Chapter 5 Fetter Ground water flow to wells

Ground water Flow To Wells

1. GROUND WATER FLOW TO WELLS
PREPARED BY :
RASHEED MATTER
JAMILA TIMRAZ
Al-Azhar University-Gaza
Master Program of Water and Environmental
Science

2. INTRODUCTION :-
A WATER WELL IS A HOLE CREATED IN THE GROUND, BY
DIGGING, OR DRILLING IN TO AN AQUIFER WITH A PIPE , A SCREEN
AND PUMP TO PULL WATER OUT OF THE GROUND .

3. WATER WELLS ARE USED FOR :-
 EXTRACTION OF GROUND WATER
 CONTROL SALT WATER INTRUSION
REMOVE CONTAMINATED WATER
LOWER THE WATER TABLE FOR CONSTRUCTION PROJECT
RELIEVE PRESSURE UNDER THE DAMS
DRAIN FARM LAND
INJECT FLUID TO GROUND WATER
ARTIFICIALLY RECHARGE AQUIFER

4. PUMPING CON. (CON. OF
DEPRESSION )
THE AREA AROUND A DISCHARGING WELL WHERE
THE WATER LEVEL IN THE AQUIFER DROPS IN THE
SHAPE OF A CON. DUE TO PUMPING

5. HYDRAULIC OF WELLS :-
STATIC WATER LEVEL [SWL] (HO )
IS THE EQUILIBRIUM WATER LEVEL BEFORE PUMPING
COMMENCES
PUMPING WATER LEVEL [PWL] (H)
IS THE WATER LEVEL DURING PUMPING
DRAWDOWN (S = HO - H)
IS THE DIFFERENCE BETWEEN SWL AND PWL
WELL YIELD (Q)
IS THE VOLUME OF WATER PUMPED PER UNIT TIME
SPECIFIC CAPACITY (Q/S)
IS THE YIELD PER UNIT DRAWDOWN

6. BASIC ASSUMPTIONS
1.The aquifer is bounded on the bottom by a confining layer.
2.All geological formations are horizontal and of infinite horizontal extent.
3.The potentiometric surface of the aquifer is horizontal prior to the start
of the pumping.
4.The potentiometric surface of the aquifer is not changing with time prior
to the start of the pumping.
5. All changes in the position of the potentiometric surface are due to the
effect of the pumping well alone.
6.The aquifer is homogeneous and isotropic.

7. 7. All flow is radial toward the well.
8. Groundwater flow is horizontal.
9. Darcy’s law is valid.
10. Groundwater has a constant density and viscosity.
11. The pumping well and the observation wells are fully penetrating;
i.E., They are screened over the entire thickness of the aquifer.
12. The pumping well has an infinitesimal diameter and is 100%
efficient

8. Computing Drawdown Causing By A
pumping Well:
un steady radial flow:
the use of polar coordinates to describe the position of
appoint in aplane . it lies the distance r from the origin and
the angle between the polar axis and aline connecting the
point and the origin is

9. Two – dimensional flow in confined aquifer has
previously been derived as equation :
THE RESULT IS EQUATION IN RADIAL COORDINATES :

10. THE TWO – DIMENSIONAL EQUATION FOR CONFINED FLOW , IF
THERE IS RECHARGE TO THE AQUIFER , IS GIVEN BY EQUATION :
CAN BE TRANSFORMED INTO RADIAL COORDINATES BECOMING
•

11. The First Mathematical Analysis Of A transient
Draw Down Effect On Confined Aquifer Was
Published By C.V. Theis

12. UNSTEADY FLOW TO A WELL IN A
CONFINED AQUIFER

14. The Integral In Equation Is Called The Exponential
Integral . It Can Be Approximated By An Infinite
Series So The Theis Equation Becomes :
•Infinite Series Term Of Equation Has Been Called The Well Function
And Is Generally Designated As W (U( :

15. THEIS EQUATION - EXAMPLE
Q = 1500 M
3
/DAY, T = 600 M
2
/DAY S = 4 X 10
-4
FIND: DRAWDOWN 1 KM FROM WELL AFTER 1 YEAR
ANSWER:
FIRSTLY FIND WELL FUNCTION U, THEN W(U(
FIND W(U) FROM APPENDIX 1


u=
r
2
S
4Tt
=
(1000m)
2
(4x10
-4
)
4(600m
2
/d)(365d)
=4.6x10
-4
()uW
T
Q
hho
p4
=-
12.7)(=uW
m
dm
dm
uW
T
Q
hho 42.112.7*
)/600(4
/1500
)(
4
2
3
===-
pp

16. THEIS EQUATION - EXAMPLE WELL
FUNCTION

u=4.6x10
-4

W(u)=7.12

17. The Data Required For The Theis Solution Are :
1 Drawdown Vs. Time Data At An Observation Well,
2 Distance From The Pumping Well To The Observation Well,
3 Pumping Rate Of The Well.

