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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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1. Ground Waterg110
Lecture prepared by Bob Hall
Revised 8/6/2000
2. 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?
3. 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?
4. 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
5. Ground Water As Part of the Hydrologic Cycle
6. Zones of Subsurface Water 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
8. What is the configuration of the water table?
10. How does ground water move? 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
11. 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)
12. 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
13. 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
15. 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
16. 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.
18. 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.
20. 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.
22. 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.
23. How do springs occur? Lateral diversion of flow
Perching
Fracture zones
24. What is karst topography? Topography with features relating to underground solution.
Collapse may also be involved.
Surface waters diverted underground.
Features: sinkholes, sinking streams, rises, caves.
29. Terms Used in the Study of Ground Water Cone of depression
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
30. Student Responsibilities on Groundwater 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.