Multi-directional flow dynamics shape groundwater quality in sloping bedrock strata

Journal Pre-proofs Research papers Multi-directional flow dynamics shape groundwater quality in sloping bedrock strata Robert Lehmann, Kai Uwe Totsche PII: DOI: Reference:

S0022-1694(19)31026-1 https://doi.org/10.1016/j.jhydrol.2019.124291 HYDROL 124291

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Journal of Hydrology

Received Date: Revised Date: Accepted Date:

26 March 2019 25 October 2019 26 October 2019

Please cite this article as: Lehmann, R., Totsche, K.U., Multi-directional flow dynamics shape groundwater quality in sloping bedrock strata, Journal of Hydrology (2019), doi: https://doi.org/10.1016/j.jhydrol.2019.124291

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© 2019 Published by Elsevier B.V.

Multi-directional flow dynamics shape groundwater quality in sloping bedrock strata

Robert Lehmann1 and Kai Uwe Totsche1*

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Hydrogeology, Institute of Geosciences, Friedrich Schiller University Jena, Burgweg 11, 07749 Jena, Germany.

*Corresponding author: Kai Uwe Totsche, Friedrich Schiller University Jena, Institute of Geosciences, Department of Hydrogeology, Burgweg 11, 07749 Jena, Germany Email address: [email protected]

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Abstract Multi-directional fluid flow and transport dynamics as intrinsic characteristics of hillslope flow regimes can strongly contribute to the quality evolution of groundwater resources and compartmentalization of subsurface ecosystems. However, their extent and importance in topographic highs (groundwater recharge areas) is typically less investigated, because productive groundwater bodies and thus monitoring activities are frequently lacking. In the Hainich Critical Zone Exploratory, we explored the hydrogeological functioning of the widely distributed setting of thin-bedded, alternating mixed carbonate-/siliciclastic bedrock, by using basic environmental timeseries. Up to 8-year records of weather parameters, ground temperatures, multi-depth hydraulic heads, and non-conservative tracers, were exploited applying a scheme for the exploratory analysis of dis-/continuous groundwater quality data. We identified transient, multi-directional flow dynamics, comprising fluctuating perched groundwater, localized recharge and groundwater mounding in the aeration zone that modify and partly reverse flow patterns in the phreatic zone. This interplay of flow dynamics within the hillslope aeration and phreatic zone causes significant re-distribution (e.g. oxygen, nitrate) that even overcome the “protective cover” of thick argillaceous strata by supplying surface-sourced substances to deep groundwater resources. As a factor for ecosystem compartmentalization, our results further suggest to carefully consider the hillslope multidirectional flow and matter exchange in biogeochemical models, as well as in resource protection and groundwater management practices.

Keywords: Aeration zone, non-conservative authigenic tracers, fractured rock, subsurface ecosystems, crossstratal flow

Abbreviations: CWB, climatic water balance; CZE, critical zone exploratory (observatory); Fm., formation; m amsl, meter above mean sea level; m bgl, meter below ground level; SWCC, sliding-windows cross-correlation

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1. Introduction Multi-directional exchange of fluids, solutes, particles and energy within the subsurface compartments of topographic highs is an often observed, yet rarely addressed factor for dynamics of bedrock groundwater quality and interdependent subsurface ecosystems. Generally, the quality and ecology of shallow groundwater is controlled to a variable extent by inputs with infiltrating precipitation and seepage (Lin 2010), by hyporheic zone exchange (Schmidt and Hahn 2012), by fluid-rock interactions (Daniele et al. 2013) and biogeochemical cycling (Lohse et al. 2009). The fluid flow, pervaded solids and thus transport of matter and energy differ largely between the aeration zone and the zone of permanent saturation. The aeration zone harbors variably active flow paths (Pronk et al. 2008) and shows pronounced bypass flow, matter re-/distribution by water level fluctuations (Legout et al. 2007, Rouxel et al. 2011), but also (discontinuous) permanently water-saturated parts. In contrast, the phreatic zone that locally contain productive groundwater resources, is not underlain by unsaturated zones. Its depth is controlled by various factors like climate, topography, bulk rock permeability and the gravity-driven organization of groundwater flow (Tóth 1995, Haitjema and Mitchell-Bruker 2005) that usually cause a large thickness of aeration zones beneath topographic highs (Salvucci and Entekhabi 1995). “Multi-directional” highlights the fact that subsurface fluid flow in both zones is characterized by transient variation of generally gravity-driven flow regimes. Whereas in the shallow aeration zone, viz. the soil mantle, variable surface-subsurface exchange is well-studied with respect to hillslope-stream connectivity, runoff generation, and stream water quality (Gabrielli et al. 2012), fluid flow dynamics within deeper bedrock are scarcely recognized (Salve et al. 2012, Rouxel et al. 2011). The deep aeration zone, however, can regularly undergo ascending cross-communication as well, when rainstorm or snowmelt infiltrations exceeds the local percolation capacity and result in perched groundwater and event-scale upwelling flow. In the phreatic zone, the partitioning of recharge into shallow local to regional flow systems varies with the dynamic phreatic surface (Goderniaux et al. 2013) and cause directions of flow and transport to be transient and even regularly reversed. So far, the importance of multi-directional flow and transport phenomena within topographic recharge areas, both in the interacting aeration and phreatic zone, for the downgradient resource quality is essentially unknown. Especially, dynamics of flow, transport and transformation in the bedrock part of the aeration zone are not in focus of resource management practices, since its monitoring in hillslope terrain is challenging (Gabrielli et al. 2012). Likewise, it is vastly unknown to what extent low-permeability strata (aquitards) take part in hillslope fluid flow. We thus explored multi-directional flow and exchange dynamics in a hillslope setting of the widely-distributed bedrock type (Westphal 2006) of limestone-mudstone alternations. By analyzing long-term environmental data

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for subsurface responses to surface and subsurface signals, we aimed to answer: 1) Where and when do conditions of multi-directional exchange occur? 2) Do these conditions significantly affect groundwater quality? In general, surface signals are “filtered, retained or eliminated” (Larocque et al. 1998) within the subsurface. The same holds for superimposing signals of subsurface origin. Their transformations though offer insights into the subsurface architecture and flow patterns. However, hydrographs, thermographs, chemographs and time series of discrete data reveal characteristics of both the surface events and of the connected compartments as a “global response” (Kiraly 2003) to the input. Furthermore, in topographic highs, aeration zone water that circulates (far) above the phreatic surface, is of predominant meteoric origin, so that isotopic water tracers gain no insights. We thus systematically analyzed multi-year and multi-depth time series of water levels and of a large set of nonconservative, including authigenic tracers to elucidate the hidden aeration zone dynamics and sub-regional impacts.

2. Material and methods 2.1 Study site Within the Hainich Critical Zone Exploratory (CZE), our study site (Fig. 1) is located at the Hainich’s gently inclined eastern hillslope that is covered by managed mixed beech forest, unmanaged woodland and shrubland (Hainich National Park) and synclinewards, pasture land, cropland and small villages. As an anticline that exhumed mixed carbonate/siliciclastic Middle Germanic Triassic bedrock (Fig. 1-B), the Hainich forms the NWSE oriented margin of the Mühlhausen-Bad Langensalza Syncline. The well transect (Fig. 1-C) accesses marine sediments of the Upper Muschelkalk (mo) lithostratigraphic subgroup. Bioclastic limestone beds with minor mudstone interbeds form the basal Trochitenkalk formation (Fm.). The hanging Meissner Fm. and Warburg Fm. contain thinner bedded alternations of calcimudstones, various mudstone types and thin bioclastic marker beds (c.f. Kohlhepp et al. 2017). These sloping strata (Fig. 1-B) are locally and in footslope positions covered by Lower Keuper (ku) sediments, Quaternary loess derivates and alluvial valley fills. The large hillslope represents fluviokarst with its transverse valleys and lineaments of caprock sinkholes (Fig. 1-D) that originate from cavitation in Middle Muschelkalk (mm) evaporites (anhydrite, halite; Jehne 1978). Karstification that is pronounced for mm strata and limited to widened fractures in carbonate rocks in mo strata is of “intrastratal karst” type (cf. Klimchouk and Ford 2000). The thin-bedded sequences host a multi-storey aquifer system of fractured to karstic-fractured aquifers and mudstone-dominated aquitards. Infiltration-recharge to the sloping strata is assumed to take place preferentially in outcrop areas with thin soil coverage (Kohlhepp et al. 2017). The main groundwater resources are bound to

