Maya Engel
SLAC National Accelerator Laboratory
ABSTRACT
Subsurface interfaces in floodplains are biogeochemical hotspots known to trigger colloid generation. Fluctuating redox conditions can induce transformation and mobilization of the colloids, with the potential to release associated compounds such as organic matter (OM), metal micronutrients, and contaminants. Observations from our Slate River (CO) floodplain site suggest that sulfidic Fe-rich colloids are abundant in the fine-grained, anoxic zones and can transport through the riparian zone into the river. Yet, the mechanisms controlling colloid dynamics are largely unsolved. Therefore, we are focusing on our mechanistic understanding of colloid dynamics by investigating the molecular and nanoscale structure and chemical composition of the colloids. We collected pore water samples from our Slate River floodplain field site in spring, summer, and autumn, to account for differences in colloid structure/composition linked to seasonally variable dissolved oxygen concentrations. All samples were filtered (220, 100 and 20 nm filters) in the field at collection to size-separate the colloids before being stored in anoxic conditions in the dark at 2 oC. Reduced Fe-rich colloids were abundant between 130-240 cm depths, where anoxic conditions prevail. Their levels peaked in May and June and dropped in July and August (2021). This likely stemmed from the intrusion of oxygenated water following July rainfall that stimulated either colloid transformation and/or colloid transport. Initial cryo-EM imaging of colloid samples showed mono-dispersed spherical colloids in the 10-50 nm range. These nano-spheres were also observed as nano-assemblages (>20 nm), composed of multiple nano-spheres. Preliminary Fe-EXAFS data suggest the colloids (both the nano-spheres and nano-assemblages) contain a mixture of FeS and ferrihydrite. Our findings will help characterize these natural colloids and unravel their role in colloid associated biogeochemical processes at the groundwater to watershed scale.
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