Page 240 - Materials Chemistry, Second Edition

Groundwater Remediation 223

Example 6.4: Estimate the Groundwater Extraction Rate from an
Unconfined Aquifer Using the Steady-State Drawdown Data
Use the following information to estimate the groundwater extraction rate of
a pumping well in an unconfined aquifer:

• Aquifer thickness = 30.0-ft (9.1-m) thick
• Well diameter = 4-in. (0.1-m) diameter
• Well perforation depth = fully penetrating
• Hydraulic conductivity of the aquifer = 400 gpd/ft 2
• Steady-state drawdown
= 2.0 ft in a monitoring well that is 5 ft from the pumping well
= 1.2 ft in a monitoring well that is 20 ft from the pumping well

Solution:
(a) First we need to determine h and h :
1
2
h = 30.0 − 2.0 = 28.0 ft
1
h = 30.0 − 1.2 = 28.8 ft
2
(b) Inserting the data into Equation (6.2), we obtain
2
Kh 2 − h 2 ) 400(28.8 2 − 28.0 )
Q = ( 2 1 = = 28.6 gpm
1,055 log( /)rr 1 1,055log(20/5)
2
6.2.2 Capture-Zone Analysis
One key element in design of a groundwater extraction system is to select
proper locations for the extraction wells. If only one well is used, the well
should be strategically located to create a capture zone that encloses the
entire plume. If two or more wells are used, the general interest is to
find the maximum distance between two consecutive wells such that no
COCs can escape through the interval between these two wells. Once
such distances are determined, one can depict the capture zone of these
wells.
To delineate the capture zone of a groundwater pumping system in an
actual aquifer can be very complicated. To allow for a theoretical approach,
let us consider a homogeneous and isotropic aquifer with a uniform thick-
ness and assume that the groundwater flow is uniform and steady. The theo-
retical treatment of this subject starts from one single well and expands to
multiple wells. The following discussion is mainly based on the work by
Javandel and Tsang [1].