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regional fracturing zones (Jehne 1978) that redirect the subsurface runoff from Eichsfeld and Hainich to NW-SE (Fig. 1-C), jointly caused by lesser karstified strata (“permeability barrier”, Treffurt 1982; Fig. 1-B/C). Here, groundwaters feed prominent sinkhole/fracture artesian springs (Fig. 1-A). Our subcatchment’s main discharge area is the Golken spring system in Bad Langensalza (Götze 1969; Fig. 1-C). Parts of the Heilbronn Fm. or both the presumably well-connected Diemel Fm. and hanging Trochitenkalk Fm. (moTK; Jehne 1978; Teichmüller 1989) are considered as the main aquifer, while the hanging Meissner Fm. (moM)/Warburg Fm. (moW)/Erfurt Fm. (kuE) represent a minor aquifer system (Teichmüller 1989). According to analogous regions (see Zech et al. 2016), the local to intermediate flow system likely span circulation down to at least the Lower Muschelkalk with the laying Röt Fm. (Upper Buntsandstein) representing an effective hydraulic barrier (Huckriede and Zander 2011).

2.2 Environmental monitoring Groundwater monitoring and aquifer tests From summit to footslope positions, currently 16 bedrock wells with a total number of 23 screen sections span different land use, relief positions (Fig. 1) and depths (see Table 1). Construction procedure and materials are reported in Küsel et al. (2016). Groundwater levels and screen temperatures are recorded with vented data loggers. Multi-parameter probes record temperature (T; resolution: 0.01 K), specific electrical conductivity (EC25; reference T: 25°C), pH, redox potential (ORP), dissolved oxygen concentration (DO; optical) and absolute pressure. Prior to data analysis, sampling-induced fluctuations and operation artefacts (offsets, drifts, gaps) were removed from the hydrographs and the natural progression was reconstructed by additional discrete or redundant level data and linear interpolation. In additional multi-channel tubes of eleven wells, multi-depth hydraulic heads were measured weekly (07/17-10/18) with a water level meter. Groundwater was extracted with submersible pumps (MP1 or SQE 5-70, Grundfos, Denmark) and PE-HD tubing at least every four weeks (Oct 2011 to Sep 2018). Prior to sampling, pumping continued until steady conditions of physicochemical parameters were reached. Upper slope wells with low yields were sampled with a low-flow bladder pump, 12V pump or with bailers (all stainless steel, PE-LD tubing). Physicochemical parameters were measured using flow-through-cells, daily calibrated meters and digital probes for EC, pH, ORP and DO. Acid neutralizing capacity (ANC4.3) by acid-titration and redox-sensitive parameters (Fe2+, NO2-, NH4+, HS-, PO43-) by colorimetry were measured at the Hainich CZE field base (Kammerforst). Laboratory analyses (duplicates) comprised major anions (SO42-, Cl-, NO32-, NO2-, PO43-; after 0.45 µm filtration, PES filter) by ion

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chromatography and elemental composition, including major cations by ICP-OES and ICP-MS. Dissolved organic carbon (DOC; 0.45 μm, PES) and total OC (TOC, unfiltered) were measured by high-temperature catalytic oxidation. The saturation index of calcite (SIcalcite) and the partial pressure of CO2 (pCO2) were calculated with PhreeqcI 3.3.9 (Parkhurst and Appelo 1999) using alkalinity, major ion concentrations, pH, T and the thermodynamic database WATEQ4F (Ball and Nordstrom 1991). Recovery periods (3-11 each well) of pronounced and cross-seasonal sampling-induced residual drawdowns, and one drawdown period of a high-discharge aquifer test (H51; pumping rate: 4 m³/h) were analyzed utilizing MLU for Windows Lite (C. J. Hemker, The Netherlands). Transmissivity and storage coefficient were obtained by the analytical solution of well flow equations of layered aquifer systems (see Hemker 1999). Screen section lengths were taken as confined aquifer thicknesses to calculate hydraulic conductivity/permeability.

Weather monitoring and climate data analysis Weather data were continuously recorded with two stations at Reckenbühl and Heuberg (Fig. 1-D) and supplemented with data from the meteorological station in Weberstedt (MeteoGroup Deutschland GmbH, Germany; 270 m amsl; Fig. 1-D). The growing season was determined as defined in Frich et al. (2002). A daily climatic water balance (CWB), also referred to as effective precipitation, was calculated as the difference between measured daily precipitation and potential evapotranspiration, calculated with the Penman-Monteith method according to Allen et al. (1998). Its cumulative form (CWBcum) depicts soil moisture surpluses and deficits (c.f. Steenhuis and Van der Molen 1986). The linear trend of the CWBcum time series was detrended for hydrograph analysis. Weather data recorded with our own stations were compared (event timing, magnitude) to Weberstedt for the period of simultaneous operation. In fact, Weberstedt underwent precipitation events that happened at the hilltop, thus, allowing substitutability.

2.3 A scheme for classification and analysis of subsurface responses We applied a scheme for classification and analysis of subsurface responses to signals of surface or subsurface origin, inherent in groundwater quality data. It utilizes the temporal fluctuations of environmental parameters to track cross-stratal exchange and to distinguish directions of exchange, rather than allow for the identification of signal sources. The scheme comprises (1) a classification of subsurface responses, (2) exploratory timeseries analyses, and a (3) response matrix, summarizing dominating responses of a suite of environmental tracers used to unravel the hydrogeological functioning.

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Classification of surface signals and subsurface responses Surface signals (Fig. 2-A) are generally introduced in the subsurface by variable input of matter and energy by atmospheric forcing (e.g. precipitation, surface temperature) or interacting surface conditions (e.g. vegetation period, tillage). The signals can be of episodic and seasonal character or both (superimposition). Subsurface responses (Fig. 2-B, C) are the event-scale or seasonal (re-)distribution of matter (incl. fluids), information and energy. Hydrographs (Fig. 2-B) show the water level fluctuation (or hydraulic head) of a monitoring well (or spring) vs. time. Generally, hydrographs may show pronounced event-scale peaks in topographic recharge areas (upslope, small contribution areas), and under conditions of low aeration zone storage, aquifer storativity, and/or localized recharge, conduit/fracture flow (see Smart and Hobbs 1986; Bakalowicz 2005). Whereas delayed (i.e. longer lag I), damped, as well as superimposed and cumulating responses indicate larger contribution areas (downslope, discharge areas), higher storativity, and/or diffuse recharge and diffuse (retarded) flow. Together with spatiotemporal head data, responses of subsurface environmental parameters (Fig. 2-C) indicate event-scale or seasonal directed exchange of matter and energy. The informative value of single parameters of authigenic (target zone) or external origin, may be limited if superimposition of signals and responses occur due to multiple relevant sources. Thus, their utilization is dependent on spatiotemporal conditions, therefore requiring alternative environmental parameters, if necessary. The response types integrate combined and interacting processes and exchange conditions. Reactive transport phenomena like advection, dispersion, retardation, and degradation control the lag time (II), and peak height/width. “Simple responses” indicate, for instance, the input of external matter (increase) or dilution (decrease) by recharging waters. Variable responses point to, for instance, variable importance of the former by variably active/composed signal sources or exchange directions. “Dual responses” can indicate piston-flow phenomena (i.e. replacement of pre-event water with minor mixing) (e.g. Williams 1983) in soil or bedrock. “Multiple responses” may be caused by the sequential activity of flow paths and sources within the contribution area (see White 2016).

