Lithological and Tectonic Control on Groundwater Contribution to Stream Discharge During Low-Flow Conditions

water Article Lithological and Tectonic Control on Groundwater Contribution to Stream Discharge During Low-Flow Conditions Stefanie B. Wirth 1,*, Claire Carlier 1,2, Fabien Cochand 1,3, Daniel Hunkeler 1 and Philip Brunner 1 1 Centre for Hydrogeology and Geothermics, University of Neuchâtel, 2000 Neuchâtel, Switzerland; [email protected] 2 Bundesanstalt für Geowissenschaften und Rohstoffe, 30655 Hannover, Germany 3 CSD Ingenieurs SA, 5000 Aarau, Switzerland * Correspondence: [email protected]; Tel.: +41-32-718-2635 Received: 19 February 2020; Accepted: 11 March 2020; Published: 14 March 2020 Abstract: Knowing how stream discharge in an ungauged catchment reacts to dry spells is a major challenge for managing water resources. The role of geology on these dynamics is poorly understood. For the Swiss Molasse basin, we therefore explored how the geology influences the groundwater contribution to stream flow during low-flow conditions. Using existing data from geological reports and maps as well as from deep boreholes, we constructed a basin-wide overview of the hydrogeological quality of the bedrock and investigated five catchments in 3D. We found that catchments with the most permeable sedimentary bedrock are least sensitive to low flows (marine 4 5 6 sandstone, K = 10− to 10− m/s, Peff = 5–10%). In contrast, if bedrock K is low (K < 10− m/s), the presence of a productive Quaternary volume becomes decisive for groundwater contribution to stream flow. Limitations exist due to a restricted database for K and Peff values of the Molasse and limited information on continuation of lithologies with depth. This emphasizes the need for more hydrogeologically relevant data for the future management of water resources. Our results highlighting what lithotypes favor groundwater contribution to stream flow are valid also in other regions for the assessment of a catchment’s sensitivity to low flows. Keywords: low flow; groundwater; geology; permeability; hydraulic conductivity; lithology; bedrock; alluvial aquifer; Molasse basin 1. Introduction Understanding the effect of prolonged dry spells on stream discharge and groundwater heads has become essential in managing water resources with ongoing climate change (e.g., [1–3]). The situation may intensify, since many regions worldwide are projected to experience more droughts and prolonged dry periods in the future due to changes in the atmospheric circulation patterns with climate warming [4]. It is therefore critical to understand the hydrological and topographic but, specifically, also the geological and tectonic factors controlling the remaining stream flow during dry spells. Such an assessment requires an integral view of the properties of a catchment (e.g., [5–8]), and in particular surface waters and groundwater should be considered as one single coupled system. If stream flow is sustained by groundwater during low-flow conditions, a catchment is more robust against, i.e., less sensitive to, dry spells. Such catchments are thus more stable regarding continuous water availability for drinking water resources, agriculture, and riverine habitats, even during dry spells. In addition to natural factors such as the geology and the climatic setting, anthropic influences such as the sealing of Water 2020, 12, 821; doi:10.3390/w12030821 www.mdpi.com/journal/water Water 2020, 12, 821 2 of 23 stream beds and other engineering measures may also influence surface-water and groundwater flows in some catchments (e.g., [9,10]). Former studies demonstrated that groundwater contribution is important for sustaining stream flow during dry spells and that the geology (bedrock and alluvial lithologies, tectonic setting) of a catchment is a crucial factor in controlling this contribution [11–16]. In particular, the permeability of the bedrock was identified as decisive for the low-flow dynamics in a catchment [17–25]. The ideal high-productivity bedrock for reducing the sensitivity of a catchment to low flows is characterized by a sufficient porosity to store water and an adequate hydraulic conductivity in the range of K = 5 6 10− to 10− m/s [19] allowing a slow but constant release of groundwater to the stream and also to alluvial aquifers during low flows. If the bedrock is impermeable, it does not have the capacity for recharging surface waters and alluvial aquifers during dry periods. In addition to the aquifer quality of the bedrock, alluvial aquifers, as productive geological volumes, may contribute groundwater during low flows [18,19,26]. However, alluvial aquifers in unconsolidated Quaternary valley fillings with 2 4 typical permeabilities of 10− to 10− m/s [26,27] drain comparably fast and might, therefore, contribute to low flows only on a limited time scale, unless the alluvial