Solute Release from Floodplains during Distinct Types of Inundation Events: Insights from Field Experiments and Reactive Transport Simulations

Abstract. Floodplains exert profound control on catchment water quality by transiently storing, transforming, and releasing solutes and particulate matter as they flow through the river corridor. These dynamic environments are shaped by periodic floods that increase hydrologic connectivity, linking shallow floodplain soils to groundwater and surface water and driving abrupt shifts in solute mobility. While floodplain inundation is often assumed to flush accumulated solutes toward groundwater and surface water, floods can arise through several different mechanisms — including overbank flow, rising groundwater levels, and rainfall- or snowmelt-induced ponding — each of which likely has a distinct impact on solute transport. Here, we combine field observations with reactive transport simulations to evaluate how distinct forms of inundation impact solute mobilization from floodplains. Results from a dye infiltration experiment reveal that precipitation-driven, top-down inundation events (such as rainfall in excess of infiltration capacity) induce preferential flow through macropores. This floodwater bypasses the soil matrix and results in minimal solute exchange. In contrast, observations from a groundwater-driven, bottom-up flood show rapid increases in solute concentrations as groundwater rose into the soil profile and dissolved evaporites within the soil matrix. To generalize beyond these field observations, we use dual permeability reactive transport simulations to test how inundation type, magnitude, and timing affect net solute exchange between the soil and underlying aquifer. Simulations show that groundwater-driven, bottom-up floods drive a net gain of solutes from the aquifer to the floodplain. Top-down floods, on the other hand, lead to solute loss from the floodplain to the aquifer, but this loss can be outweighed by summer plant water demand, which limits downward percolation of floodwater. Collectively, these results reveal that floodplain inundation cannot be interpreted as a simple flushing process. Instead, net solute exchange depends on the type of inundation event, the extent of preferential flow, and the seasonal timing of flooding with respect to plant transpiration.