Exploratory time series analysis Descriptive and multi-variate statistical analyses and time series analyses were carried out using SPSS Statistics (v. 25; IBM, USA) and Origin Pro (v. 2018; OriginLab, USA). Hereinafter, “seasonality” addresses periodic components of time series and “episodic” refers to irregular occurrence. Extreme-event thresholds for precipitation were determined from long-term data (1951-2005; Kammerforst, abandoned station, 270 m amsl), solely upon event rarity, if rarer than the 99th percentile. Furthermore, we determined “extreme subsurface events” by the 0.1th and 99.9th percentile of recorded water levels and of the

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length of groundwater surplus/deficit periods. Besides, the irregular and non-seasonal water saturation of regularly water-unsaturated zones or aeration of saturated zones, respectively, or any extreme state in local energy/matter conditions would represent subsurface extremes. The delineation of extreme or even surface/subsurface “single events”, the latter occurring only once within a time series, generally depends on the length and temporal resolution of the respective records. Longer time series and higher temporal resolution result in higher thresholds of extreme/single events and vice versa. Short-term events that are followed by fast relaxation are defined by their irregularity only and are not detectable based on sparse and short observation periods. We applied sliding-windows cross-correlation (SWCC; Delbart et al. 2014) for exploring the time-variant relation between CWBcum and groundwater level time series that were split into 6-month windows with 4-month overlaps. Time lags were picked from maxima of the cross-correlation function (> 95% confidence level) and plotted against the central dates of the 6-month windows. Periodicity was determined by periodogram inspection (sub-annual) following spectral analysis with Tukey-Hamming smoothing of daily levels (medians). Average hydrographs (season figures) were constructed from repeatedly plotted monthly medians. Subsequently, the large set of basic groundwater quality parameters, was analyzed. We inspected each (dis)continuous time series of single parameters, plotted against the hydrographs and tested the fluctuation of consecutive data (i.e. peak vs. late relaxation state) for statistical significance by applying the Mann-Whitney Utest (P <0.05). Hereby, parameters that show “non-significant responses” (Fig.2-C), though representing important information, were excluded from further analysis. In relation to each wells’ hydrograph, the reduced set of parameters was then visually analyzed for each parameter’s response characteristics, comprising the tendency (increase/decrease), timing (synchronous/delayed; Fig. 2: lag II) and also the fluctuation magnitude and relaxation behavior.

Response matrix Finally, dominating response behaviors (tendency, timing) of the groundwater parameters were aggregated in a “response matrix” whose column arrangement represent a transect within the investigated area. In the condensed and easily explorable form, predominant parameter responses are highlighted , thus, facilitating the evaluation of multi-depth and multi-site observations. As the response matrix show characteristics that were required to be reproduced within multi-year time series, generally suited tracers, their informative value and thus the hydrogeological functioning is revealed.

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3. Results 3.1 Atmospheric forcing: local climate and weather characteristics The mean annual precipitation at Kammerforst (1971-2000) is 722 mm. Extreme event thresholds for precipitation are 21.2 mm/d in winter and 30.1 mm/d in summer. Events with interruptions ≤ 1 d are referred to as “cumulative extremes”, if exceeding 85.5 mm in winter and 72.9 mm in summer. Precipitation-free periods are qualified extreme after 17 days in winter and 15 days in summer. For the monitoring period (2010-2018), in Weberstedt, the mean annual precipitation was 580 mm and the mean air temperature was 9.1 °C. The growing season lasted for an average of 237 d from 23 March to 14 November. Thus, the period 1 November to 31 October was set as the hydrological year for data analysis. The CWBcum (same period, not shown) shows a negative temporal trend, reflecting synclineward water scarcity in Weberstedt. Adding the average surplus (median: ~33%, 2003-2006) of daily precipitation that Kammerforst receives in comparison to Weberstedt led to a slight positive trend of the CWBcum (not shown), as expected for the hilltop areas. This site-correction also roughly equals out the annual sum differences. The de-trended, sinusoidal CWBcum (Fig. 3-A), the surrogate for soil moisture or deep infiltration, was found to be more suited than the precipitation record for joint time series analysis. The CWBcum shows maxima in February and minima from October to December. Second maxima, partly exceeding the previous, occurred from April to July. Seasonal maxima were pronounced in 2010/2011 (moderately wet beginning, not shown), 2014/2015 (wet previous season) and 2017/2018 (anomalous wet autumn in 2017). Recession was most pronounced in 2016 (driest hydrological year on record). Up to four snowmelts in 2013 (Fig. 3-A) that are not expressed in the CWBcum represent an anomalous wet period.

3.2 Subsurface responses 3.2.1 Groundwater level fluctuations and hydraulic properties The hillslope wells access conditions from mostly water-unsaturated (H11/12, H21/22/23), partly waterunsaturated (H13/31), mostly water-saturated (H14/32) within the aeration zone, to permanently water-saturated (H4-H5) within the phreatic zone. The water level time series exhibit an average periodicity of 366.9 d ± 16.4 (mean, standard deviation; H2-H5) and show high maxima (medians) of the SWCC cross-correlation function with CWBcum: H31 (0.61); H32 (0.82); H41 (0.74); H43 (0.74); H51 (0.76); H53 (0.57). The responses of hydraulic heads to infiltration events (CWBcum peaks) differ in timing (time lag), velocity of rise and recession, peak height, peak width (duration) and vary between wells and events. Hydrographs of the aeration zone are peaky by showing pronounced event-scale rising limbs and recessions. In contrast, smooth, damped and further delayed hydrographs within the phreatic zone show moderate rising limbs and hardly event-scale recessions (Fig.

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3-B) that are superimposed by ongoing recharge of larger contribution areas. In summer half years, however, the recession periods of phreatic zone wells are pronounced. SWCC of CWBcum and water levels revealed the lags (medians in d): 0 (H13/14/21/32), 3 (H23), 16 (H31/51/52), 20 (H43), 23 (H41), 43 (H53). The determined timing of high-frequency fluctuation by manually measuring differences between plotted CWBcum peaks and hydrograph peaks revealed fast responses to individual events (minimum-maximum in d): H13 (2-7), H14 (1-4), H21 (1-4), H31 (4-40), H32 (1-26), H41 (2-70), H43 (20-100), H51 (4-33), H53 (70-124). Maxima of eventscale level fluctuation (rise in m per day/recession in m per day) that occur within hours (summit) to a few days, are moderate in H13 (0.4/0.2), H21 (3.0/0.9), H32 (1.5/0.4), H41 (0.9/0.2), maximal in H31 (3.6/7.7) and minor (<0.4/0.3) in the other downslope wells. Maximal heads are usually reached between March and April and lowstands occur between October and December (Fig. 3-B). The highest annual heads correspond to snowmelts or heavy rain events and pronounced head fluctuations occur in winter. The fluctuation magnitudes increase from the summit/shoulder to the midslope and footslope and range between a few decimeters and more than 25 meters per high-flow event or seasonal maximum (Fig. 3-B). Rare saturation in shoulder wells are related to snowmelts (spring 2013, Fig. 3-B) or subsequent wet periods. Midslope wells are highly responsive, although the lower moTK well lacks responses to summer events. Midslope to footslope wells exhibited an 8-year maximum in 2013 after multiple snowmelts, followed by heavy rains in May. In contrast to the pronounced highstands in 2011 (not shown) and 2018, the 2013 period resulted in an extreme duration of surplus. Subsequently, fast responses to rain events occurred in summer and during the beginning dormancy period. The multi-depth head monitoring (Fig. 4) revealed a stacking of water-unsaturated and saturated zones and covered one seasonal highstand. Between sites and depths, or perched and phreatic groundwater, respectively, head responses to event-scale and seasonal forcings showed different timing. Besides responsive fluctuation, we observed head reversals (i.e. heads of deeper measurement points exceed heads of shallower points) in midslope to footslope wells that indicate event-scale and seasonal variation of the flow regime. A contour map of the pressure heads in Trochitenkalk strata (Fig. 5-A) and the constructed head surface shows episodic saturation (i.e. perched groundwater) by the steep branch above the break of slope (Fig. 5-B). The projected regional phreatic surface, being deep below the summit (~150 m) and close to the surface in footslope positions and valleys (<5-10 m), is accordance to regional data (Fig. 5-A). The synclineward convergence of the horizontal flow component likely reflects flow to the regional main discharge (Golken spring) and illustrates an increase of the wells’ contribution areas.

