Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 370

saturated zone is pumping air and/or applying vacuum to strip volatile materials from
soil pore air or groundwater. In the vadose zone this is called air sparging . When
water is used to flush contaminants, it is called in situ soil washing .
Treatment involves a variety of above-ground technologies for both water and
air that is recovered from the subsurface. Processes that utilize intrinsic bacterial
populations are useful in destroying contaminants in-place. These include natural
attenuation and intrinsic bioremediation .
Equilibrium and chemical kinetic aspects of P&T technologies for the vadose zone
and saturated zone in subsurface soil environments are explored below.

6.4.2.1 P&T for NAPL Removal from Groundwater

Figure 6.60 is a schematic for a P&T groundwater remediation. This is a method for
remediating contaminated groundwater. Clean water that is brought into the aquifer
flushes pollutants from the contaminated region. The water brought to the surface
will have to be further treated before re-injection or disposal. Although effective in
containing plume migration, P&T is ineffective in removing all of the material from
the subsurface. Once the free phase NAPL is removed, the residual NAPL is harder to
remove by water flushing. It gives rise to the “tailing-off” effect, that is, contaminants
in inaccessible regions dissolve very slowly over extended periods (Figure 6.61).
For a withdrawal rate Q from a well, the maximum removal rate of a contaminant
is Q [A], where [A] is the porewater concentration. If the contaminant is present as a
pure phase, the value of [A] is the same as the aqueous solubility [A] ∗ . As water is
pumped through the aquifer, equilibrium between the solids and porewater dictates
the maximum mass of contaminant that can be transported through the groundwater.

Off-site

Treat water

Re-inject

Remove pollutant From clean

water storage

Injection
well

Extraction
well

Injection
well

Cone of
depression

Vadose
zone

Groundwater
table

Plume

Flow

Impervious bedrock

FIGURE 6.60 A basic P&T approach for groundwater remediation.