Movement of Light, Heat, and Chemicals in Water - Freshwater Ecology: Concepts and Environmental Applications - page 34

Molecular diffusion is extremely slow on scales that humans can ob-
serve directly (Fig. 3.1). For example, a cube of sugar would take days to
dissolve completely and disperse evenly throughout a glass of water in the
absence of any movement of the water. Cooling and convection currents
are created as water cools at the surface from evaporation and drops to
the bottom of the glass. These density currents cause the sugar to dissolve
much more quickly in normal circumstances because the temperature of
the glass is not controlled precisely. If a weak agar or gelatin solution is in
the glass, convection would be damped and diffusion would be slowed
considerably.
Water currents often occur at spatial scales exceeding those of indi-
vidual molecules, and such currents can dominate material transport.
Molecular diffusion is many orders of magnitude slower than diffusion
with water movement (variously referred to as transport diffusion, ad-
vective transport, or eddy diffusion ). As discussed in Chapter 2, overall
molecular movements are on the order of several nanometers per second,
whereas velocity in streams and rivers can exceed 1 meter per second (a
billion times more rapid). Turbulent flow causes transport diffusion and
relatively high diffusion coefficients. Thus, if there is any appreciable
movement of water, transport diffusion is expected to dominate over
molecular diffusion. With very small spatial scales turbulence is not pres-
ent, and molecular diffusion is more important. Transport diffusion
overrides molecular diffusion in the water column of lakes and parts of
wetlands, groundwaters, streams, and rivers. Very near solid surfaces or
within fine sediments, molecular diffusion is the dominant mode of
diffusion.
In sediments, such as in groundwater or at lake bottoms, the rate of
chemical diffusion is also influenced by the mean path length and size of
the channels within the sediment. Short path length and large channels lead
to high permeability. When the channels are long or many dead-end chan-
nels exist, diffusion rates are slowed because molecules must take a longer
path to diffuse between two points, and transport diffusion is limited be-
cause water velocity is slow. Determining mean channel length in a sedi-
ment requires direct empirical measurements of diffusion of a tracer through
the sediment.
Solutes can interact with sediments (both the inorganic particles and
the organic materials) and further lower diffusion rates. For example, re-
moving organic contaminants from groundwater is very difficult because
they have low solubility and are adsorbed onto the surface of the sedi-
ment particles, so they have low diffusion rates. Thus, bioremediation
efforts (the use of microbes to clean groundwater) and other methods
used to remove organic contaminants from groundwaters often employ
detergents. Detergents increase diffusion rates, increase the biological
availability of organic contaminants, and speed the decontamination
process.
Water velocity nears zero as a solid surface is approached, and mo-
lecular diffusion predominates when water movement is sufficiently
slow. The region near a solid surface where molecular diffusion pre-
dominates is called the diffusion boundary layer . This is similar to, but
thinner than, the flow boundary layer discussed in Chapter 2. However,