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Hydraulic conductivities from our aquifer test data range from 7.5×10-6 (H53) to 5.1×10-5 (H51) m/s. Archive data (Fig. 5-C) of bulk rock permeabilities characterize the mo strata as moderately to low-permeable.

3.2.2 Fluctuation of environmental tracers and hydrochemical characteristics Groundwater temperature fluctuations Typically, shallow wells undergo sinusoidal temperature fluctuations in response to air temperature (not shown) that are damped and delayed with increasing depth (amplitude in K/delay to air temperature in months/depth in m bgl): 4.6/3/5; 1.5/5/7; 0.5/7/12. At summit, event-scale simple responses and dual responses (Fig. 6-A) show mostly cooling and range from 0.3 K (7 m, moM) to 0.45 K (15 m, moTK). Generally, deeper shoulder wells (2030 m bgl) and upper midslope wells (20-50 m bgl) show only minor variation <0.1 K, and cumulative responses that cause seasonal cooling trends (Fig. 6-B). Contrastingly, the lower midslope moTK well shows pronounced step-shaped temperature increases (0.3 K) with commencing head rises (Fig. 6-C). At the footslope, minimal fluctuations were recorded: 0.15 K (88 m bgl, moTK), 0.10 K (69 m bgl, moM) and 0.05 K (50 m bgl). While most shallow wells show no trends, the shallow midslope wells show minima in 2014. The deep lower midslope and footslope wells show a lasting cooling (0.1-0.2 K) since 2015 (Fig. 6-D. Generally, the rarity of high-frequent or strong temperature fluctuations indicate nearly given groundwaterbedrock thermal equilibria, However, the measurement resolution of our recording devices was generally appropriate for distinguishing surface/subsurface signal origin. Summit to midslope domains exhibit synchronous decreases that propagate down to 50 m depth. Here, cold-season infiltrating waters keep the contrast, even if weak. Summer events scarcely result in synchronous warming in summit wells. In midslope to footslope positions, synchronous increases strongly indicate ascending waters. For the deepest footslope well, temperature recordings were not sufficient for tracing multi-directional exchange, due to diminished differences in that depth. Here, the cooling trend since 2015

Hydrochemical parameters Statistical parameters of up to 8 years of groundwater quality data characterize the shallow perched and phreatic groundwater of the topographic recharge area (Table 1). Median values of EC25 and total dissolved solids, indicate a synclineward increase of mineralization with notably lowest values in shoulder wells (moM/moTK: 380/558 µS/cm; 291/492 mg/L) and a distinct maximum in footslope moTK (1070 µS/cm; 750 mg/L). Applying a Šcukarev (1974) classification, groundwaters are of Ca-HCO3+ (summit, shoulder) and Ca-Mg-HCO3+ type

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(midslope). Synclinewards, the moTK contains Ca-Mg-HCO3+-SO42- water, whereas the moM wells access MgCa-HCO3+ waters. Average pH values range from 7.0 (summit, saturated) to 7.4 (shoulder, aerated). Regarding saturated wells only, DO and ORP range from anoxic to suboxic conditions (0-0.3 mg/L) in moM strata, whereas moTK endure oxic (2.7-7.2 mg/L). DOC and TOC are maximal in shallow summit wells (~1.5 mg/L) and ~1 mg/L elsewhere. Simulated pCO2 values range from 0.01 to 0.03, being elevated in moM in shallow summit, lower midslope and deep footslope moTK. The SIcalcite ranges around equilibrium, with slightly elevated values (supersaturation) in moTK and aerated shoulder wells. Summit/shoulder wells are low in SO42-, Na, Mg, Si, K and Sr. Sulfate, Cland Sr concentrations are increasing downdip. Upper midslope wells are distinctly high in NO3-. Silicon concentrations, increasing along the transect as well, are elevated in moM wells. Noteworthy, the lower midslope moTK well (H41) is high in SO42-, but low in Cl-. It shows a broad scattering in Sr/Ca vs. Mg/Ca plots (not shown) and episodically changes water type from Ca-Mg-HCO3+ to Mg-Ca-HCO3+. The footslope moTK well (H51) shows distinct maxima of SO42-, Cl-, Ca, Sr and low Si and K concentrations. The synclineward moM wells are high in Na, Mg, K, Si, low in DO, NO3- and show moderate Cl- concentrations. Statistically significant fluctuations of basic quality parameters as environmental tracers are classified in the response matrix (Fig. 7). In perched, shallow groundwater, mainly simple, but also dual event-scale responses are contained in the continuous record. The latter indicate piston-like replacement of warmer, higher mineralized and oxygen-depleted pre-event water (DO, EC, Fig. 6-A) by infiltration-recharge. In continuous data of the deeper perched water (Fig. 6-B), only simple event-scale responses of environmental tracers were recorded. In discrete data within the phreatic zone, fluctuations cover simple responses to cumulating hydrographs, highlighting rather seasonal fluctuation (Fig. 6-C) and simple responses to single event-peaks (Fig. 6-D), if resolved.

4. Discussion 4.1 The hillslope hydrogeological functioning The Upper Muschelkalk flow system The overall head and quality fluctuations, showing a strong annual seasonality with response times <5 months, characterize the mo strata as responsive, and demonstrate the meteoric control (infiltration recharge) in the topographic high. Not contradictorily, mo strata also represent a “retarded flow system” (White 1969) due to missing conduit-flow phenomena that otherwise would cause multiple event-scale responses. As evident from drilling (Kohlhepp et al. 2017), the hillslope setting of limestone-mudstone alternations is lacking epikarst.

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Being typical for massive carbonates, such a highly karstified zone is usually absent in intrastratal karst and mixed sequences (Klimchouk 2004). In comparison to the low matrix permeabilities of tight mo rocks, bulk permeabilities show that fractures do importantly contribute to fluid flow. In the moTK limestones, flow likely occurs preferentially in partially widened, even breccia-like networks of fractures (Kohlhepp et al. 2017). In the tight moM mudstones and calcimudstones, considerable part of the flow is restricted to thinner and lesser connected fractures. All hydrographs are lacking instantaneous peaks. Obviously, event-scale pressure transfer (Lischeid et al. 2002) is a negligible phenomenon, most likely due to high hydraulic diffusivity of carbonate rocks (Mádl-Szönyi and Tóth 2015). Despite the presence of argillaceous strata, thus, even high soil/rock moisture and larger perched groundwater bodies in winter does not impede aeration zone ventilation. At the lower midslope, where surface flow in intermittent creeks occurs, the shallow groundwater thermographs are lacking short-term fluctuations and mostly reflect seasonal conductive warming/cooling. Thus, multi-directional exchange between groundwater and surface-water, typical for hyporheic zones (Schmidt and Hahn 2012) is at best, minor and retarded around the small creeks.

Atmospheric forcing and the role of surface/subsurface extremes Snowmelts and subsequent rain events in winter cause maxima of groundwater levels and thus, of recharge. Missing responses to precipitation events (e.g. in Oct/Nov 2015) and delayed maxima highlight the role of antecedent soil moisture and indicate the dominance of diffuse recharge. Summer responses also depend on antecedent saturation of the aeration zone. In general, percolation flux to the aquifers decreases during the growing season (CWBcum minima, monotonic recessions) due to higher evapotranspiration losses. Thus, the dormancy period plays a crucial role for groundwater recharge. Among the surface signals, ”weather signals” comprise multiple snowmelts, heavy rainstorms, wet periods (e.g. anomalous wet May 2013), and precipitation-free periods (e.g. spring 2018). The extreme highstands and surplus period after the snowmelt-rich spring 2013 demonstrate that episodic events strongly contribute to groundwater highstands and budget. Subsequently, seasonally filled soil storages likely favor recharge even from minor input. Regarding the anomalous wet seasons in summer 2014 (Fig. 3-A), precipitation extremes, however, do not necessarily enhance infiltration-recharge. They contribute to the climatic water balance and thus mainly to winter recharge. Thus, (multiple) snowmelt events most effectively recharge groundwater resources. The pronounced surplus period also lastingly prevented levels to drop to minima, despite the minimum precipitation in 2015.

