This study presents field experiments conducted in a contaminated aquifer in Rifle, CO, to determine the speciation and accumulation of uranium in sediments during in-situ bioreduction. We applied synchrotron-based X-ray spectroscopy and imaging techniques, as well as aqueous chemistry measurements to identify changes in U speciation in water and sediment in the first days follwing electron donor amendment. Limited changes in U solid speciation were observed throughout the duration of this study, and non-crystalline U(IV) was identified in all samples obtained. However, U accumulation rates strongly increased during in-situ bioreduction, when the dominant microbial regime transitioned from iron- to sulfate-reducing conditions. Results suggest that uranium is enzymatically reduced during Fe-reduction, as expected. Mineral grain coatings newly formed during sulfate reduction act as reduction hotspots where numerous reductants can act as electron donors (Fe(II), S(-II), microbial extracellular polymeric substances) that bind and reduce U. The results have implications for identifying how changes in the dominant reducing mechanism , such as Fe- vs. sulfate-reduction, affect trace metal speciation and accumulation. The outcomes from this study provide additional insights into uranium accumulation mechanisms in sediments that could be useful for the refinement of quantitative models describing redox processes and contaminant dynamics in floodplain aquifers.Read less <