Heterogeneous advective transport involves a secondary
phase within the bulk advective phase, such as when a particle in air or water acts
as a carrier of a chemical. In many cases we can treat heterogeneous advection the
same as homogeneous advection if we know the flow rate of the secondary phase and
the concentration of chemical in the secondary phase. In the lake example above, if
the volume fraction of suspended particles in the lake water is 10−5, the flow rate of
suspended particles is 0.01 m3/h, and the concentration of the toxicant in the solid
particles is 100 mg/m3, then the advective flow of the toxicant on suspended particles
will be 1 mg/h or the same as the homogeneous advection via water. Although the flow
rate of particles is much lower than that of water, the concentration of the toxicant is
much higher in the suspended particles than dissolved in the water. This is typical of
a hydrophobic toxicant such as DDT or benzo[a]pyrene. In soil and sedimentary systems,
colloidal particles (often macromolecular detritus) can play a very important role
in heterogeneous advective transport because they have greater mobility than larger
particles, and they often have greater capacity to sorb many toxicants because of their
higher organic carbon content and higher surface area/mass ratio. In highly contaminated
sites, organic co-solvents can be present in the water (usually groundwater) and
act as a high-capacity and high-efficiency carrier of toxicants through heterogeneous
advection in the water.