13

The driest and warmest calendar year 2018 was remarkably accompanied by 5- to 8-year highstands (Fig. 3-A). Here, groundwater quantity drew on the antecedent wet winter as well. However, we cannot exclude severe effects for matter quality and exchange throughout the aeration zone. The 85-year precipitation record (Kammerforst) showed a trend for increasing winter precipitation and a tendency for decreasing summer precipitation (not shown) that coincides with climatic studies for central and northern Europe (Beniston et al. 2007). Because of predominantly winter-timed recharge, groundwater renewal in the hillslope system might be lesser threatened by these effects of climate change. A warming trend may generally increase number of mid-winter melts (Allen et al. 2010). However, earlier and reduced melts, along with reduced summer precipitation may contrarily result in extended recession periods and pronounced lowstands (Eckhardt and Ulbrich 2003; Allen et al. 2010). Subsurface extremes (Fig. 3-B), accompanied by phases of pronounced highstands or lowstands, represent episodic, but lasting conditions. Whereas the aeration zone, consequently, undergoes harsh environmental changes in terms of water-saturation and connectivity, the phreatic zone keeps saturated. However, severely changed heads and thus flow patterns, may have also strong impacts for the subsurface ecosystems. One example is the lasting impact on the phreatic zone temperatures following the extreme recharge in 2013/14.

The topographic high’s flow regime Overall HCO3+-type groundwaters characterize a shallow local recharge-discharge system (Mádl-Szőnyi and Tóth 2015). Water-rock-interaction indicators (e.g. Sr, Mg/Ca ratio; Table 1) point to increasing flow distances or residence times, respectively, along the transect (H1moTK). Continuously recorded EC25 increases revealed surface-input in uphill areas, but ascending input in footslope domains. The simulated pCO2 values for the shallow summit well indicate loading in percolated soil and the dominance of diffuse infiltration-recharge. In the shallow lower midslope, high CO2 originate rather from microbial cycling in low-permeable, saturated overburden (cf. Schwab et al., 2019). During the summer halfs, our transect area generally represent a groundwater recharge area that is widely dominated by descending and downdip or sub-lateral flow (Fig. 8). Fast rises and slower recessions of hydraulic heads confirm a low-permeability fracture network in the upslope aeration zone. The frequent level fluctuations coincide with the moderate bulk permeability and low storage coefficients (not shown) or both (Sauter 1995). Responding with stronger, but still short-lived peaks to weather events, the upper midslope moM well shows its larger contribution area, enhanced surface-connection, and low transmissivity and minimal storativity. Highest amplitudes of level fluctuation in midslope/footslope wells are due to the larger contribution areas, resulting in

14

higher groundwater supply and head built-up. Comparatively slow rises are related to higher permeabilities and flow distances. Slow recessions likely represent regional back-flooding and thus point to the permeability barrier. For the deepest moTK well (H51), highest permeability and upward leakage was evident from its necessity for drawdown curve fitting. In the footslope domains, the moTK represents a productive aquifer. Synclinewards, buried and less altered mudstones, however, contain fine fractures (Lazar et al. 2019) that may even be more permeable than the upslope weathering remains, and thus, do less impede cross-stratal exchange. If assuming purely stratiform flow from outcropping high-permeability strata (i.e. screen sections) to buried footslope domains, travel times based on hydraulic gradients (see Fig. 5-B) and the permeability data (Fig. 5-C) would cause ~30-fold higher theoretical time lags for hydrograph responses. In turn, generally low SWCC lag times demonstrate that considerable cross-stratal (e.g. vertical) flow and recharge occur in the limestone-mudstone alternations. However, in the upslope aeration zone, diffuse infiltration feed high-permeability strata, that store and transmit perched groundwater as indicated by event-scale level fluctuations (Fig. 8-A). Mudstone strata, being dense, delithified and plastic here, apparently lack fractures. Thus, downdip (stratiform) flow is likely favored and even produces episodic rushing noise in shoulder wells. Fast level rises, and slower recessions also indicate backflooding, likely due to exceedance of vertical and downdip percolation capacity under locally low hydraulic gradients. The perched water bodies exist permanently or temporally in the uphill aeration zone. Yet, they seem to be rather patchy and discontinuous in space. In the phreatic zone, the flow regime (Fig. 8) is subject to gravity-driven flow, comprising synclineward vertically converging flow as shown by the head reversals (Fig. 4). The conceptual permeability barrier (Treffurt 1982; Fig. 1-B/C) that is characterized by reported lower permeabilities beyond (i.e. lacking productive groundwater resources), is however not necessarily the cause for groundwater rise and discharge. Both the current (sub-)regional flow regime and its developed hydrofacies zonation rather originate from self-organization of subsurface flow (Tóth 1999) and the “geologic agency” (Tóth 1995) of groundwater. The latter decreases synclinewards due to higher mineralization and additionally reduced flow and thus bequeathed less weathered or karstified strata in the syncline center.

4.2 Transient patterns of multi-directional flow and matter exchange Continuous DO recordings indicate loading by descending waters at summit (Fig. 3-B) and shallow midslope. low-loaded event waters (e.g. H2, Table 1) indicate locally fast bypass flow without pronounced solute

15

redistribution in the aeration zone of the slope shoulder. However, during seasonal or episodic infiltrationrecharge, perched water bodies form or expand in the aeration zone that fluctuate in the stratiform and/or vertical direction (Fig. 8-A). Those conditions of multi-directional flow can efficiently connect neighboring parts of the aeration zone. Upwelling waters or higher heads would generally allow for mobile matter to be supplied in lowpermeable, also bypassed rock (limestone matrices, mudstones). Thus, aerated hanging strata likely receive matter input during highstands, while laying strata may receive lasting drainage during subsequent recessions. In the deep lower midslope well, DO logging strongly indicated loading by ascending, thus mm waters. Seasonal and episodical abrupt warming and synchronous increases in sulfate indicate ascending flow from laying strata, too. Here, in the saturated zone, the moTK receives oxygen during progressed seasonal highstands (Fig. 8-C). The same applies for midslope well H31 during highstand maxima (not shown). Noteworthy, dissolved oxygen as surface-sourced matter can thus be supplied to oxygen-deficient domains by ascending waters that bypassed the tighter, oxygen-deficient overburden. At the lower footslope, DO is lacking in the continuous record. Here, increases of Ca2+, Sr2+, SO42- and Cl- point to ascending mm groundwaters. Decreasing Cl- responses in H52 (i.e. dilution) might additionally point to ultrafiltration of laying percolated argillaceous beds/membranes (Tóth 1999), leading to ion enrichment in single strata. Interestingly, lacking Cl- input from mm waters to H41, point to depletion due likely to the completed dissolution of mm evaporites in this area. Agricultural-sourced nitrate is transported by descending waters at the upper midslope, but likely also by ascending waters in the deep lower midslope. Higher Na+ concentrations in H4/H5 moM waters and lower levels of Ca2+, compared to moTK, point also to ion exchange (Tóth 1999) on clay mineral-rich strata during cross-stratal exchange or residence. Authigenic boron, negligible in upslope domains, is elevated in footslope mo strata and seasonal decreases also indicate ascending mm waters. Barium is another, likewise conditional indicator of cross-stratal exchange. It increases in H41/H51 due to input from laying strata and decreases in H52/H53 due to export by ascending flow. Thus, loading by mm waters, but also locally high concentrations in moM, especially in the upper midslope occur. Lower silicon concentrations also characterize ascending mm waters, in contrast to moM strata, due likely to faster passage. The aberrant strong level rises of the upper midslope moTK well (e.g. autumn 2013) that are accompanied by abrupt strong increases in DO, NO3-, and drops in Ca, SO42-, pCO2, may point to extreme-event activation of upslope karst input features or abrupt onset of ascending flow due to filled storages in laying strata. Typical indicators for flow variation in carbonate rocks, like carbon parameters and turbidity (cf. White 2015; Pronk et al. 2008), were shown to be not suited to understand our retarded flow system due to non-significant responses. Stable isotopes of H2O (18O, 2D, not shown in this study) do not significantly differ between groundwater, surface waters and precipitation. They are thus irrelevant for the exploration of flow dynamics far

