Page 103 - Materials Chemistry, Second Edition

86 Practical Design Calculations for Groundwater and Soil Remediation

• Aquifer thickness = 80 ft
• Pumping rate = 40 gpm
• Steady-state drawdown in the well = 7.5 ft

Solution:
(a) First let us determine the specific capacity. Use Equation (3.12),

Q 40
Specific capacity = = = 5.3 gpm/ft 2
s w 7.5
(b) The transmissivity of the aquifer can be estimated as:
T = (5.3)(1,550) = 8,220 gpd/ft
(c) The hydraulic conductivity of the aquifer can be estimated as:
K = T/b = 8,220/80 = 103 gpd/ft 2

Discussion:
The calculated hydraulic conductivity (103 gpd/ft ) from this exercise
2
has the same order of magnitude as the value specified in Example
3.10 (200 gpd/ft ).
2

3.4 Aquifer Tests
In Section 3.3, methods using the steady-state drawdown data (Equations
3.11 and 3.14) were described to estimate the hydraulic conductivity of aqui-
fers. For a groundwater-remediation project, it is often required to have a
good estimate of the hydraulic conductivity before the full-scale ground-
water extraction. Grain-size analysis of aquifer materials and bench-scale
testing on core samples can provide some limited information. For more
accurate estimates, aquifer tests are often conducted.
Pumping tests and slug tests are two common types of aquifer tests. In
a typical pumping test, groundwater is extracted from a pumping well
at a constant rate. (Other pumping schemes are also feasible, but not as
popular.) The time-dependent drawdowns (or recovery) in a pumping
well and in a few monitoring wells are recorded. The data are then ana-
lyzed to determine the hydraulic conductivity and storativity. The pump-
ing test is recommended because it provides information on subsurface
hydrogeology over a large area (the area affected by the pumping) and
gives a realistic estimate of the pumping rate for the full-scale groundwa-
ter extraction. Many remediation systems have been incorrectly designed
and installed for a flow rate much larger than the extraction wells could
yield due to lack of accurate aquifer information. In addition, analysis of