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12/26/2012 1
Chapter 7
Civil Engineering Department
Prof. Majed Abu-Zreig
Hydraulics and Hydrology – CE 352
Groundwater Hydraulics
Hydrologic cycle
Occurrence of Ground Water
• Ground water occurs when water recharges a porous subsurface geological formation “called aquifers” through cracks and pores in soil and rock
• it is the water below the water table where all of the pore spaces are filled with water.
• The area above the water table where the pore spaces are only partially filled with water is called the capillary fringe or unsaturated zone.
• Shallow water level is called the water table
Groundwater Basics -
Definitions
Recharge
Natural
• Precipitation
• Melting snow
• Infiltration by streams and lakes
Artificial • Recharge wells
• Water spread over land in pits, furrows, ditches
• Small dams in stream channels to detain and deflect water
Aquifers
Definition: A geological unit which can store and supply significant quantities of water.
Principal aquifers by rock type:
Unconsolidated
Sandstone
Sandstone and Carbonate
Semiconsolidated
Carbonate-rock
Volcanic
Other rocks
Example Layered Aquifer System
Bedient et al., 1999.
Other Aquifer Features
Groundwater occurrence in confined and
unconfined aquifer
Potentiometric Surfaces
Eastern Aquifer
Growndwater
basins
in Jordan
Unconfined Aquifers
• GW occurring in aquifers: water fills partly an
aquifer: upper surface free to rise and decline:
UNCONFINED or water-table aquifer: unsaturated
or vadose zone
• Near surface material not saturated
• Water table: at zero gage pressure: separates saturated
and unsaturated zones: free surface rise of water in a
well
Confined Aquifer
• Artesian condition
• Permeable material overlain by relatively
impermeable material
• Piezometric or potentiometric surface
• Water level in the piezometer is a measure of
water pressure in the aquifer
Groundwater Basics -
Definitions • Aquifer Confining Layer or Aquitard
– A layer of relatively impermeable material which restricts vertical
water movement from an aquifer located above or below.
– Typically clay or unfractured bedrock.
Aquifer Characteristics
• Porosity
– The ratio of pore/void volume
to total volume, i.e. space
available for occupation by air
or water.
– Measured by taking a known
volume of material and adding
water.
– Usually expressed in units of
percent.
– Typical values for gravel are
25% to 45%.
Bedient et al., 1999.,
Typical Values of Porosity
Aquifer Properties • Porosity: maximum amount of water that a rock
can contain when saturated.
• Permeability: Ease with which water will flow through a porous material
• Specific Yield: Portion of the GW: draining under influence of gravity:
• Specific Retention: Portion of the GW: retained as a film on rock surfaces and in very small openings:
• Storativity: Portion of the GW: draining when the piezometric head dropped a unit depth
Figures from Hornberger et al. (1998)
Unconfined aquifer
Specific yield = Sy
Confined aquifer
Storativity = S
b
h h
Storage Terms
S = V / A h
S = Ss b
Ss = specific storage
Aquifer Characteristics
• Hydraulic Conductivity – Measure of the ease with which water can flow through an
aquifer.
– Higher conductivity means more water flows through an
aquifer at the same hydraulic gradient.
– Measured by well draw down or lab test.
– Expressed in units of mm/day, ft/day or gpd/ft2.
– Typical values for sand/gravel are 2.5 cm/day to 33 m/day
m1 (1 to 100 ft/day).
– Typical values for clay are 0.3 mm/day (0.001 ft/day). That
is why is is an aquifer confining layer.
• Transmissivity (T = Kb) is the rate of flow through a
vertical strip of aquifer (thickness b) of unit width
under a unit hydraulic gradient
•
Aquifer Characteristics
• Hydraulic gradient – Steepness of the slope of the water table.
– Groundwater flows from higher elevations to lower elevations
(i.e. downgradient).
– Measured by taking the difference in elevation between two
wells and dividing by the distance separating them.
– Expressed in units of ft/ft or ft/mi.
– Typical values for groundwater are .0001 to .01 m/m.
Aquifer Characteristics
• Groundwater Velocity – How fast groundwater is moving.
– Calculated by conductivity multiplied by gradient divided by
porosity.
– Expressed in units of ft/day.