16

above the phreatic surface or of shallow groundwater that is of meteoric-dominated origin. Contrastingly, our results demonstrate that large suites of complementary environmental tracers are necessary to reveal cross-stratal exchange and multidirectional flow patterns within sub-regional flow systems and particularly, within mixed lithologies. The exploitability of temperature signals and responses that was limited in the deep phreatic zone could be increased by high-spatial resolution fibre sensor measurements for continuous tracing of fluid migration.

Groundwater mounding boosts shallow flow paths Taken together, the environmental time series, environmental tracers, and multi-depth head measurements further allow for the reconstruction of transient flow patterns within the aeration and phreatic zone. Different timing of multi-depth head responses, and lacking responses in superimposed screen sections to samplinginduced drawdowns (H4/5), point to a rather slow hydraulic equilibration within the stratified, but hydraulically continuous bedrock in the phreatic zone. Thus, in the aeration zone, we assume downdip runoff in sloping highpermeable strata to cause localized recharge and concomitant groundwater mounding. Such mounds modify the local to sub-regional flow patterns (Fig. 8-C), as groundwater circulation in short local paths relatively increase with elevated relief of the phreatic surface (Goderniaux et al. 2013). Consequently, recharge/discharge areas expand or contract seasonally. As multiply evidenced by the head reversals and cross-stratal breakthrough of matter, for instance the oxygenation of the main aquifer by ascending waters, the phreatic zone also undergoes transient variation of flow directions and flow intensity.

4.3 Compartmentalization and nutritional supply of the subsurface ecosystem Beneath the topographic high of the Hainich low-mountain range, the important recharge area of downslope used groundwater resources, we identified a typically large aeration zone. The pronounced stacking of saturated (perched) and unsaturated zones from summit to midslope positions, the configuration of sloping bedrock strata and multi-directional flow dynamics, both in the aeration zone and phreatic zone, jointly control the nutritional supply and exchange between subsurface ecosystems. This consequently cause a compartmentalization of the hillslope subsurface. Regarding surface-sourced input and subsurface re-distribution, we can distinguish different degrees of nutritional supply or isolation (see Fig. (8-C), respectively. High isolation can be expected in (I) parts of the aeration zone that are bypassed and not regularly connected by water flow. Furthermore, high isolation is given in saturated zones that only receive cross-stratal fluid flow that was filtered by (II) thick hanging (e.g. H42/3) or

17

(III) laying sequences (e.g. H53). For such isolated, although shallow domains, Schwab et al. (2019) recently found that groundwater-dispersed microbes also utilize sedimentary carbon and that autotrophic lifestyle is even common, due likely to limited input of fresh organic matter. Except for the shallowest summit wells that show elevated organic carbon (Table 1) in groundwater, neither surface-origin nor event-scale subsurface exchange was indicated by time series of carbon sums due to non-significant fluctuation. At the upper midslope, the shallow well receive more oxygen and nitrate (Table 1, Fig. 3-C) by descending waters than the shallow wells of summit or lower midslope. Here, favorable strata lithology, weathering/fracturing and karst input features likely enhance the surface-connection. Generally higher, but variable transfer of surface-sourced nutrients, takes place in high-permeable strata (e.g. moTK), both in perched water bodies within the aeration zone and in phreatic regions that undergo pronounced cross-stratal exchange. Here, the ascend of oxygenated waters into oxygen-deficient regions that rather receive only cycled or filtered input during recession periods, highlight the interplay of aeration zone dynamics and phreatic flow for the evolution of groundwater quality. Likewise, subsurface-sourced sulfate (Fig. 6-C/D,) and surface-sourced nitrate (not shown) can enter the groundwater bodies by ascending flow, rather than by downward percolation. Limited oxygen and nitrate in descending waters is likely caused by microbial turnover. Accordingly, Herrmann et al. (2017) found diverse denitrifier communities in the Hainich groundwater with higher potentials for heterotrophic nitrate reduction in the Trochitenkalk Fm. and high autotrophic potential in groundwater of the tighter hanging strata. For the latter, Kumar et al. (2017) reported high anammox activity from anoxically incubated groundwater. Consequently, the hillslope configuration of bedrock and flow exhibit an intrinsic protection against agricultural loading from the footslope cropland. Because of even adverse aspects (nitrate loading) by cross-stratal exchange, the quality of the main aquifer benefit from solely forestry use in the upslope areas. For the main aquifer, Nawaz et al. (2018) identified a fungal core genus that also cover potential plant pathogens and thus confirmed a higher surface-connectivity, independently of our presented flow patterns.

5. Conclusions Our study illustrates the immense value of multi-depth exploration and long time series of basic environmental data for elucidating the scarcely addressed role of multi-directional flow phenomena beneath topographic highs. We improved the understanding of the local Muschelkalk flow system, mainly by exploring the high responsivity of perched and phreatic groundwater bodies and their groundwater quality to atmospheric forcing, and by demonstrating the overall cross-stratal exchange, and the functional differences of argillaceous strata along the hillslope for the first time. Noteworthy, the hillslope interplay of aeration zone dynamics and flow

18

paths of variable temporal activity in the phreatic zone was found to reduce the isolating or “protective” cover of thick argillaceous strata, thus increasing the intrinsic vulnerability, and resulted in supply of even deep regions by surface-sourced matter. Regarding limestone-mudstone alternations, representing a widely distributed bedrock type that hosts important groundwater resources, we can conclude: 

Multi-directional flow phenomena are intrinsic features within multi-strata hillslope settings. Phenomena like expansion/contraction of perched groundwater in the aeration zone and transient cross-stratal exchange through low-permeability strata (aquitards) in the phreatic zone interact and shape the quality of groundwater resources.



The hillslope configuration of sloping strata facilitates the formation of (discontinuous) perched groundwater bodies and of a multiply zoned aeration zone.



Episodic and seasonal multi-directional fluid flow patterns and transport contribute both to ecosystem compartmentalization and to the nutritional supply and exchange between subsurface ecosystems.

The developed scheme for exploratory time series analysis (response classification and response matrix) helps to systematically exploit indispensable basic and dis-/continuous groundwater quality data. For hydrogeological site investigations in mixed lithologies, we suggest: 

Large sets of conditional tracers, including authigenic signals are required (I) to explore crosscompartment exchange, and thus (II) to determine the hydrogeological functions of argillaceous strata, due to superimposing signals of surface and subsurface origins.



Continuous recording of groundwater quality parameters is generally required for the retrieval of response characteristics of episodic forcings, whereas discrete data, for instance on a monthly cycle, is appropriate for identifying seasonal quality variation in comparable (retarded) flow systems.

The extent of matter re-distribution by multi-directional flow phenomena within topographic highs, including dispersal of organisms, should be further investigated in detail. Attributing the microbial diversity of groundwater resources solely on fluid/habitat properties (i.e. from single aquifers), disregarding cross-stratal import/export of both microbes and nutrients hampers the predictability, for instance of ecosystem services. We thus suggest to carefully consider the hillslope multi-directional flow and matter exchange in ecological studies, biogeochemical modeling as well as in resource management practices.