18. The Non-equilibrium Reverse Type Curve (Theis
Curve) For A Fully Confined Aquifer
Theoretical Curve W(u) Versus 1/U Is Plotted On A Log-log Paper.
This Can Be Done Using Tabulated Values Of The Well Function
(See Appendix 1).

19. Field Data Plot On Logarithmic Paper For Theis
Curve-marching Technique :
The Field Measurements Are Similarly Plotted On A Log-log Plot With
(T) Along The X-axis and (Sw) along the Y-axis

20. MATCH OF FIELD DATA PLOT TO THEIS TYPE CURVE :
keeping the axes correctly aligned, superimposed the type curve on the
plot of the data (the data analysis is done by matching the observed
data to the type curve)
select any convenient point on the graph paper (a match point) and
read off the
coordinates of the point on both sets of axes. this gives coordinates
( 1/u, w(u)) and (t,sw) use the previous equations to determine t and s.

21. The log function lets us plot this as a straight line on semi-
log paper
Jacob method of solution of pumping-test data for a fully confined
aquifer. Drawdown is plotted as a function of time on semi-logarithmic
paper

22. Thiem equation for steady radial flow in confide
aquifer :
•In the case of steady radial flow in a confide aquifer , the following
assumptions are necessary :-
•1- the aquifer is confide top and bottom
•2- the well is pumped at a constant rate
•Equillibrium has been reached that is , there is no farther change in draw
down with time.
•T= kb = Q/ 2 P (s1-s2 ) * ln ( r2/r1)
•h0-h = q / 2 p t * ln ( r2/r1)

23. FLOW IN A LEAKY, CONFIDE AQUIFER :
FLOW EQUATION :
Most Confide Aquifers Are Not Totally Isolated From Sources Of
Vertical Recharge . Aquitards , rather Above Or Below The Aquifer ,
Can Leak Water Into The Aquifer If The Direction Of The Hydraulic
Gradient Is Favorable :-

25. WELL PUMPING FROM A LEAKY AQUIFER

26. Transient flow in a leaky , confide aquifer with no
storage :
Hantush infection – point method
Walton graphic method

28. Log-log plot for hantush method

29. Unsteady radial flow in an unconfined aquifer
(non-equilibrium radial flow)
•The flow of water in an unconfined aquifer toward a pumping well is described by the
following
Equation (neuman & witherspoon 1969)

30. Neumann's solution assumes the following, in addition to
the basic assumptions:
Flow description :
Initial flow : the Theis – like ( horizontal ) from release of compression ,
specific storage component
Intermediate flow :
has gravity drainage component ( both horizontal & vertical flow )
Late – time flow :
becomes horizontal again from specific yield components

31. Type curves of drawdown versus time illustrating the effect of delayed
yield for pumping tests in unconfined aquifers.

33. CROSS-SECTION OF A PUMPED UNCONFINED
AQUIFER

34. FLOW EQUATIONS :

36. Type curves for unconfined aquifers

37. RECOVERY TEST

38. DESIGN OF PUMPING TESTS :
PARAMETERS :
. Test well location, depth, capacity (unless existing well used).
. Observation well number, location, depth.
. Pump regime
GENERAL GUIDANCE :
•CONFINED AQUIFERS:
•Transmissivity more important than storativity: observation wells not
always needed (although accuracy lost without them!).
•Unconfined aquifers: storativity much larger, and has influence over
transmissivity estimates: observation wells important as is larger
test duration. Care needed if aquifer only partly screened

39. THE IMPORTANCE OF PUMPING TESTS
* Pumping tests are carried out to determine:
1- how much groundwater can be extracted from a well based on long-
term yield, and well efficiency?
2 - the hydraulic properties of an aquifer or aquifers.
3 - spatial effects of pumping on the aquifer.
4 - determine the suitable depth of pump.
5- information on water quality and its variability with time.

40. PUMPING TEST IN THE FIELD