Transport Within the Aqueous Phase

The three-dimensional dispersion of a completely soluble organic solute within a volume of pure water will be governed by its rates of diffusion within the water column and by the flow characteristics of the water itself (also called convection or advection). In actual water bodies, complicating factors include the presence of particles of various sizes within the aqueous, phase and the effects of boundary layers such as those associated with the air-water and sediment-water interfaces. Further complications occur in soil-water and groundwater systems in which the aqueous phase is a minor component in the presence of an excess of solid material (Thibodeaux, 1979). 

Movement of a soluble chemical throughout a water body such as a lake or river is governed by thermal, gravitational, or wind-induced convection currents that set up laminar, or nearly frictionless, flows, and also by turbulent effects caused by inhomogeneities at the boundaries of the aqueous phase. In a river, for example, convective flows transport solutes in a nearly uniform, constant-velocity manner near the center of the stream due to the mass motion of the current, but the friction between the water and the bottom also sets up eddies that move parcels of water about in more randomized and less precisely describable patterns where the instantaneous velocity of the fluid fluctuates rapidly over a relatively short spatial distance. The dissolved constituents of the water parcel move with them in a process called eddy diffusion, or eddy dispersion. Horizontal eddy diffusion is often many times faster than vertical diffusion, so that chemicals spread sideways from a point of discharge much faster than perpendicular to it (Thomas, 1990). In a temperature- and density-stratified water body such as a lake or the ocean, movement of water parcels and their associated solutes will be restricted by currents confined to the stratified layers, and rates of exchange of materials between the layers will be slow. 

The other method of diffusion of a chemical through a liquid phase, molecular diffusion, is driven by concentration gradients. It is normally orders of magnitude slower in natural waters than eddy-driven processes, unless the water body is abnormally still and uniform in temperature (Lerman, 1971). Such situations are found only in isolated settings such as groundwaters and sediment interstitial waters. Even here, however, empirical measurements often indicate that actual dispersion exceeds that calculated from molecular diffusion alone. 

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