– Typical values for gravel or sand are 0.15 to 16 m/day (1 to 50
ft/day).
• Water table: the
surface separating
the vadose zone
from the saturated
zone.
• Measured using
water level in well
The Water Table
Fig. 11.1
• Precipitation
• Infiltration
• Ground-water recharge
• Ground-water flow
• Ground-water discharge to
– Springs
– Streams and
– Wells
Ground-Water Flow
• Velocity is
proportional to
– Permeability
– Slope of the water
table
• Inversely
Proportional to
– porosity
Ground-Water Flow
Fast (e.g., cm per day)
Slow (e.g., mm per day)
• Infiltration
– Recharges ground
water
– Raises water table
– Provides water to
springs, streams
and wells
• Reduction of
infiltration causes
water table to drop
Natural Water
Table Fluctuations
• Reduction of infiltration causes water table to drop
– Wells go dry
– Springs go dry
– Discharge of rivers drops
• Artificial causes
– Pavement
– Drainage
Natural Water
Table Fluctuations
• Pumping wells
– Accelerates flow
near well
– May reverse
ground-water flow
– Causes water table
drawdown
– Forms a cone of
depression
Effects of
Pumping Wells
• Pumping wells
– Accelerate flow
– Reverse flow
– Cause water
table drawdown
– Form cones of
depression Low river
Gaining
Stream
Gaining
Stream
Pumping well
Low well
Low well
Cone of
Depression
Water Table
Drawdown
Dry Spring
Effects of
Pumping Wells
Dry river
Dry well
Effects of
Pumping Wells
Dry well
Dry well
Losing
Stream • Continued water-
table drawdown
– May dry up
springs and wells
– May reverse flow
of rivers (and
may contaminate
aquifer)
– May dry up rivers
and wetlands
Ground-Water/
Surface-Water
Interactions
• Gaining streams
– Humid regions
– Wet season
• Loosing streams
– Humid regions, smaller
streams, dry season
– Arid regions
• Dry stream bed
Figure taken from Hornberger et al. (1998)
Darcy column
h/L = grad h
q = Q/A
Q is proportional
to grad h
x
hAKQ
x
hAQ
Darcy’s Law Henry Darcy’s Experiment (Dijon, France 1856)
AQxQhQ ,1,
x
hAKQ
x
hAQ
Q
Q: Volumetric flow rate [L3/T]
Darcy investigated ground water flow under controlled conditions
h
h1 h2
h
x
h1
Slope = h/x
~ dh/dx h
x h2
x1 x2
K: The proportionality constant is added to form the following equation:
K units [L/T]
A
: Hydraulic Gradient xhh
A: Cross Sectional Area (Perp. to flow)
Calculating Velocity with Darcy’s
Law • Q= Vw/t
– Q: volumetric flow rate in m3/sec
– Vw: Is the volume of water passing through area “a” during
– t: the period of measurement (or unit time).
• Q= Vw/t = H∙W∙D/t = a∙v
– a: the area available to flow
– D: the distance traveled during t
– v : Average linear velocity
• In a porous medium: a = A∙n
– A: cross sectional area (perpendicular to flow)
– n: porous For media of porosity
• Q = A∙n∙v
• v = Q/(n∙A)=q/n
Vw
v
x
h
n
Kv
Darcy’s Law (cont.)
• Other useful forms of Darcy’s Law
dx
dhKq
Q
A =
Q
A.n = q
n = dx
dh
n
Kv
Volumetric Flux (a.k.a. Darcy Flux or
Specific discharge)
Ave. Linear
Velocity
Used for calculating
Q given A
Used for calculating
average velocity of
groundwater transport
(e.g., contaminant
transport Assumptions: Laminar, saturated flow
dx
dhAKQ Volumetric Flow Rate
Used for calculating
Volumes of groundwater
flowing during period of
time
Figure from Hornberger et al. (1998)
Linear flow
paths assumed
in Darcy’s law
True flow paths
Average linear velocity
v = Q/An= q/n
n = effective porosity
Specific discharge
q = Q/A
Steady Flow to Wells in Confined Aquifers
• Radial flow towered wells
• Aquifers are homogeneous (properties are uniform)
• Aquifers are isotropic (permeability is independent of flow direction)
• Drawdown is the vertical distance measured from the original to the lowered water table due to pumping
• Cone of depression the axismmetric drawdown curve forms a conic geometry
• Area of influence is the outer limit of the cone of depression
• Radius of Influence (ro) for a well is the maximum horizontal extent of the cone of depression when the well is in equilibrium with inflows
• Steady state is when the cone of depression does not change with time
Horizontal and Vertical Head Gradients
Freeze and Cherry, 1979.
