Dissolved Organic Carbon Sources and Transport and Its Role in Regulating Dissolved Arsenic Levels.

Arsenic and other trace metals commonly are released through the reductive dissolution of iron and manganese oxides by microorganisms as part of energy metabolism. Both dissolved and sedimentary carbon substrates can be used by these microorganisms. Recent evidence implies that dissolved organic carbon (DOC) from the surface is efficiently transported to depth, where it stimulates in situ microbial communities. Other studies that correlate low aqueous As concentrations with older, recalcitrant organic carbon in older sediments. This work examines processes that affect which of these substrates contribute to reduction, and identifies potential characterizes how organic carbon moves through the subsurface to better understand this apparent contradiction.  Based on laboratory experiments and field monitoring using soils and sediments from Vietnam, Cambodia and Bangladesh, we propose a conceptual model for DOC transport in which DOC transport is retarded by adsorption, and in which it re-equilibrates with sedimentary (particulate) organic carbon. In many cases, desorbed sedimentary organic forms a distinct DOC carbon pool with properties and reactivity that are similar to that of young DOC and much higher than predicted based on its age. Under conditions in which this sedimentary DOC is efficiently mobilized, the associated reducing conditions can stimulate arsenic release and in some cases, down-gradient transport.