19

Acknowledgements The work has been funded by the German Research Foundation (DFG) CRC 1076 “AquaDiva” and the state of Thuringia “ProExzellenz” initiative AquaDiv@Jena (107-1). The authors thank the Hainich National Park, Thüringer Landesamt für Umwelt, Bergbau und Naturschutz, ThüringenForst, TUPAG Holding AG and the municipality of Kammerforst for provision of archive data and enabling field work. We thank Christine Hess, Anna Späthe, Anna Rusznyák and Maria Fabisch for scientific coordination, Heiko Minkmar for groundwater sampling, Paul Seeber for support of groundwater monitoring and valuable field reports, Falko Gutmann for supporting samplings and on-site analyses and Thomas Ritschel for technical support and discussion.

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Küsel K., Totsche K. U., Trumbore S. E., Lehmann R., Steinhäuser, C., Herrmann, M., 2016. How Deep Can Surface Signals Be Traced in the Critical Zone? Merging Biodiversity with Biogeochemistry Research in a Central German Muschelkalk Landscape. Front Earth Sc 4 (32), 1-18. Larocque, M., Mangin A., Razack, M., Banton, O., 1998. Contribution of correlation and spectral analyses to the regional study of a large karst aquifer (Charente, France). J Hydrol 205, 217-231. Lazar, C.S., Lehmann, R., Stoll, W., Rosenberger, J., Totsche, K.U., Küsel, K., 2019. The endolithic bacterial diversity of shallow bedrock ecosystems. Sci Total Environ 679, 35-44. Legout, C., Molenat, J., Aquilina, L., Gascuel-Odoux, C., Faucheux, M., Fauvel, Y., Bariac, T., 2007. Solute transfer in the unsaturated zone-groundwater continuum of a headwater catchment. J Hydrol 332(3), 427-441. Lin, H., 2010. Earth's Critical Zone and hydropedology: concepts, characteristics, and advances. Hydrol Earth Syst Sc 14(1), 25-45. Lischeid, G., Kolb, A., Alewell, C., 2002. Apparent translatory flow in groundwater recharge and runoff generation. J Hydrol 265(1), 195-211. Lohse, K. A., Brooks, P. D., McIntosh, J. C., Meixner, T., Huxman, T. E., 2009. Interactions between biogeochemistry and hydrologic systems. Annu Rev Environ Resour 34, 65-96. Mádl-Szőnyi, J., Tóth, Á., 2015. Basin-scale conceptual groundwater flow model for an unconfined and confined thick carbonate region. Hydrogeol J 23(7), 1359-1380. Nawaz, A., Purahong, W., Lehmann, R., Herrmann, M., Totsche, K. U., Küsel, K., Wubet, T., Buscot, F., 2018. First insights into the living groundwater mycobiome of the terrestrial biogeosphere. Water Res 145, 50-61. Parkhurst, D. L., Appelo, C. A. J., 1999. User's guide to PHREEQC (Version 2). USGS Water-Resources Investigations Report 99-4259. Rouxel, M., Molénat, J., Ruiz, L., Legout, C., Faucheux, M., Gascuel-Odoux, C., 2011. Seasonal and spatial variation in groundwater quality along the hillslope of an agricultural research catchment (Western France), Hydrol Process 25(6), 831-841. Salve, R., Rempe, D.M., Dietrich, W.E., 2012. Rain, rock moisture dynamics, and the rapid response of perched groundwater in weathered, fractured argillite underlying a steep hillslope. Water Resour Res 48(11). Salvucci, G.D., Entekhabi, D., 1995. Hillslope and climatic controls on hydrologic fluxes. Water Resour Res 31(7), 1725-1739. Sauter, M., 1995. Delineation of a karst aquifer using geological and hydrological data and information on landscape development. Carbonate Evaporite 10(2), 161-170.

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Schmidt, S. I., Hahn, H. J., 2012. What is groundwater and what does this mean to fauna? – An opinion. Limnologica 42, 1-6. Schwab, V. F., Nowak, M. E., Elder, C. D., Trumbore, S. E., Xu, X., Gleixner, G., Lehmann, R., Pohnert, G., Muhr, J., Küsel, K., Totsche, K., 2019. 14C-free carbon is a major contributor to cellular biomass in geochemically distinct groundwater of shallow sedimentary aquifers. Water Resour Res 55(3), 2104-2121. Šcukarev, S. A., 1974. In: Sydykov, Zh. S., Davletgalieva K. M.: Gidrohimicheskie klassifikacii i grafiki. Smart, P.L., Hobbs, S.L., 1986. Characterisation of carbonate aquifers: a conceptual base. In: Proceedings of the Environmental Problems in Karst Terranes and Their Solutions Conference. National Water Well Association, Dublin OH, 1-14. Steenhuis, T. S., Van der Molen, W. H., 1986. The Thornthwaite-Mather procedure as a simple engineering method to predict recharge. J Hydrol 84(3-4), 221-229. Teichmüller, U., 1989. Ergebnisbericht mit Grundwasservorratsberechnung, Detailerkundung Bad Langensalza. VEB Hydrogeologie, unpublished exploration report. Tóth, J., 1995. Hydraulic continuity in large sedimentary basins. Hydrogeol J 3(4), 4-16. Tóth, J., 1999. Groundwater as a geologic agent: an overview of the causes, processes, and manifestations, Hydrogeol J 7(1), 1-14. Treffurt, D., 1982. Ergebnisbericht mit Vorratsberechnung Dingelstedt 1982, Vorerkundung/Detailerkundung. VEB Hydrogeologie, unpublished exploration report. Westphal, H., 2006. Limestone-marl alternations as environmental archives and the role of early diagenesis: a critical review, Int J Earth Sci 95(6), 947-961. White, W. B., 1969. Conceptual Models for Carbonate Aquifers, Ground Water 7(3), 180-186. White, W. B., 2015. Chemistry and karst. Acta Carsologica 44(3), 349-362. Williams, P.W., 1983. The role of the subcutaneous zone in karst hydrology. J Hydrol 61(1-3), 45-67. Zech, A., Zehner, B., Kolditz, O., Attinger, S., 2016. Impact of heterogeneous permeability distribution on the groundwater flow systems of a small sedimentary basin. J Hydrol 532, 90-101.

Declarations of interest: none

23

Table 1: Quality parameters of Upper Muschelkalk groundwaters (Hainich CZE). Well name

