Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 366

TABLE 6.14
Comparison of the Characteristic Times of Sediment
Transport Processes
Mechanism Characteristic Time (Years)

Molecular diffusion (unretarded by sorption) 0.5 (hypothetical)

Molecular diffusion (retarded by sorption) 1900

Colloidal-enhanced diffusion 1500

Sediment erosion (1 cm/y erosion) 10

Capped sediment (30 cm effective cap) 21,000

Bed load transport (sediment movement) 42 h

Advection (aquifer interactions) 4000

Surface roughness (local advection) 69,000

Bioturbation 10

Note: Characteristic times are order of magnitude estimates of time required to
leach a typical hydrophobic organic compound (e.g., trichlorobiphenyl)
from a 10-cm layer of sediment by each of various transport mechanisms.
In most cases, the characteristic time is 1/ e times or half-lives. For advective
processes, the characteristic times represent time for complete removal.
The above refer to typical sediment conditions represented in Reible et al.
(1991). Although absolute values refer to a particular set of conditions, the
ranked magnitudes are probably indicative of generic behavior.

6.4.1.3 Soil-Air Exchange of Chemicals

Figure 6.58 represents the chemical transport pathway from soil to the atmosphere.
The difference from the saturated case (groundwater) is that in this unsaturated case,

Water

Contaminant
diffusion

z = 0

x

Contaminated sediment

z

FIGURE 6.57 Schematic of the diffusive transport of a contaminant from a bed sediment to
the overlying water column.