Although diffusion of compounds is important in sediment porewaters and on small spatial scales of less than a meter, rates ofdiffusion alone do not account for all solute transport in most systems. Advection, the movement of the fluid itself, accounts for much of the solute transport over larger distances and in overlying waters. Specifically, advection represents a flow of sediment or water. For a solute, the advective flux Fa (mol cm 2 s 1) in a 1D model can be defined as

with u the speed of water flow (cms-1). Here, advection can be due to burial, compaction, and/or external hydro-logical flow.

The movement ofsolutes in sediments and porewaters is often primarily via diffusion, but principally via advection in aquatic and atmospheric systems. Both processes, diffusion and advection, are important at the sediment-water or sediment-air interface. Rates of solute transport within sediments by diffusion to the sediment-water interface are largely influenced by the concentration gradient between the sediments and the overlying waters. Rates of diffusion out of sediments are increased when the solutes are then advected away into the overlying waters, as this transport prevents their accumulation at the interface and maintains a large concentration gradient across the sediment-water interface, thus facilitating higher diffusive fluxes than would occur without advection. Advection can also act to replenish solutes (e.g., O2) in overlying waters at this interface and facilitate faster diffusion rates in the reverse direction into sediments. The direction of the concentration gradient of nutrients and contaminants, relative to overlying waters, will determine whether sediments will act as a source or sink for these compounds.

A conceptual diagram of transport processes occurring at the sediment-water interface is shown in Figure 2.

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