Shifts in phosphorus chemistry in a wetland stream during episodic rain events: co-variation and association with uranium.

Phosphorus (P) transport in riparian wetland systems is strongly influenced by hydrological events and interactions with suspended solids, yet association with its speciation and coupling with contaminant transport remain unclear. Here, we investigated P dynamics using high-frequency time-series sampling during five storm events in a riparian-wetland located on the Savannah River Site, South Carolina. Total P was fractionated into dissolved and particulate forms, with particulate P further separated into four operationally defined species identified by sequential extraction techniques. Total P was dominated by particulate P (>85%), with storm-driven increases primarily associated with elevated suspended solids. Marked shifts in P speciation occurred between early and late stormflow stages. Early-stage samples were enriched in reactive particulate P (e.g., exchangeable- and Fe-bound P), whereas late-stage samples contained higher proportions of occluded P and phosphate. These patterns corresponded to changes in particle composition, with early samples associated with Fe/organic matter (OM)-rich flocs and late samples contributed by less reactive mineral particles and aggregates. Strong correlations between P partitioning coefficients and solid-phase Fe and Mn highlight the likely central role of Fe-rich particles in regulating P transport. Significant correlations between stream P and uranium (U), one of key contaminants on the site, indicate their co-variation transport via shared associations with reactive suspended solids, suggesting that P may serve as an indicator of contaminant-associated particle transport under the studied conditions. Overall, storm-driven formation and mobilization of Fe/OM-rich particulates play an important role in regulating P speciation and transport in riparian-wetland systems.