4 - Aquifers - University of Iowauser.engineering.uiowa.edu/~gw102/aquifers.pdf · 1 Aquifers Names based on geologic time period formed • Precambrian: 4 billion years 10.5 months

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Aquifers Names based on geologic time period formed

• 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

Geologic Time

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

http://www.igsb.uiowa.edu

Iowa Groundwater Information

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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 aquiferMaquoketa aquiclude*Cambrian-Ordovician aquiferSt. Lawrence aquiferJordan sandstone aquiferDresbach 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 givenlayer 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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S = Ss b

T = K b

Storativity

Transmissivity

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”

Specific Yield (Sy) - ratio of volume of water that drains by gravity tototal volume of rock in an unconfined aquifer.

Second – unconfined aquifer – partly saturated – now WT moves!

Specific yield known as “water table” or “phreatic” storage

Specific Retention (Sr) - ratio of volume of water retained tototal volume of rock in an unconfined aquifer.

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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 gof 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?