H13

H14

H31

H32

H41

H42

H51

H52

H53

Final depth

15,5

7,0

46,7

22,0

47,5

12,5

88,0

69,0

50,0

median

SD

median

SD

median

SD

median

SD

median

SD

median

SD

median

SD

median

SD

median

SD

635

48

692

15

763

21

764

8

777

22

742

10

1075

33

746

16

732

13

pH

7,3

0,2

7,0

0,1

7,3

0,1

7,3

0,1

7,2

0,1

7,1

0,1

7,1

0,1

7,3

0,1

7,3

0,1

ORPNHE mV

423

56

396

65

398

73

379

56

373

65

162

60

384

51

230

32

242

43

DO

mg/l

10,9

0,7

0,3

0,5

7,2

2,0

2,4

0,7

5,1

1,7

0,0

0,1

2,7

0,3

0,0

0,0

0,0

0,0

mg/l

0,02

0,06

0,02

0,04

0,02

0,02

0,02

0,02

0,08

0,08

0,14

0,07

0,02

0,02

0,37

0,15

0,53

0,25

(m bgl) EC25

µS/cm

NH4+

mg/l

365

41

409

15

397

18

406

25

418

31

467

18

374

22

397

22

415

18

CO2

mg/l

22,5

14,8

60,8

8,3

23,5

7,6

27,5

6,6

31,5

6,9

42,2

9,4

34,5

8,1

25,7

6,1

25,5

5,3

DOC

mg/l

1,3

0,7

1,5

0,8

0,9

1,0

1,0

1,0

1,0

1,1

1,0

1,1

0,9

1,1

1,0

1,1

1,0

1,0

TOC

mg/l

1,5

0,8

1,4

0,8

1,2

1,1

1,2

1,1

1,1

1,2

1,2

1,3

1,1

1,1

1,2

1,4

1,1

1,1

mg/L

6,7

2,7

1,1

1,9

30,3

14,5

26,5

10,1

8,5

4,9

0,1

0,1

7,0

3,8

1,0

4,0

0,8

2,0

SO4

mg/L

34,2

3,5

26,0

3,6

76,2

17,7

73,1

8,1

91,2

28,9

34,4

4,0

292,9

43,2

90,9

9,9

66,7

8,0

Cl-

mg/L

4,9

0,3

4,1

0,5

5,8

0,6

6,2

0,7

6,3

0,5

9,0

0,6

12,2

1,3

8,7

1,9

6,8

1,5

Al

µg/

15,9

6,9

17,0

7,8

15,0

25,5

16,3

6,9

15,0

16,3

15,0

13,1

15,0

10,5

15,2

6,9

15,0

7,4

B

µg/

7,4

3,0

7,2

2,7

38,4

13,6

24,3

3,3

111,8

72,9

100,4

8,5

57,8

10,2

426,6

44,7

581,0

63,6

Ba

µg/

36,8

6,1

13,5

2,7

50,5

5,9

55,7

4,5

45,1

8,9

96,3

4,5

31,7

4,6

32,5

13,0

46,4

5,2

Ca

mg/L

132,3

12,0

143,0

3,2

97,9

8,7

95,5

3,6

110,7

14,4

83,4

2,4

183,0

9,3

68,6

2,0

60,7

3,2

Co

µg/

0,14

0,05

0,15

0,06

0,11

0,02

0,12

0,03

0,15

0,05

0,14

0,04

0,21

0,06

0,56

0,08

0,53

0,13

Fe

µg/L

2,00

4,21

3,20

12,40

3,02

13,34

2,70

26,59

2,10

45,74 160,00 63,69

2,65

11,92

19,00

29,57

14,40

14,36

K

mg/L

0,81

0,08

0,66

0,10

2,70

0,49

2,74

0,16

4,90

2,16

6,50

0,28

1,85

0,25

9,78

0,47

13,50

0,93

Li

µg/L

2,78

0,37

1,92

1,05

20,52

3,93

16,75

1,90

22,60

9,28

29,10

2,79

14,91

1,38

38,68

3,92

66,20

8,91

Mg

mg/L

3,43

0,10

3,56

0,21

43,37

3,50

47,60

2,24

39,75

5,33

48,40

1,55

40,40

3,48

52,90

2,08

52,95

2,06

Mn

µg/L

0,53

0,61

1,23

1,92

0,50

0,86

0,68

1,07

1,23

1,84

9,82

7,73

4,05

2,48

3,87

1,29

2,91

1,11

Na

mg/L

3,53

0,18

3,97

0,22

5,20

0,94

4,57

0,65

7,73

1,69

9,73

0,47

6,84

0,43

14,96

0,81

16,17

0,82

S

mg/L

10,99

0,78

8,70

0,73

25,80

5,60

24,47

1,83

30,88

6,48

11,50

0,50

99,70

7,74

29,95

2,25

21,90

2,19

Si

mg/l

3,55

0,07

3,42

0,08

4,90

0,31

4,68

0,25

5,52

0,77

7,56

0,24

4,41

0,19

6,76

0,30

7,12

0,26

Sr

mg/L

0,15

0,01

0,15

0,00

0,44

0,10

0,25

0,01

1,24

0,15

0,34

0,02

2,78

0,14

2,28

0,16

2,34

0,07

Sc

µg/L

0,87

0,26

0,88

0,27

1,22

0,39

1,13

0,36

1,33

0,46

1,80

0,51

1,08

0,32

1,58

0,46

1,71

0,50

U

µg/L

0,37

0,02

0,36

0,02

0,60

0,18

0,35

0,02

0,51

0,14

0,03

0,01

1,04

0,10

0,40

0,10

0,20

0,08

TDS

mg/L

552

57

597

18

673

57

671

26

698

35

670

23

935

51

658

22

648

28

0,24

0,17

0,03

0,08

0,13

0,11

0,11

0,07

0,11

0,12

-0,02

0,08

0,17

0,10

-0,04

0,09

-0,01

0,10

-

HCO3

-

NO3

2-

SIcalcite

24

25

26

27

28

29

30

31

32

Fig. 1: Study site. (A) Location of the Hainich CZE. (B) General regional flow system and hydrochemical zonation (after Treffurt 1982). (C) Map of the study area. Data sources: contour lines, faults © TLUBN; DEM © GDI-Th; DLM250 © GeoBasis-DE/BKG 2016; dl-de/by-2-0, http://www.govdata.de/ dl-de/by-2-0; coordinate system: ETRS89/UTM, zone 32N.

Fig. 2: Classification of subsurface responses to signals of surface and/or subsurface origin. (A) Surface signals: precipitation and the soil moisture surrogate (cumulative climatic water balance) are two examples of external forcings. (B) Hydrographs of monitoring wells or springs respond to recharge with event-scale peaks (left) or cumulating responses, forming seasonal trends. (C) Response types of conditional tracers. Schematic responses (chemographs) are generalized after White (2015) and Bakalowicz (2005).

Fig. 3: Time series of surface signals and groundwater levels along the hillslope well transect. (A) Cumulative precipitation, cumulative climatic water balance (site-corrected, detrended) and extreme events. Black circles mark extreme precipitation events. Rectangles mark extreme cumulative precipitation minima (red) and maxima (green). (B) Average hydrographs (grey lines) indicate phases of surpluses (green areas) and deficits (red areas). Groundwater highstands (green) and lowstands (red) are marked with circles. SM: snowmelts.

Fig. 4: Multi-level pressure head fluctuation and saturation state in the aeration zone and phreatic zone along the eastern Hainich hillslope.

Fig. 5: Hydraulic conditions and properties in the Triassic strata. (A) Contour map of groundwater levels: highstand (February 2018) in the Trochitenkalk Fm. (main aquifer, blue lines) in comparison to the average regional level (black dashed lines; data source: see Fig. 1). (B) Cross section of the well transect showing heads and fluctuation magnitudes in the main aquifer (see Fig.1 for C-C’). (C) Bulk rock permeabilities and matrix permeabilities (Lazar et al. 2019) of the Hainich area. Bulk rock permeabilities obtained from sampling-related drawdown (recovery) analysis in short-screen research wells scatter less than data from borehole tests (Götze 1969) in exploratory drillings.

33

Fig. 6: Example time series of continuous and discrete groundwater data in different depths along the hillslope well transect (A-D). DO and T data were smoothed (Savitzky-Golay filter). Pronounced event-scale responses are recorded in the perched groundwater bodies of the aeration zone. In the phreatic zone and due to larger contribution areas, seasonal trends predominate. SR: simple response; DR: dual response.

Fig. 7: Response matrix showing dominating, reproduced response behaviors of non-conservative environmental tracers to groundwater level fluctuation in the observation points (monitoring wells). White cells mark nonsignificant responses.

Fig. 8: Conceptual flow regime and ecosystem compartmentalization in the eastern Hainich hillslope. (A) The flow regime is characterized by cross-stratal exchange and interacting flow dynamics of the aeration and phreatic zone. The aeration zone shows a multi-storey zonation by permanent and temporary perched groundwater. (B and C) Seasonal or event-scale flow patterns differ during lowstands and highstands. (C) Runoff in highpermeable strata cause localized recharge and groundwater mounding that boosts local (shallow) flow routes. The interplay of aeration/phreatic zone fluid flow dynamics cause ecosystem compartmentalization (I to III, see text).

34

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

35

36



Multi-directional flow phenomena are intrinsic features of topographic highs



Multi-directional flow shapes groundwater quality and intrinsic vulnerability



We present a scheme for exploratory analysis of dis-/continuous groundwater data



Large sets of environmental tracers are required in mixed lithology-flow systems



Interacting aeration zone/phreatic flow dynamics shape ecosystem compartmentalization

37