Download Aquifers • Precambrian: 4 billion years 10.5 months • Paleozoic: 540

Aquifers
Names based on geologic time period formed
Geologic Time
• Precambrian: 4 billion years
10.5 months
• Paleozoic: 540 – 250 million years
1 month
• Mesozoic: 250 – 65 million years
2 weeks
• Cenozoic: 65 million years
1 week
Devonian Fossil Gorge
“When the 1993 flood abated, the eroded
Gorge surface revealed a succession of
375 million year old bedding planes with
diverse and abundant fossils commonly
standing out in relief.”
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U.S.G.S Groundwater ATLAS
of the United States
http://capp.water.usgs.gov/gwa/
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Major Types of Aquifers
1.
unconsolidated sediments
2. semiconsolidated sediments
3.
sandstone / carbonate aquifers
4.
basaltic and other volcanic rock aquifers
Some Example Aquifers
• The High Plains Aquifer, Midwest U.S.
• New Jersey Coastal Plain, East Coast U.S.
• The Dakota Sandstone Aquifer, Midwest U.S.
• Columbia Plateau Basalts, NW U.S.
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High Plains Aquifer
Ogallala Aquifer
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Cambrian-Ordovician Aquifer
Iowa Groundwater Information
http://www.igsb.uiowa.edu
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Bedrock Geologic Map of Iowa
Silurian-Devonian Aquifer
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Surficial Geologic Map of Iowa (Principal Aquifers of Iowa)
Iowa City Aquifers
*Glacial Drift aquifer
*Silurian-Devonian aquifer
Maquoketa aquiclude
*Cambrian-Ordovician aquifer
St. Lawrence aquifer
Jordan sandstone aquifer
Dresbach aquifer
Two Important Aquifer Characteristics
1. to store ground water, and
2. to transmit water to well.
Transmissivity: measure of how easily water moves through an
aquifer. Similar to K, but applies over the thickness of a given
layer in the aquifer.
Ti = biKi
Storativity: volume of water released or taken into an aquifer per unit
head change per unit surface.
S
=
volume of water
(unit area)(unit head change)
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Storativity
S = Ss b
Transmissivity
T = Kb
Water Storage is Different in Unconfined and Confined Aquifer
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Why? First – Saturated conditions – Confined aquifer
1 - fluid compressibility, βw
2 - compression of porous media, βp
Combined known as “elastic storage”
Second – unconfined aquifer – partly saturated – now WT moves!
Specific Yield (Sy) - ratio of volume of water that drains by gravity to
total volume of rock in an unconfined aquifer.
Specific Retention (Sr) - ratio of volume of water retained to
total volume of rock in an unconfined aquifer.
Specific yield known as “water table” or “phreatic” storage
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Confined Aquifer
Water released estimated using specific storage
Ss = ρg (βp + nβw)
Unconfined Aquifer
Water released estimated using specific storage AND specific yield
S = Sy + bSs
Water Storage is Different in Unconfined and Confined Aquifer
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Example – Water Storage in Unconfined Aquifer
A glass funnel filled with a volume of 400 cm3 is filled with 763.2 g
of soil (the soil solids have a density of 2.65 g/cm3). The soil is then saturated with
water. Following saturation, the water is allowed to drain under gravity from the soil.
The amount of water that has drained is 60 g. What is the porosity, specific yield, and
specific retention of the sample?
The aquifer underlies an area of 300 sq. miles and has a saturated thickness of 20 ft.
Estimate how much water is in the aquifer in gallons and cubic ft. What is the total amount
of obtainable water in gallons? If the water table in the aquifer was uniformly lowered over
its entire area by 5 ft, how much water would the aquifer have yielded?
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Example: Water Storage in a Confined Aquifer - Dakota Sandstone
1. Volume of water from fluid expansion?
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2. Volume of water from compression of porous media?
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