Pedological and hydrogeological setting and subsurface flow structure of the carbonate-rock CZE Hainich in western Thuringia, Germany

Abstract

Abstract. The quality of near surface groundwater reservoirs is to a large extent controlled by land use and the properties of the soils in the recharge areas. Studies on groundwater quality and vulnerability, therefore, call for a thorough and holistic analysis of the buildup and hydraulic properties of the full range of surface and subsurface compartments involved in the interactions of water with immobile surfaces of the soils and rocks. This study provides a comprehensive characterization of the soils senso stricto, the unsaturated zone, aquifer stratigraphy and hydrochemistry in a hillslope karst environment of a carbonate-rock subcatchment of the Hainich Critical Zone Exploratory (CZE). This CZE is located within the Upper Muschelkalk Formations in the Hainich low mountain range, Thuringia, central Germany. We investigated the soils, surface geology, land use types and the infiltration properties based on a field survey. Aquifer stratigraphy, lithology and structure were analyzed based on drill core analysis, mineralogical analysis and geophysical borehole logs. Hydrogeochemical data from 15 permanent monitoring wells along a 5.4 km long hillslope transect were analyzed for major and minor ions, total and dissolved organic and inorganic carbon during a 4 year monitoring period and were statistically evaluated. The geological succession of the interlayered and laterally continuous limestone (karst/) fracture aquifers and marlstone aquitards of the Upper Muschelkalk results in two main aquifer assemblages (HTL and HTU = Hainich transect lower/upper aquifer assemblage), which comprise ten previously undocumented individual aquifer storeys. The geologically inferred stratification of the subsurface was confirmed by principal component analysis and cluster analysis of groundwater chemistry. According to groundwater compositions within the HTL and HTU assemblages, there are 5 main clusters (2 in HTU and 3 in HTL). Soil properties are related to infiltration rates and the initial chemistry of recharge waters. The reconstructed recharge zones for the aquifer storeys are characterized by predominantly forest land use on "carbonate series" soils and cropland/pasture land use on "siliceous series" soils, the latter developed from quarternary aeolian loess and alluvial valley fills. Based on the local geological structure, aquifers in the lower positions of the litho/hydrostratigraphy (i.e. HTL) outcrop in higher slope positions, show greater catchment sizes and a greater limestone/marlstone-ratio. The latter leads to thin soils, low water retention potential and predominantly forest land use, resulting in presumably high groundwater recharge rates and little anthropogenic influence on groundwater quality. As the subsurface stratification enforces confined groundwater flow, intrastratal karstification and the resulting hydraulic conductivity increases (and groundwater residence time decreases) with increasing bed thickness and limestone/marlstone ratio towards the lower portions of the stratigraphic succession. By implication, aquifers in the upper litho/hydrostratigraphic positions (HTU) exhibit small and partly agriculturally managed catchments with thick soils, low infiltration potential and, based on thinly bedded aquifer/aquitard successions, low hydraulic conductivities and long groundwater residence times. All in all, the complex interplay of geological structure, lithology, soil group and land use type results in distinct hydrochemical, and presumably ecological conditions in the different aquifers.

Carbon isotopes of dissolved inorganic carbon reflect utilization of different carbon sources by microbial communities in two limestone aquifer assemblages

“…Mesozoic rocks are partly to totally covered by Pleistocene Loess loam in the midslope/footslope area. Footslope valleys are filled with unconsolidated alluvium Kohlhepp et al, 2016). Agricultural areas with different management intensities surround the largely unmanaged forest area at the eastern hillslope of the Hainich low mountain range.…”

“…The Hainich CZE comprises here a number of surface and belowground observational plots along a 5.4 km long hillslope transect in an intensively investigated area of about 29 km 2 (Kohlhepp et al, 2016;Küsel et al, 2016). A groundwater well transect consists of five locations (H1 (upslope) to H5 (toeslope)) that also span a land-use gradient from deciduous managed forest (H1), unmanaged woodland (H2), and grassland/pasture (H3) to cropland agriculture (H4 and H5) longitudinal to the assumed groundwater flow direction (Fig.…”

“…1). The wells provide access to two main aquifer assemblages, referred to as HTL and HTU (Hainich transect lower/upper aquifer assemblage, respectively ), which consist, respectively, of one (HTL) and nine (HTU) individual aquifer storeys Kohlhepp et al, 2016). The HTU aquifers are sampled at depths ranging between 0.6 m (mid-slope; H3) and 54 m (downslope; H5), while the HTL aquifers are sampled at 2 m (upslope; H1) and 89 m (H5) (Fig.…”

“…HTU comprises mainly an layered aquifer complex within the overlying Meissner formation (moM), comprising fracture aquifers with fine fissures and less pronounced karstification due to the finely alternating succession of limestones and low-permeable marlstone beds ( Fig. 1; Küsel et al, 2016;Kohlhepp et al, 2016). Vertical exchange is strongly inhibited by frequent and lowpermeable marlstone interbeds resulting in confined flow conditions and a layer-cake architecture (Kohlhepp et al, 2016).…”

“…In limestone landscapes like the Hainich CZE, groundwater recharge rates can be fast due to karstification or slower due to thick soils developed on Quaternary loess deposits (Kohlhepp et al, 2016). In such regions, recharge waters containing biogenic soil CO 2 that is mostly of recent origin near the surface react with the carbonates in aquifer rock according to the stoichiometry of Eq.…”