t ests and tests in well fields Aquifers may be pumped at variable discharge rates either deliberately or due to characteristics of the pump Aquifers can be pumped stepwise always pumped but pumping rates vary or may be pumped intermittently not always pumped and pumping rates can va ID: 320884
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Slide1
Chapter 12:Variable-discharge tests and tests in well fields
•Aquifers may be pumped at variable discharge rates either deliberately or due to characteristics of the pump.
•Aquifers can be pumped step-wise (always pumped but pumping rates vary) or may be pumped intermittently (not always pumped and pumping rates can vary). Slide2
Confined Aquifers, Birsoy-Summer’s MethodBirsoy and Summers present an analytical solution for the drawdown response in a confined aquifer that is pumped step-wise or intermittently .
They apply the principle of superposition (ch.6) to Jacob’s approximation of the
Theis
equation (3.7) (shown below)
Variable-discharge testsSlide3
The drawdown in the aquifer at time t during the nth pumping period of intermittent pumping is shown by the following expression: where
Variable-discharge tests
Confined Aquifers,
Birsoy
-Summer’s Method
where
whereSlide4
Intermittent PumpingSlide5
For step-wise (uninterrupted pumping):Variable-discharge tests
t’
(i-1)
=
t
i
, and the ‘adjusted time’ becomes
where
Confined Aquifers,
Birsoy
-Summer’s MethodSlide6
If the intermittent pumping rate is constant (Q=Q1 =Q2=…….Qn) then the adjusted time becomes:Dividing both sides of equation 12.1 by Qn gives an expression for drawdown:
Variable-discharge tests
Confined Aquifers,
Birsoy
-Summer’s MethodSlide7
Step-wise PumpingSlide8
Variable-discharge testsConfined Aquifers, Birsoy-Summer’s Method
Assumptions
(from Ch.3):
1.)The aquifer is confined
2.)The aquifer has a seemingly infinite areal extent3.)The aquifer is homogeneous, isotropic, and of uniform thickness over the area influenced by the test
4.)Prior to pumping, the
piezometric
surface is horizontal (or nearly so) over the area influenced by the test
5.)The aquifer is pumped step-wise or intermittently at a variable discharge rate or is intermittently pumped at a constant discharge rate
6.)The well penetrates the entire thickness of the aquifer and thus receives water by horizontal flow
The following conditions are added
:t
he flow to the well is in an unsteady state
R is small and t is sufficiently largeSlide9
•The sharpest decrease in discharge occurs soon after the start of pumping. Aron and Scott take this into account. They show that when:Variable-discharge tests
Confined Aquifers,
Aron
-Scott’s Method
where
s
n
=
drawdown at a certain moment
t
n
,
; se=excess drawdown caused by the earlier higher discharge Slide10
Variable-discharge tests
Confined Aquifers,
Birsoy
-Summer’s Method
•Determine the slope of the straight lineSlide11
Confined Aquifers, Aron-Scott’s Method
Variable-discharge tests
If the fully developed drawdown is considered to extend to the distance
r
i
at which
then the s
e
(excess drawdown) can be approximated by:Slide12
Assumptions:All same as in chapter 3, except:5.)The discharge rate decreases with time, the sharpest decrease occurring soon after pumpingthe following condition is added:
Variable-discharge tests
Confined Aquifers,
Aron
-Scott’s MethodSlide13
Free-Flowing WellsBased on the conditions that the drawdown in the well is constant and discharge decreases with time.To satisfy these conditions, the well is shut until pressure becomes static, then at t=0 the well is opened and the water level in the well drops instantaneously to a constant drawdown level which is equal to the outflow. The well discharges at a decreasing rate.Slide14
Confined aquifer, unsteady-state flow, Hantush’s Methodwhere
Free-flowing wellsSlide15
AssumptionsAll same as in chapter 3, except:5.)At the start of the test (t=0), the water level in the free-flowing well drops instantaneously. At t>0, the drawdown in the well is constant, and its discharge is variable.The following condition is added:the flow to the well is in an unsteady state
Free-flowing wells
Confined aquifer, unsteady-state flow,
Hantush’s
MethodSlide16
Leaky aquifer, steady-state flow, Hantush-DeGlee’s method
where
Free-flowing wellsSlide17
AssumptionsAll assumptions that underlie the standardmethods for leaky aquifers, except:5.)At the beginning of the test (t=0), the water level in the well drops instantaneously. At t>0, the drawdown in the well is constant, and it’s discharge is variable. Slide18
Well-fields