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Ground Water g110. Lecture prepared by Bob Hall Revised 8/6/2000. Points to Be Covered on Ground Water. How is ground water formed? What are the zones of subsurface water and their characteristics? What is a water table, and how is it configured in humid vs arid climates?

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Ground water g110 l.jpg

Ground Waterg110

Lecture prepared by Bob Hall

Revised 8/6/2000


Points to be covered on ground water l.jpg
Points to Be Covered on Ground Water

  • How is ground water formed?

  • What are the zones of subsurface water and their characteristics?

  • What is a water table, and how is it configured in humid vs arid climates?

  • How does ground water move? What is porosity? permeability? Primary vs seondary p and p?


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Points to Be Covered (cont.)

  • What is an aquifer, and what kinds are there?

  • Be able to discuss the use of wells in ground water, including potential problems.

  • What is karst topography?


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Origin of Ground Water

  • When precipitation falls on the land surface, what happens to it?

    • Evaporates and/or is transpired back to atmosphere

    • runs over land surface

    • infiltrates the land surface

      and

    • percolates downward

      and

    • becomes ground water



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Zone of Aeration

pores filled with both air and water

Water held against gravity by surface tension

Soil water

Zone of Saturation

pores filled only with water

Water drained through soil under influence of gravity.

Ground Water

Zones of Subsurface Water



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What is the configuration of the water table?

Humid Area

Arid Area

R = Recharge Area

R = Recharge area



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Porosity: % by volume of an earth material that is pore space.

Primary porosity depends upon:

- shape of grains

- arrangement of grains

- size distribution

- compaction/cement’n

Permeability: ability of an earth material to transmit water

Depends upon

- porosity

- degree and size of interconnecting pores between larger pores

How does ground water move?


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What is secondary porosity and permeability?

  • Secondary porosity developed after the material was formed . Depends upon:

  • Primary porosity and permeability were created by the same processes that formed the material.

    - degree of fracturing

    - amount of solution (for limestone)


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What are some typical values of porosity and permeability?

  • Porosity

    clay 45-55 %

    sand 30-40

    sandstone 10-20

    shale 1-2

    limestone 1-10 (or larger)

  • Permeability: varies over several orders of magnitude. Expressed as a rate, e.g. ft/day


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What is Darcy’s Law?

An explanation of the factors determining the rate (velocity) of ground water flow.

V = K (h/l) where K = coefficient of

permeability

dh/dl = hydraulic gradient


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Configuration of Water Table

in a humid climate

Et


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How does the geology control the existence of ground water?

  • What is an aquifer?

    A permeable, water-containing unit.

    - Water enters from recharge.

    - Temporarily stored.

    - Leaves by flow to streams (baseflow) or springs, or to wells


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What is an unconfined aquifer?

  • They are not sealed

    off at any point.

  • Recharge can occur

    anywhere.

  • Water at w.table

    under atm pressure.

  • Must pump.



What is a confined or artesian aquifer l.jpg
What is a confined (or artesian) aquifer?

  • Sealed off

  • Transmits water

    down from R.A.

  • Water confined in

    aquifer unless

    drilled.

    - Water under hydrostatic

    pressure.

    - Water rises; well may

    flow.



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What is a cone of depression?

  • Steepens local hydraulic gradient, increases flow rate.

  • Created by drawing down water table by pumping a well.

  • Overdraft occurs where pumping is too rapid, well goes dry.



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What are some other problems associated with the use of wells?

  • Lack of filtering in karst regions.

  • Limited amounts of g.w. in some rock types.

  • Subsidence

  • Salt-water invasion.


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How do springs occur? wells?

E

E

E

E

E

E

E

e

j

  • Lateral diversion of flow

  • Perching

  • Fracture zones


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What is karst topography wells??

  • Topography with features relating to underground solution.

  • Collapse may also be involved.

  • Surface waters diverted underground.

  • Features: sinkholes, sinking streams, rises, caves.




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Cave wells?


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Cave wells?


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Cone of depression wells?

Confined aquifer

Darcy’s Law

Ground water

Karst

Permeability

Porosity

Secondary p & p

Sinkhole

Soil moisture

Spring

Unconfined aquifer

Water table

Zone of aeration

Zone of saturation

Terms Used in the Study of Ground Water


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Student Responsibilities on Groundwater wells?

Know how groundwater occurs and moves.

Know the factors that determine the direction and rate of movement.

Know how we can use groundwater safely, as well as the dangers of using it unwisely.


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