Flow to Wells
Steady Radial Flow to a Well-
Confined
Q
Cone of Depression
s = drawdown
h r
Steady Radial Flow to a Well-
Confined
• In a confined aquifer, the drawdown
curve or cone of depression varies with
distance from a pumping well.
• For horizontal flow, Q at any radius r
equals, from Darcy’s law,
Q = -2πrbK dh/dr
for steady radial flow to
a well where Q,b,K are
const
Steady Radial Flow to a Well-
Confined • Integrating after separation of variables, with
h = hw at r = rw at the well, yields Thiem Eqn
Q = 2πKb[(h-hw)/(ln(r/rw ))]
Note, h increases
indefinitely with
increasing r, yet
the maximum head
is h0.
Steady Radial Flow to a Well-
Confined
• Near the well, transmissivity, T, may be
estimated by observing heads h1 and h2
at two adjacent observation wells
located at r1 and r2, respectively, from
the pumping well
T = Kb = Q ln(r2 / r1)
2π(h2 - h1)
Steady Radial Flow to a Well-
Unconfined
Steady Radial Flow to a Well-
Unconfined
• Using Dupuit’s assumptions and applying Darcy’s law
for radial flow in an unconfined, homogeneous,
isotropic, and horizontal aquifer yields:
Q = -2πKh dh/dr
integrating,
Q = πK[(h22 - h1
2)/ln(r2/ r1)
solving for K,
K = [Q/π(h22 - h1
2)]ln (r2/ r1)
where heads h1 and h2 are observed at adjacent
wells located distances r1 and r2 from the pumping
well respectively.
Steady Flow to a Well in a Confined
Aquifer
2rw
Ground surface
Bedrock
Confined
aquifer
Q
h0
Pre-pumping
head
Confining Layer
b
r1
r2
h2
h1
hw
Observation
wells
Drawdown curve
Q
Pumping
well
Q = Aq = (2prb)Kdh
dr
rdh
dr=
Q
2pT
h2 = h1 +Q
2pTln(
r2r1
)
Theim Equation
In terms of head (we can write it in terms of drawdown also)
Example - Theim Equation
• Q = 400 m3/hr
• b = 40 m.
• Two observation wells,
1. r1 = 25 m; h1 = 85.3 m
2. r2 = 75 m; h2 = 89.6 m
• Find: Transmissivity (T)
T =Q
2p h2 - h1( )ln
r2r1
æ
è ç
ö
ø ÷ =
400 m3/hr
2p 89.6 m - 85.3m( )ln
75 m
25 m
æ
è ç
ö
ø ÷ =16.3 m2 /hr
h2 = h1 +Q
2pTln(
r2r1
)2rw
Ground surface
Bedrock
Confine
d
aquifer
Q
h0
Confining Layer
b
r1
r2
h2 h1
hw
Q
Pumping
well
Steady Flow to a Well in a Confined Aquifer
Steady Radial Flow in a Confined
Aquifer
• Head
• Drawdown
h r( ) = h0 +Q
2pTln
r
R
æ
è ç
ö
ø ÷
s r( ) =Q
2pTln
R
r
æ
è ç
ö
ø ÷
s(r) = h0 - h r( )
Steady Flow to a Well in a Confined Aquifer
Theim Equation
In terms of drawdown (we can write it in terms of head also)
Example - Theim Equation
• 1-m diameter well
• Q = 113 m3/hr
• b = 30 m
• h0= 40 m
• Two observation wells, 1. r1 = 15 m; h1 = 38.2 m
2. r2 = 50 m; h2 = 39.5 m
