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

Stefanie Wirth,Philip Brunner,Claire Carlier,Fabien Cochand,Daniel Hunkeler

doi:10.3390/w12030821

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 sandstone, K = 10−4 to 10−5 m/s, Peff = 5–10%). In contrast, if bedrock K is low (K < 10−6 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.

Highlights

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., [1,2,3])
We categorized the Molasse into five principal lithologies, based on the S–N grain-size evolution in the Molasse basin as described in Section 3.2 and their hydrogeological quality described in Section 3.3: marine sandstones (M1), coarse-grained alluvial deposits (M2), fan conglomerates (M3), fine-grained alluvial deposits (M4), and the Subalpine Molasse (M5) (Figure 1b; Table 1)
In this study we approached an important knowledge gap regarding the influence of the geology, in particular of the occurring lithologies and the tectonic structures, on stream flow during low-flow conditions in a certain catchment

Summary

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., [1,2,3]). It is critical to understand the hydrological and topographic but, 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., [5,6,7,8]), 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. In addition to natural factors such as the geology and the climatic setting, anthropic influences such as the sealing of

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