Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 291

Higher permeability zones
(darker zones)

Lower permeability zones
(white zones) (a)

(b)

FIGURE 8.10
Effects of diffusion over time within a contaminant plume. (a) depicts an immature contaminant plume and (b)
depicts a mature contaminant plume and the effects of diffusion. (Adapted from Payne, F.C. et al., Remediation
Hydraulics , CRC Press, Boca Raton, FL, 2008. With permission.)

Several examples of this affinity for contaminants to migrate within more permeable
layers are shown in this topic (Figures 2.24, 2.28, and 2.36). Zones with higher permeability
move water more quickly and have a higher flux density. If more water moves through
these higher permeability zones, then a potentially greater contaminant mass also moves
through. Figure 8.9 shows this relationship at the right of the diagram, downgradient from
the source.
As contamination continues to migrate along these flow paths of higher permeability
dictated by the subsurface geology, diffusion of contaminants into less permeable zones
occurs (Figure 8.10). The top portion of the figure represents the flow paths of contami-
nants in the early stages of migration, and the later stages of migration are shown in the
figure's bottom portion. Over time, the contaminant (solute) has diffused into the less per-
meable, lower hydraulic conductivity geologic materials (shown as in Figure 8.10b; Payne
et al. 2008).
The transport of contaminants in groundwater is also influenced by many of the same
factors affecting the migration of contaminants in unsaturated soil or the vadose zone:

• Physical chemistry of the contaminants
• Solubility
• Molecular weight
• Vapor pressure
• Stability and persistence
• Sorption potential
• Type, distribution, and amount of microorganisms
• Tendency to biodegrade
• Dissolved oxygen content of groundwater
• Geological factors
• Stratigraphy (including thickness and distribution of geological units down to
microstratigraphic scales at the centimeter or even millimeter)