| 25 Aug 2026
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.
The manuscript by Perzan et al. presents a thoughtful and well executed analysis of how differences in floodplain inundation processes drive solute mobilization and mineral dissolution and precipitation patterns. The study reports primarily on reactive transport simulations of vertical columns that represent floodplain soils in the arid Wind River Range. The simulations incorporate field measurements of soil properties and are qualitatively compared with observed solute patterns. Overall, the study is robust and presents important information on how top-down and bottom-up inundation events result in different geochemical processes; namely, top-down flooding results in mineral dissolution and leaching while bottom-up flooding enriches soils in solutes that can then precipitate out as evaporite minerals, particularly when water is removed though transpiration. The results have implications for predicting seasonal and climatic influences on parameters such as soil and water salinization.
The manuscript would benefit from additional discussion of a few key points as well as some specific edits to improve clarity. First, the study focuses on a semi-arid environment where evaporites can accumulate in soils, and it is not clear whether these results are applicable to humid environments. Therefore, the paper should be clear about under what conditions and in which environments these results apply. The role of antecedent conditions in driving the observed behaviors is particularly interesting and could be expanded. Second, the observed field patterns are qualitatively supported by the results of the simulations but not quantitatively compared. How well do the simulations replicate the observed field measurements in soil moisture and/or solute concentrations?
Specific comments
l.113. Figure 4 here should be Figure 2.
l.130. Were the infiltration rates measured or used here?
l.140. Many analytes are measured but not reported in the text. Are they present in the associated dataset? If not, they can be removed from the methods.
l.171. The methods describe incorporating the geometry of the macropores and percent coverage into the simulations. Were these parameters determined from the dye infiltration tests, and if so, please provide more information on the image analysis used to determine these parameters.
l.210. The sentence “Resultant water content and concentration profiles mimic those…” needs support from a published paper and/or model-data comparison.
l.217. Here and elsewhere, the term “discharge” is confusing. Does “discharge to the surface” refer to an infiltration rate of the flooded water into the soil column?
l.429 (and l. 264). The only field observations for a top-down flood are from the dye infiltration test, and solute concentrations are only provided for the bottom-up flood. As such, no observations of solute flushing are available for a top-down event. Conversely, no field observations of soil saturation for a bottom-up event are available. These limitations should be described more thoroughly in the discussion.
Figure 2. Only five porewater samplers are shown but six are described in the text. Also, please differentiate the different sampler types and their installations in the diagram.
Figure 10. At what position in the soil profile do the plotted concentrations represent?