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Colloidal mobilization is a well-known mechanism of metal transport in groundwater and surface water. Inthe subsurface, colloids can be generated under variable redox, which drives changes in solution chemistry(like pH and ionic strength), as well as mineral dissolution. Much research has focused on the inorganiccomposition of colloids, especially reactive iron(III) oxy(hydr)oxide minerals. However, the organic composition of colloids is less well studied. Organic matter in colloids can associate with minerals, complexmetal ions, and serve as an electron donor for microbial respiration. Its composition is therefore of highinterest.In the current work our objective was to examine the composition of colloidal organic matter (< 0.6 micron)in porewater along a redox gradient in a riparian soil located along the Slate River in Crested Butte, Colorado, USA. Porewaters were collected as a function of depth (90, 130, 200, and 350 cm bgs). All depthswere oxic to suboxic, except 200-cm, where the products of iron and sulfate reduction were observed concomitant with an increase in dissolved/colloidal organic carbon, pH, alkalinity, and conductivity.The composition of porewater colloids was probed using correlated scanning transmission X-ray microscopy combined with transmission electron microscopy, which revealed organic matter enmeshed with noncrystalline iron. We observed a change in the organic matter chemistry from carboxylate-rich at the 200-cm depth (where the diversity and concentration of organic matter was also highest) to phenol-rich at theother depths. Soil porewater can flow down into the gravel bed (that lies below the soil and is connected tothe river) during baseflow conditions, thus we speculate whether anoxic porewater could transport carboxylate-rich organic colloids into the gravel bed and from there into surface waters.
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