Conceptual model of Hanford 300 Area riverbed POM dynamics. Hydrologic (river stage) driven advective fluid flux into the riverbed drives the input attached and suspended POM (organic-rich “fluff”, upper right photo) in the near-shore environment at the 300 Area, leading to its accumulation in a sediment underlying the amoring layer of large pebbles (upper right cartoon). The near-surface layer is illustrated by the green and black zones at the sediment surface in the main diagram, with the green zone illustrating that portion of the riverbed that may experience fluid inflow or outflow depending on river stage. Hydrolysis and decomposition of incoming polymeric organic materials (red arrows) has the potential to release soluble (mobile) DOC and nutrients into the underlying hyporheic zone. Some of the POM and soluble degradation products may be entrained in return flow toward the river (dark blue arrows) when the direction of fluid flow reverses. The upper left diagram illustrates of the operation of the organic matter components of a biogeochemical simulation model of POM transport and metabolism in permeable riverbed sediments. Metabolism of dissolve organic matter (DOM) released during degradation of POM drives aerobic respiration and other redox reactions associated with organic carbon oxidation.