Holocene Floodplain Sediments: From depositional processes to biogeochemical pollutant turnover

Aquifer sediments, formed under varying depositional conditions, exhibit significant heterogeneity in their sedimentary architecture causing variability in their hydraulic and biogeochemical properties. The spatial arrangement of these properties controls the net turnover of biogeochemically reactive and environmentally relevant solutes in floodplains. However, the interlinkage between reactive and hydraulic properties is still enigmatic. This study proposes using sedimentary facies analyses to reconstruct the paleoenvironmental conditions that control the abundance and spatial distribution of aquifer materials, their potential as electron donors, and their hydraulic conductivity. The approach is applied to a Holocene aquifer in the Ammer floodplain in South-West Germany, which consists mainly of organic-rich tufa successions with varying contents of total organic carbon (TOC), peat lenses, as well as of gravel- and clay layers. The spatial extent of sedimentary features and baseline reactive properties (TOC, hydraulic conductivity) were constrained by combining sedimentological observations and bulk geochemical analyses. Based on the insights gained from the paleoenvironmental reconstruction, a facies-based virtual aquifer resembling the sedimentological makeup of the Ammer floodplain was generated and used to perform flow and transport simulations, using exposure of groundwater to TOC as proxy for reactivity. The study demonstrates that the spatial arrangement of facies and their combined biogeochemical and hydraulic properties determine over which range of times the breakthrough of nitrate is to be expected, highlighting the importance of sedimentological insights for groundwater-quality projections.