Page 120 - Materials Chemistry, Second Edition

Plume Migration in Aquifer and Soil 103

• Groundwater gradient = 0.005
• Dry bulk density of aquifer materials = 1.6 g/cm 3
• Fraction of organic carbon of the aquifer materials = 0.015
• K = 0.63 K ow
oc
Briefly discuss your results and list possible factors that may cause your
estimate to differ from the true value.

Solution:
(a) Use Equation (3.1) to find the Darcy velocity:
v = k = (30)(0.005) = 0.15 m/d
i
(b) Use Equation (3.3) to find the groundwater velocity (i.e., the seep-
age velocity, or the interstitial velocity):
v = v/ϕ = (0.15)/(0.4) = 0.375 m/d
s
(c) Use Equation (3.34) and the values of R from Example 3.19 to
determine the migration speeds of the plumes:
v = (0.375)/(6.10) = 0.061 m/d = 22.4 m/yr (benzene)
p
v = (0.375)/(2.28) = 0.164 m/d = 60.0 m/yr (1,2-DCA)
p
v = (0.375)/(2,864) = 0.000131 m/d = 0.048 m/yr (pyrene)
p
(d) The time for 1,2-DCA to travel 250 meters can be found as:
t = (distance)/(migration speed)
= (250 m)/(60.0 m/yr) = 4.17 yr = 4 years and 2 months
So 1,2-DCA entered the aquifer in July of 2009.
(e) The time for benzene to travel 50 meters can be found as:
t = (50 m)/(22.4 m/yr) = 2.23 yr = 2 years and 3 months
So benzene entered the aquifer in June of 2011.

Discussion:
1. The estimates are the times when benzene and 1,2-DCA first
entered the aquifer. The data given are insufficient to estimate
the time the leachates traveled through the vadose zone and,
consequently, the time the leaks started from the sources (e.g.,
leaky USTs).
2. The retardation factor of 1,2-DCA is smaller; therefore, its migra-
tion speed in the vadose zone would be faster. This helps to
explain the fact that 1,2-DCA entered the aquifer earlier than
benzene.