A mechanistic understanding of U sorption in natural soils and sediments is useful for determining its transport and bioavailability in the environment. X‐ray absorption spectroscopy (XAS) was used to determine the mechanisms by which U(VI) sorbs to three heterogeneous subsurface media reacted under static and dynamic flow conditions. Regardless of the media chosen, ternary surface complexes were the dominant type of sorption complex. Uranyl phosphate complexes were formed in subsurface media from more acidic environments. In contrast, uranyl carbonate ternary surface complexes formed in media from more neutral conditions. The complexes are predominantly inner‐sphere, although some outer‐sphere complexes may also be present, and appear to be on iron (hydr)oxides and possibly aluminosilicates. Additionally, the uranyl phosphate and carbonate complexes are highly disordered, which contributes to their reversible sorption properties.

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