Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 277

dispersion. Advection is the horizontal transport of any property by the atmosphere and
water. Common examples are the transfer of heat by wind and sediment transport within
a flowing stream. Convection is the vertical advection of air, water, or other fluid as a
result of thermal differences. We introduced the concept of convection in Chapter 2 as the
driving force behind plate tectonics. Molecular diffusion is the movement of a chemical
from an area of higher concentration to an area of low concentration due to the random
motion of the chemical molecules. Dispersion (also referred to as hydrodynamic disper-
sion) is the tendency for contaminants to spread out from the path of the expected advec-
tive flow (USGS 2006a). Occasionally, the effects of diffusion and dispersion are treated
together, but for the purposes of this topic we treat them separately.
The rate of advective transport of a contaminant is often expressed in terms of flux den-
sity. Flux density is the mass of a chemical transported across an imaginary surface of a
unit area per unit of time. Equation 8.2 shows this relationship (Hemond and Fechner-
Levy 2000), which is independent of the media involved (soil, surface water, groundwater,
or the atmosphere):

J CV = (8.2)

where
J is the flux density = (mass/[length × width] × time) or [M/L 2 × T]
C is the concentration of the chemical per cubic liter or meter of media [M/L 3 ]
V is the velocity [length/time] or [L/T]

An example of molecular diffusion is shown in Figure 8.2 (Payne et al. 2008). From the
release time to infinity, a contaminant released into a fluid such as air or water will diffuse
throughout the fluid at random locations.
Fick's First Law of Diffusion (Equation 8.3) can be used to predict the diffusive flux of a
contaminant (solute) across an imaginary plane as a function of the rate of change in con-
centration with distance (Hemond and Fechner-Levy 2000).

D dC
dx

(

J = − one dim ension ) (8.3)

Infinity

0

Elapsed time

FIGURE 8.2
Molecular diffusion. (Adapted from Payne, F.C. et al., Remediation Hydraulics , CRC Press, Boca Raton, FL, 2008.
With permission.)