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Water in the Ground

Objectives:

    1.    Explain how the porosity and permeability affect the storage and movement of groundwater.

    2.    Describe the water table and features associated with it.

    3.    Explain how artesian formations affect groundwater.

    4.    Distinguish among hot springs, geysers, and fumaroles.

 

Notes:

Water stored in the Earth's crust is known as groundwater.  Groundwater is an important resource as it is pumped out of the ground to supplement the world's fresh water supplies.  The amount and quality of our groundwater supply is affected by the following:

climate - wet vs. dry
topography - flat or inclined
land use - industrial or rural
soil type - sand, clay, or silt

The porosity of the soil is the percent of the soil that is air space.  Porosity ultimately affects the amount of water a particular rock type can hold and depends on a couple of different factors.

particle shape - round pebbles have greater porosity than angular ones
sorting - when particles are all the same size porosity is greatest

(www.protectyourwater.net/ glossary/porosity.gif)

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The ability of the ground water to pass through the pore spaces in the rock is described as the rock's permeability.  Permeable layers of rock that store and transport water are called aquifers.  While porosity and permeability usually go hand-in-hand, though some porous rocks are not permeable and some impermeable rocks are porous.  Permeability is affected mostly by the size and arrangement of the grains in the soil.

grain size - larger grains are more permeable

The Water Table

As rain and runoff enter the soil the water begins to fill the pore spaces in the ground.  The water will continue to work its way down until it accumulates  above an impermeable layer (bedrock).  Just like the soil horizons the areas of saturated an partially saturated soils can be defined as layers (bottom up) . 

(www.geo.lsa.umich.edu/~crlb/COURSES/ 205/Lec18/WaterTable.jp)

bed rock - impermeable layer below the soil
zone of saturation - layer of saturated (filled) soil above the  impermeable layer 
capillary fringe - thin partially filled space separating the zone of saturation and the zone of aeration.  Location of the water table.
zone of aeration - porous layer of rock that is filled with air instead of water
organic layer - above the topsoil where plant roots absorb water

The water table is an area at the top of the zone of saturation and is a common term used to describe the availability of available groundwater for use.  The water table's distance from Earth's surface depends on many factors including:

rain fall - adequate amounts are necessary to soak the ground past the organic layer
duration of the rain - sustained rain is needed to replenish dropping water tables
amount of time between rains - time is needed between rainfalls to allow surface water to absorb past the zone of aeration
season - influence evaporation rates
slope of the surface - influences runoff
climate - nearer to the surface in humid climates / farther in desolate climates
humans - affect the table by drilling and overusing wells

types of wells
ordinary well - a well dug to the water table that does not reach the surface
spring - where the water table meets the hillside
can be formed from a perched aquifer (layer of impermeable rock perched on a mountainside)
artesian well - a natural well that is formed between 2 impermeable layers on a sloping surface
water is moved along by gravity
shale is usually the upper layer with sandstone in the center
usually contains massive amounts of water

(www.bartleby.com/images/ A4images/A4artwel.jpg)

Geysers and hot springs occur as water comes into contact with igneous structures
hot springs - water comes into contact with surface igneous activity and boils at the surface
Geyser - a hot sprint that occasionally shoots hot water and gas into the air as it boils beneath Earth's surface

(doors.stanford.edu/~sr/ california/geyser.jpg)

 

 

 

 

 

 

Last modified: November 18, 2003