Ken Rainwater, Department of Civil, Environmental, & Construction Engineering, Texas Tech University

1. Groundwater Hydrology 101
Ken Rainwater, Ph.D., P.E., BCEE, D.WRE,
CFM
Department of Civil, Environmental, and
Construction Engineering
Texas Tech University 1

2. Subsurface Layer Terms
 Aquifer – geologic layer that holds water
and allows flow to wells
 Unconfined – bounded below only
 Confined – bounded above and below
 Aquitard – layer with low permeability
that allows very little flow
 Aquiclude – layer with no permeability
2

3. 3

4. Aquifer Parameters
 Hydraulic conductivity – K (L/t)
 Measure of permeability of geologic
material to water
 Porosity – n
 Void volume divided by total volume
 Storage coefficient – S or Sy
 Amount of water released per unit decline
in head (larger for unconfined aquifer)
 Saturated thickness – b (L) 4

5. Hydraulic Head
 Unconfined – water table
 Aquifer base may be datum
 Saturated thickness changes
 Confined – well water level
 Datum is usually sea level
 Saturated thickness constant
5
elevationheadpressureh +=
http://pubs.usgs.gov/circ/circ1217/html/boxa.html

6. Head Gradient
6
-0.0057
m350
m2-
dx
dh
==
http://studentweb.usq.edu.au/home/w0028223/webdes/html/gwatere.htm
x

7. Darcy’s Law
7
alsodirectionszy,incomponentsVelocity
directionxintyconductivihydraulicK
(L)directionhorizontalx
(L/t)poresinvelocityactualv
dx
dh
n
K
v
x
x
x
x
−=
=
=
−=

8. Well
Drawdown
8
 Pump lowers
head at pumping
well
 3-D head gradient
to pumping well
 “Drawdown cone”

9. Better Aquifer Definition
 Geologic stratum or layer that
 holds groundwater within saturated thickness, b
 has sufficient hydraulic conductivity, K, for flow
 has enough transmissivity, T = Kb, for useful
production at pumping wells
9

10. Regional Aquifer Flow

11. Moderate Pumping

12. Long-term Depletion

13. Projections of Aquifer Life
 Some aquifers are being mined
 Withdrawals exceed recharge, lateral flow
 Groundwater owners choose to pump
 Center pivot requirements

400-1200 gpm for quarter-mile

1200-2500 gpm for half-mile
 Municipal supply requirements
 When is aquifer too thin? 30 ft, 50 ft?
13

14. Estimating Recharge
 Historical regional estimates
 Model calibrations
 Difficult under irrigated lands
 Must know withdrawals
 Irrigation return flow
 Geochemical estimates
14

15. Long-term Drawdown
15
 Effects of single pumping well
 Initial saturated thickness, ho
to
Q
ho

16. Long-term Drawdown
16
 Effects of single pumping well
 Initial saturated thickness, ho
 Transient drawdown increases
to
t1
Q
ho

17. to
t1
t2
Q
ho
ro
Long-term Drawdown
17
 Effects of single pumping well
 Stabilizes within radius of influence, ro
 Can “break suction” if drawdown > ho

18. Drawdown Calculation
 Equilibrium for unconfined aquifer
 Drawdown, s
 Pumping well radius, rw
 Flow rate, Q
 Hydraulic conductivity, K
 ro, ho
18
( )
21
w
o2
oow
r
r
ln
πK
Q
hhrs 











−−=

19. Simplifying Assumptions
 Horizontal aquifer base
 Initially horizontal water table
 Homogeneous K, specific yield, Sy
 Constant Q
 No aquifer boundaries, recharge
 Single value of ro
19

20. Minimum ho for Q, K Values
 Assume ro = 1000 ft, rw = 1 ft
20
0
50
100
150
200
250
0 10 20 30 40 50 60
MinimumSaturatedThickness(ft)
Hydraulic Conductivity (ft/d)
500 gpm
350 gpm
240 gpm
100 gpm

21. Actual Conditions
 Aquifer base topography
 Water table gradient
 More precise shape with capture zone
analysis
 Heterogeneous K, specific yield, Sy
 Multiple wells
 Overlapping drawdown
 Different times of operation
21

22. More Complex Situations
 Computer modeling required
 Limited by size of simulated cells and
time steps
 Calibration challenges
22

23. Actions for Well Owner
 Understand aquifer behavior
 Pump tests for aquifer parameters
 Observe seasonal behavior
 Selection of irrigation rates and timing
 Crop choices, input costs
 How many wells are needed to provide
the demand?
 Costs of new wells, pump replacements
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24. Groundwater Issues
 Water quality variations
 Fresh, brackish
 Arsenic, fluoride, perchlorate, others
 Recharge rate determines renewability
 Fast (Edwards)
 Slow (Ogallala)
 Withdrawal > Recharge = Depletion

25. Nine Major Aquifers

26. 21 Minor Aquifers

28. GCD Challenges
 Education of Groundwater Owners
 Well interference, depletion, pumping distribution
 Changes in water stored
 Best practices for use and conservation
 Water marketing (agricultural vs. other users)
 GCDs share aquifers
 Concern for future conditions, regional planning
 Recharge, surface water interaction uncertainties
 Some areas not currently included in GCDs
 Groundwater management areas

30. Conservation District Actions
 Well permit requirements
 Well spacing (production/area)
 Annual limits on saturated thickness
change
 Production monitoring and reporting
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31. Opportunities for Groundwater
Scientists and Engineers
 Improve quantitative aquifer descriptions
 Water in storage
 Hydraulic parameters  well capacities
 Regional and local flow models
 Water quality variations
 Inform GCD rule-making process
 Stewardship of shared resources
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