Groundwater Science, James Beach
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Transcript of Groundwater Science, James Beach
Groundwater Science Aquifers
Wellsetc.etc.
James Beach, PGLBG-Guyton Associates
Special thanks to Robert Mace of TWDB Special thanks to Robert Mace of TWDB
for many slides
Outline
• big picture
• technical definitions (sorta)
• flow in aquifers• flow in aquifers
• pumping an aquifer
• miscellaneous important stuff
Groundwater in the World
From Freeze and Cherry (1979)
Major aquifers
Minor aquifers
Texas groundwater factoids
• 60% of the water used in Texas
• 81% of groundwater is used for irrigation
• groundwater provides 39% of water to cities
groundwater use in Texas (1937 to 2003)
TECHNICAL DEFINITIONS
Definitions
• Aquifer
• Aquitard/confining layer
• Vadose zone/unsaturated zone
• Water table• Water table
• Recharge
• Water level
• Unconfined aquifer
• Confined aquifer
• an aquifer is geologic media that can yield economically usable amounts of
what is an aquifer?
yield economically usable amounts of water.
Porous Media
Hickory Aquifer, sandstone
Edwards-Trinity (Plateau) Aquifer, limestone
Ogallala Aquifer, sand and gravel
• an aquitard is geologic media that can not yield economically usable amounts of water.
what is an aquitard?
of water.
• A confining layer is an aquitard that bounds an aquifer.
what is a confining layer?
the unsaturated zone
Unsaturated zone
the water table
Unsaturated zone
recharge
recharge – typical
recharge – not typical
the water level
water flows down gradient (to lower potential energy)
Groundwater Flowpaths (TOP view)
Groundwater Flowpaths (SIDE view)
Flat Ground
With Hills and Topography
RegionalSprings
LocalSpring
• An unconfined aquifer is an aquifer that is bounded by a confining layer at its bottom but not at its top.
unconfined aquifer
an unconfined aquifer
confined (or “artesian”) aquifer
Flowing “artesian” well
Catfish Farm Well Flowed 24,000 gpm
July, 1991
catfish farm well
Edwards aquifer
• flowing well at 40,000 gpm
• 1/4 of San Antonio’s use
• 9% of Annual Recharge
• world’s largest artesian well
National Geographic (1993)
confined or unconfined?
confined or unconfined?
same aquifer: unconfined and confined
Major aquifers
same location: confined and unconfined aquifers
Confined/Dipping Aquifer
Warning:Not to Scale
Confined/Dipping Aquifer
Edwards
Gulf Coast
Carrizo-Wilcox
Carrizo Aquifer
Leaky Aquifers
Water Table is Higher
Semi-Confined water level is lower
Why do you care?
Unconfined Confined
FLOW IN AQUIFERS
fundamentals
• Terms that describe the ease with which water flows through geologic
what is permeability?
hydraulic conductivity ?transmissivity?
which water flows through geologic media.
What determines flow?
• Hydraulic properties
Flow = KA dh / dl
• Hydraulic properties
• Water level variation
• Aquifer geometry
Edwards Group
Upper Trinity aquifer
Middle Trinity aquiferGuadalupe
River
SE model boundary
A2400
2200
2000
1800
1600
1400
Trinity cross-section
Middle Trinity aquifer River
CanyonLake
Edwards aquifer(BFZ)
No flow
No flowA’
1200
1000
800
600
400
200
sea level
Spring flow
0 5 10 15 mi
Recharge
Cross-formational flow
Surface water-groundwater interaction
Groundwater flow
DrainPumping
water flows down gradient (to lower potential energy)
PUMPING AN AQUIFER
pumping a well: unconfined
pumping a well: confined
Aquifer may not be
depleted
Pressure is decreased
• Storage properties describe the amount of water released from an aquifer when water level drops
what is storativity?
• Confined : “storativity”
• Unconfined : “specific yield”
• Unconfined aquifers0.01 to 0.3
Storage yields from aquifers
• Confined aquifers0.005 to 0.00005
Simple regional flow (sand)
Simple regional flow + well
Drawdown caused by well
Drawdown caused by well
Groundwater movement to well
After 50 years
Groundwater movement to well
After 100 years
MISCELLANEOUS IMPORTANT
STUFF
Miscellaneous
• spring and stream impact
• droughts and groundwater
• long-term impacts
• water quality • water quality
Springflow ImpactEdwards Aquifer
High correlation
• Observed springflow increase increase following several wet winter months
Outcrop Well
Page 71
Pumping Well
Middle Wilcox orSimsboro Aquifer
Outcrop
Outcrop Well
Outcrop and Downdip Hydrographsin Brazos and Robertson Counties
De
pth
to
Wa
ter
(fe
et)
Page 72
State Well Number, Screened Interval
Pumping WellDe
pth
to
Wa
ter
(fe
et)
Total water leveldeclines in themajor aquifers
800
400
600
Water level declines
-200
0
200
1880 200019601920
Year
Woodbine
Paluxy
TrinityLower
Water Levels in Carrizo Wells0
50
100
150
200
Dep
th t
o W
ate
r (f
eet)
Static Water Levels in Carrizo Aquifer Wells (Angelina and Nacogdoches Counties)
250
300
350
400
Jan
-45
Jan
-47
Jan
-49
Jan
-51
Jan
-53
Jan
-55
Jan
-57
Jan
-59
Jan
-61
Jan
-63
Jan
-65
Jan
-67
Jan
-69
Jan
-71
Jan
-73
Jan
-75
Jan
-77
Jan
-79
Jan
-81
Jan
-83
Jan
-85
Jan
-87
Jan
-89
Jan
-91
Jan
-93
Jan
-95
Jan
-97
Jan
-99
Dep
th t
o W
ate
r (f
eet)
Water Quality
~40,000 wellsDeep saline units not included
Water Quality
Edwards Aquifer
Drought Sensitivity - Major
Minor
Drought and Groundwater
• Most aquifers are drought tolerant
• Higher than usual water level declines in drought can be caused by increaseddrought can be caused by increasedpumping
• Important to discern between local short-term and regional long-term impacts
Take Home
1. Laws of physics are constant – however– geology, aquifers and groundwater can vary significantly over short distances
2. Good science is critical for developing appropriate rules and management
3. Good science takes time and money