Structural controls on the hydrogeological functioning of a floodplain

• Published in: Hydrogeology journal Vol. 28; no. 8; pp. 2675 - 2696
• Main Authors: Martin, Simon, Klingler, Stefan, Dietrich, Peter, Leven, Carsten, Cirpka, Olaf A.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.12.2020; Springer Nature B.V
• Subjects: Alluvial fans; Aquatic Pollution; Aquifer permeability; Aquifers; Bedrock; Catchment area; Clay; Earth and Environmental Science; Earth Sciences; Environmental changes; Floodplains; Geological structures; Geology; Geophysics/Geodesy; Glaciation; Glaciology; Gravel; Groundwater; Groundwater barriers; Groundwater flow; Groundwater recharge; Gypsum; Holocene; Hydrogeology; Hydrology; Hydrology/Water Resources; Permeability; Quaternary; Regional climates; Sediment; Sediments; Temperate climates; Valleys; Waste Water Technology; Water balance; Water Management; Water Pollution Control; Water Quality/Water Pollution; Weathering; Sedimentary architecture; Biogeochemical turnover; Groundwater flow; Conceptual models; Floodplain
• ISSN: 1431-2174; 1435-0157
• DOI: 10.1007/s10040-020-02225-8

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 fine-grained, 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.