Page 89 - Materials Chemistry, Second Edition

72 Practical Design Calculations for Groundwater and Soil Remediation

Example 3.3: Traveling Speed of Leachate through
a Compacted Clay Liner
A compacted clay liner (CCL) was installed as the bottom liner of a landfill.
The thickness of the CCL is 2 ft, with hydraulic conductivity of ≤10 cm/s
−7
and effective porosity of 0.25. If the leachate thickness on top of the liner is
to be kept ≤1 ft, estimate the time needed for leachate to travel through this
liner.

Solution:

(a) We need to determine the hydraulic gradient first:
i = dh/dl = (head loss) ÷ (length of the flow path)
= (thickness of the CCL + leachate thickness) ÷ (thickness of the
CCL)
= (2 + 1)/(2) = 1.5
Darcy velocity (v ) = Ki
d
= (10 cm/s)(1.5) = 1.5 × 10 cm/s
−7
−7
(b) Seepage velocity (v ) = v /ϕ
d
s
= (1.5 × 10 )/(0.25) = 6.0 × 10 cm/s = 5.2 × 10 cm/day
−7
−7
−2
(c) Time = distance/velocity
= (2 ft)(30.48 cm/ft) ÷ (5.2 × 10 cm/day)
−2
= 1,176 days = 3.2 years
Discussion:
1. The maximum leachate thickness (1 ft) and the maximum
hydraulic conductivity of the CCL (10 cm/s) were used for the
−7
worst scenario.
2. Assuming the CCL is intact, it will take 3.2 years for leachate to
travel through the 2-ft CCL.
3. The total traveling time will be inversely proportional to the
hydraulic gradient and the hydraulic conductivity, but it will be
proportional to the thickness of the CCL.

3.2.3 Intrinsic Permeability versus Hydraulic Conductivity

In the soil-venting literature, one may encounter a statement, such as
“the soil permeability is 4 darcys”; while in groundwater-remediation lit-
erature, one may read about “the hydraulic conductivity is equal to 0.05
cm/s.” Both statements describe how permeable the formations are. Are
they the same? If not, what is the relationship between the permeability
and hydraulic conductivity?