Floodplains as Barriers to Groundwater Flow: Findings from the Hydrogeological Site Characterization of the Ammer Valley Floodplain

In the hillslope-to-stream continuum of temperate regions, floodplains are often assigned two primary environmental functions: (1) buffers which dampen the effects of agricultural infiltrates with a bathtub of organic-rich reduced sediments, and (2) hydraulic conduits connecting the landscape to the draining water body and regulating the magnitude and timing of streamflow generation. For the former to be of relevance to catchment-scale solute turnover, significant amounts of water must be exposed to the reducing conditions, which directly conflicts with the fine-grained, less transmissive nature of these floodplain sediments. In this study, we focused our hydrogeological investigation on the Ammer Valley Floodplain near Tübingen, Germany, which was treated as a temperate analog for fine-grained organic-rich floodplains and concluded that the entire floodplain acts as a barrier to groundwater flow from the surrounding hillslopes. We present results from a robust four-year hydrogeological site characterization, applying geological, geophysical and geochemical techniques, elucidating a highly complex floodplain hydrostratigraphy and spatiotemporally dynamic groundwater system. External hydraulic and geochemical forcings are abound, both from climate (precipitation and evapotranspiration) and the surrounding landscape (bedrock hillslopes and tributary valleys). Unfortunately, though the dynamics resulting from these forcings are attention grabbing for many research communities (e.g. steep redox gradients), water balance calculations suggest that the majority of groundwater recharge on the hillslope and floodplain itself, likely bypasses the floodplain groundwater system to the deeper groundwater and surface water catchments, respectively. Our interpretation brings up an infrequently discussed question: are floodplains acting as barriers to flow? We believe this to be case in many temperate settings where fine-grained unconsolidated material predominates over their coarser counterparts, and that this hydro(geo)logical functioning has broader implications for our conceptualizations of the regional groundwater system. In the context of Tothian regional groundwater flow, thinking of floodplains as aquitards which are forcing more water towards the intermediate and/or regional aquifers, is a perspective that is often neglected or not considered in our conceptual hydrogeological models.