Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 290

Precipitation

Gasoline

n Well

Land
surface

Diffusion

Recharge

Unsaturate d
zo ne

Biodegradation

Volatilization

Water lable

Groundwater flow direction:
advective transport

FIGURE 8.8
Advective transport and other processes effecting the migration of contaminants in groundwater. (From United
States Geological Survey (USGS), Simulating transport of volatile organic compounds in the unsaturated zone
using the computer model R-UNSAT, USGS Fact Sheet 019-98, Washington, DC, 1998.)

Source plane

Bounding envelope

Higher permeability zones
carry more contaminant mass

Center of mass

FIGURE 8.9
Contaminant migration in heterogeneous and anisotropic geologic media. (Adapted from Payne, F.C. et al.,
Remediation Hydraulics , CRC Press, Boca Raton, FL, 2008. With permission.)

This concept is represented in Figure 8.9, where a contaminant (solute) is shown migrat-
ing in the more permeable layers. More highly permeable layers have a higher hydraulic
conductivity and behave as preferred groundwater and contaminant migration pathways.
These layers are essentially super highways for groundwater and contaminant transport,
and in some instances the hydraulic conductivity is from 100 to sometimes 1000 times
greater over distances of just a few centimeters.