Groundwater



This photo is of Mammoth Hot Springs in Yellowstone National Park-- the action of groundwater can form caves, springs, and sink holes which can cause much damage to buildings and other surface structures


I. Basic information

A. Definition and explanation of groundwater
  • water below the Earth's surface occurring in open spaces including fractures, joints, faults or other open spaces between rock particles

  • the amount of groundwater is strongly influenced by the amount of infiltration which in turn can be affected by the other paths taken by precipitation in the hydrologic cycle

  • the following is a list showing the order of availability of fresh water on Earth: ice sheets and glaciers, groundwater, lakes and reservoirs, and river water
  • click here to see fresh water availability on Earth

B. Kinds of groundwater
1. Connate
  • entrapped brackish water found in the open spaces in the rocks

2. Juvenile
  • water originating directly from a magma

3. meteoric
  • fresh circulating water in the open spaces in rocks and is the type we often refer to as "the groundwater"

C. Qualities of rock materials
  • refers to the qualities of rock materials in respect to the contained groundwater

1. Porosity
  • is a measure of the water capacity of a rock material--a substance with a high porosity can hold much water

2. Permeabilty
  • is a measure of the ability of a rock material to transmit water and is proportional to the degree of interconnection of voids between particles in the rock--larger open spaces in a rock will favor a higher degree of interconnected spaces--permeability in a rock substance allows water to circulate thereby remaining pure through filtration--a rock with a high degree of permeability is a rock which should yield quality groundwater
  • a substance with high porosity does not necessarily possess good permeability
  • most sandstones are good sources of quality groundwater because they usually have a relatively high degree of porosity and permeability
  • rocks such as pumice, scoria, shale, clay stone, and siltstone may have good porosity but have poor permeability and would not be sources of quality groundwater

3. Specific yield
  • is a measure of the amount of water obtained from a rock material

4. Specific retention
  • is a measure of the amount of water retained by a rock (after extracting water from it)
  • an example of a non-rock substance which reflects specific yield and retention is a sponge--after soaking in water a sponge can yield an amount of water by squeezing it (specific yield) but will retain a certain amount (specific retention)

5. Aquifer and aquiclude (aquitard)
  • an aquifer is a permeable rock substance
  • an aquiclude or aquitard is an impermeable rock substance

II. Unconfined and confined groundwater

A. Definitions

1. Unconfined
  • is groundwater in an aquifer with a lower barrier for water movement but not an upper barrier--which means there is an aquiclude or impermeable substance located immediately below the aquifer---water can fluctuate to a small or large degree up and down in the upper portion of the aquifer

2. Confined
  • is groundwater in an aquifer with a lower and upper barrier for water movement--which means there is an aquifer located between a lower and upper aquiclude so water in the aquifer is limited or confined to upper and lower movement

B. Unconfined groundwater

1. Zone of saturation
  • is the area of open space in the rocks filled with water--the area in the rocks with the highest groundwater concentration

2. Water table
  • is the upper level of the zone of saturation--unconfined groundwater has a water table which can fluctuate up and down over relatively large distances

3. Zone of aeration (Vadose zone)
  • the area above the water table not saturated with water and filled primarily with air--water is present in this area because of specific retention and by capillary action (rising of water from the water table in tiny thread-like openings between particles)--capillary action is greatest in the capillary fringe area as shown on page 250 below
click here to see groundwater zones

4. Changing levels of water table
  • the lowering (discharge) of the water table may take place if the following usage is greater than the replenishment of water to the zone of saturation: 1. continued pumping of groundwater; 2. plant usage; 3. leaking of water onto the surface into streams and lakes

  • the rising (recharge) of the water table may take place if the rate of groundwater replenishment (primarily by infiltration) is greater than the rate of depletion

5. Groundwater interrelationship with streams
  • in some cases streams can directly supply groundwater and vice versa

6. Pumping of groundwater

C. Confined groundwater
  • artesian water is a special type of spring associated with confined groundwater--flowage of subsurface water in a confined aquifer causes a pressure surface level which defines the level or elevation to which water can rise naturally--if a surface opening is connected to the aquifer and is below the pressure surface water can flow naturally at the surface, otherwise it must be pumped
  • click here to see artesian water

III. Groundwater effects in carbonate rocks

A. Subsurface solution and precipitation effects

1. Chemical mechanism
  • when the underlying rock is carbonate in composition acid groundwater dissolves the rock to form openings (caves, caverns, etc.)--when the groundwater rich in calcium bicarbonate attains a less acid condition, precipitation of CaCO3 will occur forming cave precipitates

2. Caves and cave precipitate forms
  • some examples of famous caves are: Mammoth Cave, Kentucky; Shenandoah Caverns, Virginia; Carlsbad Caverns, New Mexico; Onondaga and Merrimac Caverns, Missouri
  • Missouri has more mapped caves than any other state and has been called "the Cave State"

  • the cave precipitate forms are: 1. stalactites which grow from the cave roof; 2. stalagmites which grow upwards from the cave floor; 3. columns which form when stalactites and stalagmites grow together--stalagmites, stalactites and columns are known as speleothems
  • click here to see speleothems

B. Surface features

IV. Groundwater and stream pollution problems

A. Saltwater encroachment and pollution of groundwater

B. Septic tank leaks

C. Landfills and Industry

1. Landfills
  • landfills have evolved from "dumps" to a "state of the art" way to accumulate and contain our wastes--layers of clay and plastic liners at the bottom of the fill have recently been used to help contain leach ate--landfills have been constructed on top of impermeable layers of rock as shale to help contain leach ate

2. Industry
  • industry has often been guilty of dumping pollutants into streams and groundwater and also adding by-product gases to the atmosphere resulting in acid rain--waste water treatment facilities have also emitted elevated levels of phosphates causing high algae concentrations to form

D. Storm water runoff
  • urbanization can cause a decrease of natural infiltration of precipitation resulting in more runoff--if runoff is not controlled flooding can occur as well as the spreading of pollution

click here to read more about groundwater pollution

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