Influence of dam on the distribution of solid iron species in riparian sediment particles at different sizes

Abstract

The impact of dam on the distribution and transformation of elements in riparian zones has received increasing attention. However, previous studies mainly focused on the site scale, with limited investigation at the particle scale, although the particle size is an important sediment property affecting solute and contaminant adsorption and desorption. In this study, we collected sediment particles from a dam-adjacent riparian zone and analyzed the content of iron (Fe) species and sediment particle surface morphology. Results showed that dam significantly altered sediment particle surface morphology, but had no significant influence on Fe mineral types or the relative sequence of different Fe species. In the upstream riparian zone, Fe contents in particles of varying sizes followed the sequence: 250−1000 μm > 5−20 μm > 20−50 μm > 50−250 μm. Conversely, in the downstream riparian zone, this sequence shifted to 5−20 μm > 20−50 μm > 50−250 μm > 250−1000 μm. Additionally, the total Fe contents in various-sized particles were higher in the upstream riparian zone than in the downstream riparian zone. These differences were attributed to the sediment particle surface properties influenced by dam-regulated hydrodynamic processes. In the upstream riparian zone, the sediment particle surface exhibited rough textures and were covered with numerous Fe-rich fine particles, resulting in higher Fe content, particularly in 250–1000 μm sediment particles. In contrast, in the downstream riparian zone, sediment particle surfaces were smoother, leading to lower Fe content. In both upstream and downstream riparian zones, the relative content of different Fe species in sediment particles of varying sizes generally followed the sequence: Silicate-associated Fe ≫ crystalline Fe oxides > poorly crystalline Fe oxides > carbonate-associated Fe/metastable Fe(II) sulfides. However, the mass contents of Fe oxides and carbonate-associated Fe/metastable Fe(II) sulfides in 5–50 μm sediment particles exceeded those in 50–1000 μm sediment particles. In addition, the rates of Fe(II)/Fe(III) cycling in 5–50 μm sediment particles were higher. This study elucidated the previously overlooked role of dam in influencing the distribution and transformation of Fe in riparian sediments at the particle scale.