Page 128 - Materials Chemistry, Second Edition

Plume Migration in Aquifer and Soil 111

used as the relative measure to compare the migration rates of compounds
with different retardation factors. For soil-venting application, the air-phase
retardation factor is also the minimum number of pore volumes that must
pass through the impacted zone to clean it up. It is considered as the mini-
mum because this approach ignores the effects of mass-transfer limitations
among the phases, subsurface heterogeneity, and unequal travel time from
the outer edge of the plume to the vapor-extraction well [9].
As shown in Equation (3.43), the air-phase retardation factor increases
with ϕ and K , but decreases with Henry’s constant. Higher moisture con-
p
w
tent means a larger water reservoir to retain the COCs, and a larger K value
p
indicates that soil has a larger organic content or the COC is more hydropho-
bic. On the other hand, a compound with a larger Henry’s constant would
have a stronger tendency to volatilize into the air void. The Henry’s constant
increases with increasing temperature and, thus, a smaller air-phase retarda-
tion factor at a higher temperature. Therefore, for a soil-venting application,
at higher temperatures, fewer pore volumes of air need to be moved through
the impacted zone to remove the COCs.

Example 3.26: Determination of the Air-Phase Retardation Factor
The vadose zone underneath a site is impacted by several organic com-
pounds, including benzene, 1,2-dichloroethane (DCA), and pyrene.
Estimate the air-phase retardation factor using the following data from the
site assessment:

• Vadose zone soil porosity = 0.40
• Volumetric water content = 0.15
• Dry bulk density of soil = 1.6 g/cm 3
• Fraction of organic carbon of soil = 0.015
• Temperature of the formation = 25°C
• K = 0.63 K ow
oc

Solution:
(a) From Table 2.5,
H = 5.55 atm/M for benzene (at 25°C)
Use Table 2.4 to convert it to a dimensionless value:
H* = H/RT = (5.55)/[(0.082)(298)] = 0.227 (for benzene)
Similarly for 1,2-DCA (the Henry’s constant value in the table
is for 20°C, so we use this value for 25°C as an approximate
value) and pyrene:
H* = H/RT = (0.98)/[(0.082)(298)] = 0.04 (for 1,2-DCA)
H* = H/RT = (0.005)/[(0.082)(298)] = 0.0002 (for pyrene)