• Find: Head and drawdown in the well
2rw
Ground surface
Bedrock
Confine
d
aquifer
Q
h0
Confining Layer
b
r1
r2
h2 h1
hw
Q
Pumping
well Drawdown
Adapted from Todd and Mays, Groundwater Hydrology
T =Q
2p s1 - s2( )ln
r2r1
æ
è ç
ö
ø ÷ =
113m3/hr
2p 1.8 m - 0.5 m( )ln
50 m
15 m
æ
è ç
ö
ø ÷ =16.66 m2 /hr
s r( ) =Q
2pTln
R
r
æ
è ç
ö
ø ÷
Steady Flow to a Well in a Confined Aquifer
Example - Theim Equation
2rw
Ground surface
Bedrock
Confine
d
aquifer
Q
h0
Confining Layer
b
r1
r2
h2 h1
hw
Q
Drawdown
@ well
Adapted from Todd and Mays, Groundwater Hydrology
hw = h2 +Q
2pTln(
rwr2
) = 39.5 m +113m3 /hr
2p *16.66 m2 /hrln(
0.5 m
50 m) = 34.5 m
sw = h0 - hw = 40 m- 34.5 m = 5.5 m
h2 = h1 +Q
2pTln(
r2r1
)
Steady Flow to a Well in a Confined Aquifer
Steady Flow to Wells in
Unconfined Aquifers
Steady Flow to a Well in an Unconfined
Aquifer
Q = Aq = (2prh)Kdh
dr
= prKdh2
dr
2rw
Ground surface
Bedrock
Unconfined
aquifer
Q
h0
Pre-pumping
Water level
r1
r2
h2 h1
hw
Observation
wells
Water Table
Q
Pumping
well
rd h2( )
dr=
Q
pK
h02 - h2 =
Q
pKln
R
r
æ
è ç
ö
ø ÷
h2(r) = h02 +
Q
pKln
r
R
æ
è ç
ö
ø ÷
h2 = h1 +Q
2pTln(
r2r1
)
Confined aquifer Unconfined aquifer
Steady Flow to a Well in an Unconfined
Aquifer
2rw
Ground surface
Bedrock
Unconfined
aquifer
Q
h0
Prepumping
Water level
r1
r2
h2 h1
hw
Observation
wells
Water Table
Q
Pumping
well
2 observation wells: h1 m @ r1 m h2 m @ r2 m
K =Q
p h22 - h1
2( )ln
r2r1
æ
è ç
ö
ø ÷
h2(r) = h02 +
Q
pKln
r
R
æ
è ç
ö
ø ÷
h22 = h1
2 +Q
pKln
r2r1
æ
è ç
ö
ø ÷
• Given:
– Q = 300 m3/hr
– Unconfined aquifer
– 2 observation wells,
• r1 = 50 m, h = 40 m
• r2 = 100 m, h = 43 m
• Find: K
K =Q
p h22 - h1
2( )ln
r2r1
æ
è ç
ö
ø ÷ =
300 m3 /hr / 3600 s /hr
p (43m)2 - (40 m)2[ ]ln
100 m
50 m
æ
è ç
ö
ø ÷ = 7.3x10-5 m /sec
Example – Two Observation Wells in an
Unconfined Aquifer
2rw
Ground surface
Bedrock
Unconfined
aquifer
Q
h0
Prepumping
Water level
r1
r2
h2 h1
hw
Observation
wells
Water Table
Q
Pumping
well
Steady Flow to a Well in an Unconfined Aquifer
Pump Test in Confined
Aquifers
Jacob Method
Cooper-Jacob Method of Solution
Cooper and Jacob noted that for small values of r
and large values of t, the parameter u = r2S/4Tt
becomes very small so that the infinite series can be
approx. by: W(u) = – 0.5772 – ln(u) (neglect higher terms)
Thus s' = (Q/4πT) [– 0.5772 – ln(r2S/4Tt)]
Further rearrangement and conversion to decimal logs yields:
s' = (2.3Q/4πT) log[(2.25Tt)/ (r2S)]
Cooper-Jacob Method of Solution
A plot of drawdown s' vs.
log of t forms a straight line
as seen in adjacent figure.
