Page 121 - Materials Chemistry, Second Edition

104 Practical Design Calculations for Groundwater and Soil Remediation

3. The migration speed of pyrene is extremely slow, 0.042 m/yr;
therefore, it was not detected in the downstream monitoring
wells. Most, if not all, of the pyrene compounds will be adsorbed
onto the soil in the vadose zone. The pyrene may travel in the
aquifer by adsorbing onto the colloidal particles.
4. The estimates are crude because many factors may affect the
accuracy of the estimates. Factors include uncertainty in the val-
ues of the hydraulic conductivity, porosity, groundwater gradient,
K , f , etc. Neighborhood pumping will affect the groundwater
oc
ow
gradient and, consequently, the migration of the plume. Other
subsurface reactions such as oxidation and biodegradation may
also have large impacts on the fate and transport of these COCs.

Example 3.21: Migration Speed of the Dissolved Plume in Groundwater
Results of a recent quarterly groundwater monitoring (July of 2013) at a site
indicate that the edge of the dissolved TCE plume has advanced 200 m in
the past 5 years. The groundwater gradient was determined to be 0.01 from
this round of monitoring. Using a value of 4.0 for the retardation factor and
effective aquifer porosity of 0.35, what would be your estimate of the aqui-
fer hydraulic conductivity? Also, because of the drought, an adjacent facility
(downgradient from the site) pumped out a great amount of groundwater in
2010. How will this affect your estimate?

Solution:
(a) The migration speeds of the plume, v :
p
= (distance)/(time) = (200)/5 = 40 m/yr
(b) Use Equation (3.34) and the value of R to find the groundwater
velocity, v :
s
v = v /R = 40 = v /4 → v = 160 m/yr
s
p
s
s
(c) Use Equation (3.3) to find the Darcy velocity, v :
d
v = v /ϕ = 160 = (v )/(0.35) → v = 56 m/yr
d
d
d
s
(d) Use Equation (3.1) to find the aquifer hydraulic conductivity:
v = Ki = (K)(0.01) = 56 m/yr → K = 5,600 m/yr = 15.3 m/day
d
Discussion:
The neighborhood pumping during the drought would increase the
natural groundwater gradient. During the pumping period, the
groundwater moved faster, and so did the plume. In other words,
the plume would have traveled a shorter distance without the pump-
ing. The hydraulic conductivity of the aquifer would be smaller than
this estimate, 15.3 m/day.