwater reservoirs is controlled, but also threatened, by manifold surface–subsurface interactions. Vulnerability studies typically evaluate
the variable interplay of surface factors (land management,
infiltration patterns) and subsurface factors (hydrostratigraphy,
flow properties) in a thorough way, but disregard
the resulting groundwater quality. Conversely, hydrogeochemical
case studies that address the chemical evolution
of groundwater often lack a comprehensive analysis of
the structural buildup. In this study, we aim to reconstruct
the actual spatial groundwater quality pattern from a synoptic
analysis of the hydrostratigraphy, lithostratigraphy, pedology
and land use in the Hainich Critical Zone Exploratory
(Hainich CZE). This CZE represents a widely distributed yet
scarcely described setting of thin-bedded mixed carbonate–
siliciclastic strata in hillslope terrains. At the eastern Hainich
low-mountain hillslope, bedrock is mainly formed by alternated
marine sedimentary rocks of the Upper Muschelkalk
(Middle Triassic) that partly host productive groundwater
resources. Spatial patterns of the groundwater quality of
a 5.4 km long well transect are derived by principal component
analysis and hierarchical cluster analysis. Aquifer
stratigraphy and geostructural links were deduced from lithological
drill core analysis, mineralogical analysis, geophysical
borehole logs and mapping data. Maps of preferential
recharge zones and recharge potential were deduced from
digital (soil) mapping, soil survey data and field measurements
of soil hydraulic conductivities (Ks). By attributing
spatially variable surface and subsurface conditions, we were
able to reconstruct groundwater quality clusters that reflect
the type of land management in their preferential recharge
areas, aquifer hydraulic conditions and cross-formational exchange
via caprock sinkholes or ascending flow. Generally,
the aquifer configuration (spatial arrangement of strata, valley
incision/outcrops) and related geostructural links (enhanced
recharge areas, karst phenomena) control the role
of surface factors (input quality and locations) vs. subsurface
factors (water–rock interaction, cross-formational flow)
for groundwater quality in the multi-layered aquifer system.
Our investigation reveals general properties of alternating sequences
in hillslope terrains that are prone to forming multilayered
aquifer systems. This synoptic analysis is fundamental
and indispensable for a mechanistic understanding of ecological
functioning, sustainable resource management and protection.
