Organic Matter Chemistry in Floodplain Soils and Sediments – Implications for Uranium Retention.

Floodplains are constantly evolving depositional environments, where surficial organic rich sediments may become buried at various stages of decomposition, through sudden events such as overbank flooding and through the slower progression of river meandering. This creates a discontinuous subsurface distribution of organic rich sediments, which are often fine-grained and therefore poorly oxygenated due to diffusion limitation. The organic-rich sediments are hotspots for microbial activity and thereby central to the subsurface cycling of contaminants (e.g. U) and biologically relevant elements (e.g. C, N, P, Fe). However, the organic matter itself is poorly characterized. Consequently, little is known about the relevance of its properties for the biogeochemical processes it fuels.

In an investigation of soil/sediment cores from five former uranium ore processing sites on floodplains distributed across the Upper Colorado River Basin we confirmed consistent co-enrichment of U with organic-rich layers in all profiles. However, using C K-edge X-ray Absorption Spectroscopy (XAS) coupled with Fourier-Transformed Ion-Cyclotron-Resonance Mass-Spectroscopy (FT-ICR-MS) on bulk sediments and density-separated organic matter fractions, we did not detect any chemical difference in the organic rich sediments compared to the surrounding coarser-grained aquifer material within the same profile, even though there were differences in organic matter composition between the 5 sites. This suggests that U retention and reduction to U(IV) is independent of C chemical composition on the bulk scale. Instead it appears to be the abundance of organic matter in combination with reducing conditions arising from microbial decomposition in oxygen-limited environments that result in U enrichment, predominantly as non-crystalline U(IV). Thus, the chemical composition of organic matter is subordinate to the physical environment and total organic matter content for controlling U reduction and retention processes.