Hydrol. Earth Syst. Sci., 21, 6091–6116, 2017 https://doi.org/10.5194/hess-21-6091-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Aquifer configuration and geostructural links control the groundwater quality in thin-bedded carbonate–siliciclastic alternations of the Hainich CZE, central Germany Bernd Kohlhepp1, Robert Lehmann1, Paul Seeber1, Kirsten Küsel2,3, Susan E. Trumbore4, and Kai U. Totsche1 1Friedrich Schiller University Jena, Institute of Geosciences, Chair of Hydrogeology, Burgweg 11, 07749 Jena, Germany 2Friedrich Schiller University Jena, Institute of Ecology, Chair of Aquatic Geomicrobiology, Dornburger Strasse 159, 07743 Jena, Germany 3German Centre for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Deutscher Platz 5d, 04103 Leipzig, Germany 4Max Planck Institute for Biogeochemistry Jena, Department of Biogeochemical Processes, Hans-Knöll-Str. 10, 07745 Jena, Germany Correspondence to: Kai U. Totsche ([email protected]) Received: 21 July 2016 – Discussion started: 5 September 2016 Revised: 4 October 2017 – Accepted: 17 October 2017 – Published: 1 December 2017 Abstract. The quality of near-surface groundwater reser- recharge zones and recharge potential were deduced from voirs is controlled, but also threatened, by manifold surface– digital (soil) mapping, soil survey data and field measure- subsurface interactions. Vulnerability studies typically eval- ments of soil hydraulic conductivities (Ks). By attributing uate the variable interplay of surface factors (land manage- spatially variable surface and subsurface conditions, we were ment, infiltration patterns) and subsurface factors (hydros- able to reconstruct groundwater quality clusters that reflect tratigraphy, flow properties) in a thorough way, but disre- the type of land management in their preferential recharge gard the resulting groundwater quality. Conversely, hydro- areas, aquifer hydraulic conditions and cross-formational ex- geochemical case studies that address the chemical evolu- change via caprock sinkholes or ascending flow. Generally, tion of groundwater often lack a comprehensive analysis of the aquifer configuration (spatial arrangement of strata, val- the structural buildup. In this study, we aim to reconstruct ley incision/outcrops) and related geostructural links (en- the actual spatial groundwater quality pattern from a synop- hanced recharge areas, karst phenomena) control the role tic analysis of the hydrostratigraphy, lithostratigraphy, pedol- of surface factors (input quality and locations) vs. subsur- ogy and land use in the Hainich Critical Zone Exploratory face factors (water–rock interaction, cross-formational flow) (Hainich CZE). This CZE represents a widely distributed yet for groundwater quality in the multi-layered aquifer system. scarcely described setting of thin-bedded mixed carbonate– Our investigation reveals general properties of alternating se- siliciclastic strata in hillslope terrains. At the eastern Hainich quences in hillslope terrains that are prone to forming multi- low-mountain hillslope, bedrock is mainly formed by al- layered aquifer systems. This synoptic analysis is fundamen- ternated marine sedimentary rocks of the Upper Muschel- tal and indispensable for a mechanistic understanding of eco- kalk (Middle Triassic) that partly host productive groundwa- logical functioning, sustainable resource management and ter resources. Spatial patterns of the groundwater quality of protection. a 5.4 km long well transect are derived by principal com- ponent analysis and hierarchical cluster analysis. Aquifer stratigraphy and geostructural links were deduced from litho- logical drill core analysis, mineralogical analysis, geophys- ical borehole logs and mapping data. Maps of preferential Published by Copernicus Publications on behalf of the European Geosciences Union. 6092 B. Kohlhepp et al.: Aquifer configuration and geostructural links control the groundwater quality 1 Introduction partments involved in the flow of water and transport of mat- ter in the subsurface of the Hainich CZE. The goals of our Near-surface groundwater reservoirs are increasingly threat- study are (1) to understand the hydrogeological and biogeo- ened by anthropogenic impact, like the intensification of chemical functioning of the subsurface in a setting provided agricultural land use and water production, while their eco- by the Hainich CZE that so far has not been adequately ad- logical functioning is still rather unexplored. Generally, dressed, (2) to explore the links and feedbacks between the the hydrogeochemical and (micro)biological compositions surface and subsurface, and (3) to demonstrate that a holistic of shallow (< 200 m) groundwater systems are a result of multi-method approach that considers