aquifers are constantly recharged by a bedrock aquifer. Even though the high relevance of the geological properties is recognized by the research community, geology-focused studies applying a thorough assessment of the hydrogeological properties of different lithologies and including the tectonic setting have been scarce and were mostly based on the surface (2D) geology of a catchment using geological maps (Swiss Molasse basin [20]; other regions [17,22,28]). Yet, for advancing research on geological controls on low flows, it is fundamental to also incorporate changes in lithology with depth and the presence of tectonic structures influencing groundwater flow [24] in low-flow assessments. For instance, tectonic fault zones may lead to a hydrogeological compartmentalization of the bedrock, and increased water circulation along fault zones may lead to exfiltration or transport of water to deeper depths. Furthermore, inclined layers as in fold limbs may drain groundwater into a certain direction, possibly causing important exfiltration or drainage to a neighboring catchment and thus groundwater loss. Moreover, a thorough understanding of the geology and the tectonics in 3D is not only essential for low-flow research but also for other fields, such as deep geothermal applications [29]. Our study area, the Swiss Plateau or, geologically, the Swiss Molasse basin (foreland-basin of the Alps), is the most densely populated area of Switzerland. Changes in water availability with climate change therefore have to be thoroughly assessed. It is projected that dry summers will occur more frequently with climate warming due to the northward shift and an expansion of the Mediterranean high-pressure zone [30,31]. The dry and hot summers of 2003, 2011, and 2015 have already demonstrated that such dry conditions may lead to an essential reduction of stream flow and to a drop in groundwater tables with recovery times of several years [32,33]. However, not all monitored catchments in the Swiss Molasse basin have shown the same degree of sensitivity to prolonged dry spells. While rivers run dry in some catchments, stream flow is continuous in others [29]. Thus, missing knowledge about the factors that sustain stream flow during dry spells may lead to surprises that impair the availability of drinking and irrigation water and that affects the riverine biosphere when streams run dry or have low water levels. Previous hydrogeological studies at the catchment-scale in the Molasse basin showed that groundwater contribution from the sedimentary bedrock is indeed important for the hydrological budget of a catchment and for sustaining stream flow during dry periods. Balderer [34] compared two neighboring catchments with the same Molasse bedrock. One of the catchments also had an alluvial aquifer, the other catchment did not. Since the water balance of the two catchments was nearly identical, Balderer [34] concluded that groundwater contribution from the Molasse bedrock presents an important water volume in both catchments. Naef, Margret, and Floriancic [20] explored the influence of the sedimentary properties on the low-flow behavior of catchments and found that a porous slow-draining character of the sedimentary bedrock is the critical factor for groundwater contribution Water 2020, 12, 821 3 of 23 during low flows. As the most favorable bedrock lithology for sustaining stream flow during dry spells, Naef, Margret, and Floriancic [20] identified marine sandstones acting as porous aquifers. The overarching objective of this study is to delineate important geological properties determining the sensitivity of catchments in the Swiss Molasse basin to low flows and to ensure that this knowledge is transferrable to other areas worldwide. The latter will be achieved by defining the lithological properties and the type of aquifers (e.g., porous vs. fissured) that are advantageous for sustaining stream flow during low-flow conditions via groundwater contribution. Given the results from previous low-flow studies outlined above, we hypothesize that the hydrogeological properties of the bedrock and the occurrence of productive alluvial aquifers along the river valleys must play an important role. The criteria found here could already be incorporated in guidelines for practical applications in water management [35]. An important consideration in this context is that such guidelines have to be applicable in practice by water managers. They should be straightforward and based on already existing and publicly available geological data and should not include time-consuming geological 3D modeling. Hence, our objective is also to provide easy-to-use criteria for the applied sector. The following workflow was applied: (1) Categorizing the Molasse into a manageable number of principal lithologies and attributing

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