Structural controls on the hydrogeological functioning of a floodplain
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PAPER
Structural controls on the hydrogeological functioning of a floodplain Simon Martin 1 & Stefan Klingler 1 & Peter Dietrich 1,2 & Carsten Leven 1
&
Olaf A. Cirpka 1
Received: 28 April 2020 / Accepted: 4 August 2020 # The Author(s) 2020
Abstract Floodplains are often conceptualized as homogeneous sediment bodies which connect streams with their respective catchment and buffer agricultural inputs. This has led to a general bias within the hydrological community towards research on sites where the floodplain is a clear conduit for groundwater flow. In humid temperate regions of central Europe, floodplains have experienced rapid environmental changes since the last glaciation, yielding significant bedrock weathering and predominantly finegrained, highly stratified hillslope and floodplain sediments. Such heterogeneous sedimentary architecture leads to conceptual ambiguities in the interpretation of the hydrogeological functioning of floodplains, thus raising the question: Do floodplains act as barriers or conduits to groundwater flow? This study analyzes the Ammer floodplain close to Tübingen in south-western Germany as a representative mid-section floodplain in a temperate climate where the regional bedrock-geology is dominated by mudstones. Geological, geophysical, and geochemical characterization and monitoring techniques were combined to shed light on the internal geological structure as a key control modulating the floodplain hydrology. Two partially separate groundwater systems were identified: a gravel body at the bottom of the Quaternary sediments and a Holocene confined tufaceous aquifer, separated by low-permeability clays. Despite flow being predominantly along-valley, sulfate concentrations in the floodplain aquifers showed evidence of a strong connection to the gypsum-bearing hillslope, particularly where tributary valley sediments are present (e.g., alluvial fans). Results from a floodplain water balance suggest the hillslope- and floodplain-aquifer material act as a barrier to hillslope groundwater recharge, where a large fraction may be bypassing the local floodplain groundwater system. Keywords Groundwater flow . Conceptual models . Floodplain . Sedimentary architecture . Biogeochemical turnover
Introduction Floodplains connect rivers with their catchments via groundwater pathways. The hydrological functioning of floodplains exerts a strong control on the timing and magnitude of streamflow generation (Jencso and McGlynn 2011), flood wave propagation (Pinder and Sauer 1971), and stream-ecosystem behavior (Jacobs and Gilliam 1985). In addition, floodplain sediments often exhibit a high electron donating capacity in * Carsten Leven [email protected] 1
Center for Applied Geoscience, University of Tübingen, Schnarrenbergstr. 94-96, 72076 Tübingen, Germany
2
Helmholtz Center for Environmental Research GmbH–UFZ, Department of Monitoring and Exploration Technologies, Permoserstrße 15, 04318 Leipzig, Germany
combination with subsurface residence times that yield favorable red
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