Page 102 - Materials Chemistry, Second Edition

Plume Migration in Aquifer and Soil 85

2. The same equation can also be used to determine the radius of
influence, where drawdown is equal to zero. More discussions
on this topic are given in Chapter 6.

Example 3.11: Estimate Hydraulic Conductivity of an Unconfined
Aquifer from Steady-State Drawdown Data

Use the following information to estimate the hydraulic conductivity of an
unconfined aquifer:

• Aquifer thickness = 30.0 ft (9.1 m)
• Well diameter = 4 in. (0.1 m)
• Well perforation = fully penetrating
• Groundwater extraction rate = 20 gpm
• Steady-state drawdown = 2.0 ft observed in a monitoring well 5 ft
from the pumping well
= 1.2 ft observed in a monitoring well 20 ft
from the pumping well
Solution:
First we need to determine h and h :
2
1
h = 30.0 – 2.0 = 28.0 ft
1
h = 30.0 – 1.2 = 28.8 ft
2
Inserting the data into Equation (3.14), we obtain:
(1,055)(20)log(20/5)
K = 2 2 = 280 gpd/ft 2
(28.8 − 28 )

Discussion:
Drawdown and flow rate data in Examples 3.8 and 3.11 (one for a con-
fined aquifer and the other for an unconfined aquifer) are the same;
however, the calculated hydraulic conductivity values are different.
In these examples, the hydraulic conductivity of the unconfined
aquifer is smaller, but it delivers the same flow rate with the same
drawdown because the unconfined aquifer has a larger storage
coefficient. Refer to Section 3.2.4 for the discussion on the storage
coefficient.

Example 3.12: Estimate Hydraulic Conductivity of an
Unconfined Aquifer Using Specific Capacity

Use the pumping and drawdown data in Example 3.10 to estimate the
hydraulic conductivity of the aquifer: