Page 119 - Materials Chemistry, Second Edition

102 Practical Design Calculations for Groundwater and Soil Remediation

(d) Use Equation (3.33) to find the retardation factor:
b ρ K p (1.6)(1.275)
R =+ =+ = 6.10 forbenzene
1
1
φ 0.4
b ρ K p (1.6)(0.32)
R =+ =+ = 2.28 for 1,2-DCA
1
1
φ 0.4
b ρ K p (1.6)(717)
R =+ =+ = 2,869 forpyrene
1
1
φ 0.4
Discussion:
Pyrene is very hydrophobic with a large K value, and its retardation
p
factor is much larger than those of benzene and 1,2-DCA.

3.5.4 Migration of Dissolved Plume

The retardation factor relates the plume migration velocity to the ground-
water seepage velocity as

R = V s or V p = V s (3.34)
V p R

where V is the groundwater seepage velocity and V is the velocity of the dis-
s
p
solved plume. When the value of R is equal to unity (for inert compounds),
the compound will move at the same speed as the groundwater flow without
any “retardation.” When R = 2, for example, the COC will move at half of the
groundwater flow velocity.

Example 3.20: Migration Speed of Dissolved Plume in Groundwater

The aquifer underneath a site is impacted by several organic compounds
including benzene, 1,2-dichloroethane (DCA), and pyrene. A recent ground-
water monitoring in September 2013 indicated that 1,2-DCA and benzene
have traveled 250 m and 20 m downgradient, respectively, while no pyrene
compounds were detected in the downgradient wells.
Estimate the time when the leachates first entered the aquifer. The follow-
ing data were obtained from the site assessment:

• Effective aquifer porosity = 0.40
• Aquifer hydraulic conductivity = 30 m/day