the surface and subsur- biogeochemical processes and the fluid–rock/soil interac- face factors is indispensable for the mechanistic understand- tions while travelling through the overburden of the aquifers ing of the coupling of surface and subsurface compartments, (Urich, 2002; Canora et al., 2008). Infiltrating precipita- fluid dynamics, biogeochemical element cycling and ecology tion, as the main component of recharge of the surface in the critical zone. near groundwater systems, first passes the soils. Thereby, To reach these goals, the following research questions the seepage collects among others dissolved organic carbon are answered. (i) How is the critical zone comprised and (Guggenberger and Kaiser, 2003), colloidal, mineral, organic connected in the hillslope setting of a thin-bedded, mixed and organo-mineral suspended particles (Münch et al., 2002; carbonate–siliciclastic succession? (ii) How do spatial ar- Totsche et al., 2006, 2007; Schrumpf et al., 2013; Gleixner, rangement, particularly outcrop patterns and geostructural 2013) and even biota (Dibbern et al., 2014). The residence links (karst features like caprock sinkhole lineaments), im- time of the percolating water in the soils as well as in the pact compartment connection (intrinsic vulnerability) and unsaturated and saturated zones regulates the chemical and groundwater quality? Does groundwater quality in contrast- biological quality by the extent to which the partial equilib- ing summit/midslope and footslope wells reflect surface in- rium of the rate-limited dissolution, retardation and release fluences? And finally: (iii) what are the main control pa- processes is achieved (Weigand and Totsche, 1998; Münch et rameters for groundwater quality? What are the reasons for al., 2002). Residence times and thus groundwater quality de- particular hydrogeochemical conditions within the multi- pend on various surface factors (duration and intensity of pre- storey/hillslope aquifer system? cipitation, land management/vegetation, presence of macro- pores) and subsurface factors (i.e. soil thickness, hydraulic parameters and preferential flow paths). 2 Site description However, subsurface factors are spatially very variable in fractured rock and carbonate-rock terrains (Cook, 2003), and The Hainich Critical Zone Exploratory covers 430 km2 of a their relative importance for recharge and groundwater qual- hillslope subcatchment of the Unstrut River in north-western ity is still poorly understood (Gleeson et al., 2009). Sus- Thuringia, central Germany (Fig. 1). It is bounded by the re- tainable management and resource protection therefore call cipient Unstrut River and the distribution area of the Upper for a holistic investigation of surface–subsurface architec- Muschelkalk formations. The Hainich represents a NW–SE ture and interactions for the reconstruction of the hydroge- orientated geological anticline that is developed topograph- ological functioning and groundwater quality. Concepts for ically as a low-mountain range with a steep western flank vulnerability assessment (Hölting et al., 1995; Goldscheider and a moderately inclined eastern flank (Jordan and Weder, et al., 2000; Witkowski et al., 2003; Vías et al., 2006), for 1995). The study area is located at the eastern flank of the instance, already consider the variable interplay of surface– Hainich hillslope that shows a geostructural buildup with NE subsurface factors and compartments in a thorough way, but dipping strata in the direction of the syncline of Mühlhausen– disregard the evolution of pristine groundwater quality. Con- Bad Langensalza (Kaiser, 1905; König, 1930; Patzelt, 1998; versely, hydrogeochemical case studies (i.e. Suk and Lee, Wätzel, 2007). Tectonic uplift, faulting and tilting of strata 1999; Moore et al., 2009) often lack a comprehensive anal- are assumed for the Late Cretaceous in analogy to the sur- ysis of the structural buildup and composition of the en- rounding horst structures Thüringer Wald (in the south) and tire compartments, whereas the important “biogeoreactor” Harz (in the north; compare to Voigt et al., 2004; Kley and of the overburden soils is vastly neglected. Studies that in- Voigt, 2008). The outcropping strata in the study area com- clude land use and soil groups (Pronk et al., 2009; Steward prise sedimentary rocks (Middle/Upper Muschelkalk and et al., 2011; Allocca et al., 2014) typically leave groundwater Lower Keuper – subgroups of the Middle Triassic). Accord- chemistry out of their focus. In this study, we follow a holistic ing to the German stratigraphy (Deutsche Stratigraphische approach for reconstructing the pristine groundwater qual- Kommission, 2002), the Diemel formation (Middle Muschel- ity in the aquifers of the Hainich