Page 84 - Materials Chemistry, Second Edition

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Plume Migration in Aquifer and Soil

3.1 Introduction
In Chapter 2, we illustrated necessary calculations for site assessment and
remedial investigation. Generally, from remedial investigation (RI) activi-
ties, the extent of the contaminated plume in subsurface soil and/or aquifer
would be defined. If the compounds of concern (COCs) are not removed,
they may migrate farther under common field conditions, and the extent of
the plume(s) will enlarge.
In the vadose zone, the COCs will move downward as free product and,
in the meantime, get dissolved in infiltrating water and then move down-
ward by gravity. The downward-moving liquid may come into contact with
the underlying aquifer and create a dissolved plume. The dissolved plume
will move downgradient in the aquifer. In addition, the COCs, especially
the volatile organic compounds (VOCs), will volatilize into the air void of
the vadose zone and travel under advective forces (with the air flow) and
concentration gradients (through diffusion). Migration of the vapor can be
in any direction, and the COCs in the vapor phase, when coming in contact
with the soil moisture and groundwater, may get absorbed into them. For
site remediation or health risk assessment, understanding the fate and trans-
port of COCs in subsurface is important. Common questions related to the
fate and transport of COCs in subsurface include:
1. How long will it take for the plume in the vadose zone to enter the
aquifer?
2. How far and how fast will the vapor COCs in the vadose zone travel?
In what concentrations?
3. How fast does the groundwater flow? In which direction?
4. How fast will the plume migrate? In which direction?
5. Will the plume migrate at the same speed of the groundwater flow?
If different, what are the factors that would make the plume migrate
at a different speed?
6. How long has the plume been present in the aquifer?

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