A projection of the line back
to s' = 0, where t = t0 yields
the following relation:
0 = (2.3Q/4πT) log[(2.25Tt0)/ (r2S)]
Semi-log plot
Cooper-Jacob Method of Solution
Cooper-Jacob Method of Solution
So, since log(1) = 0, rearrangement yields
S = 2.25Tt0 /r2
Replacing s' by s', where s' is the drawdown
difference per unit log cycle of t:
T = 2.3Q/4πs'
The Cooper-Jacob method first solves for T and
then for S and is only applicable for small
values of u < 0.01
Cooper-Jacob Example
For the data given in the Fig.
t0 = 1.6 min and s’ = 0.65 m
Q = 0.2 m3/sec and r = 100 m
Thus:
T = 2.3Q/4πs’ = 5.63 x 10-2 m2/sec
T = 4864 m2/sec
Finally, S = 2.25Tt0 /r2
and S = 1.22 x 10-3
Indicating a confined aquifer
Jacob Approximation
• Drawdown, s
• Well Function, W(u)
• Series
approximation of
W(u)
• Approximation of s
s u( ) =Q
4pTW u( )
W u( ) =e-h
hu
¥
ò dh » -0.5772 - ln(u)+ u -u2
2!+
u =r2S
4Tt
W u( ) » -0.5772 - ln(u) for small u < 0.01
s(r,t) »Q
4pT-0.5772 - ln
r2S
4Tt
æ
è ç
ö
ø ÷
é
ë ê ê
ù
û ú ú
s(r,t) =2.3Q
4pTlog10(
2.25Tt
r2S)
Pump Test Analysis – Jacob Method
Jacob Approximation
s =2.3Q
4pTlog(
2.25Tt
r2S)
0 =2.3Q
4pTlog(
2.25Tt0
r2S)
t0
1=2.25Tt0
r2S
S =2.25Tt0
r2
Pump Test Analysis – Jacob Method
Jacob Approximation
t0
S =2.25Tt0
r2
t1 t2
s1
s2
s
logt2
t1
æ
è ç
ö
ø ÷ = log
10* t1t1
æ
è ç
ö
ø ÷ =1
1 LOG CYCLE
1 LOG CYCLE
Pump Test Analysis – Jacob Method
Jacob Approximation
S =2.25Tt0
r2=
2.25(76.26 m2/hr)(8 min*1 hr /60 min)
(1000 m)2
= 2.29x10-5
t0
t1 t2
s1
s2
s
t0 = 8 min
s2 = 5 m s1 = 2.6 m s = 2.4 m
Pump Test Analysis – Jacob Method
Multiple-Well Systems
• For multiple wells with drawdowns that overlap, the principle of superposition may be used for governing flows:
• drawdowns at any point in the area of influence of several pumping wells is equal to the sum of drawdowns from each well in a confined aquifer
Multiple-Well Systems
Injection-Pumping Pair of Wells
Pump Inject
Multiple-Well Systems
• The previously mentioned principles also
apply for well flow near a boundary
• Image wells placed on the other side of the
boundary at a distance xw can be used to
represent the equivalent hydraulic condition
– The use of image wells allows an aquifer of
finite extent to be transformed into an
infinite aquifer so that closed-form solution
methods can be applied
Multiple-Well Systems
•A flow net for a pumping
well and a recharging
image well
-indicates a line of
constant head
between the two wells
Three-Wells Pumping
A
Total Drawdown at A is sum of drawdowns from each well
Q1
Q3
Q2
r
Multiple-Well Systems
The steady-state drawdown
s' at any point (x,y) is given
by:
s’ = (Q/4πT)ln
where (±xw,yw) are the
locations of the recharge and
discharge wells. For this
case, yw= 0.
(x + xw)2 + (y - yw)2
(x - xw)2 + (y - yw)2
Multiple-Well Systems
The steady-state drawdown s' at any point (x,y) is given by
s’ = (Q/4πT)[ ln {(x + xw)2 + y2} – ln {(x – xw)2 + y2} ]
where the positive term is for the pumping well and the
negative term is for the injection well. In terms of head,
h = (Q/4πT)[ ln {(x – xw)2 + y2} – ln {(x + xw)2 + y2 }] + H
Where H is the background head value before pumping.
Note how the signs reverse since s’ = H – h
7.5 Aquifer Boundaries
The same principle
applies for well
flow near a
boundary
– Example:
pumping near a
fixed head stream
well near an impermeable boundary