Chapter 10 - GROUNDWATER


Facts about global fresh water resources and groundwater:



POROSITY: percentage of "empty" space (pores) in a rock or sediment.

PERMEABILITY: ability of rock or sediment to transmit water.

General permeability and porosity characteristics of common rocks and sediments.

In general the WATER TABLE parallels the surface of the land and intersects the surface at LAKES and STREAMS.

A stream that is gaining water from the ground is an EFFLUENT STREAM.

A stream that is loosing water from the ground is an INFLUENT STREAM.

AQUIFER - a porous layer of sediment or rock WITH high permeability.

AQUICLUDE - a layer with low permeability that impedes groundwater movement.

SPRING - where the water table reaches the surface, allowing the discharge of water.

PERCHED WATER TABLE - an aquaclude which allows water to accumulate high on a hill, it forms intermittent springs.

HOW WELLS WORK: pumping water out of the ground results in the DRAW DOWN of the water table, creating a CONE OF DEPRESSION in the water table.

ARTESIAN WELL - aquifers may be confined above and below by impermeable layers; if the source of infiltrating ground water into an aquifer is HIGHER than the top of a well draining the aquifer, the hydrostatic pressure will cause water to flow out of a well naturally.


1. The primary concern of groundwater use is CONTAMINATION. Any substance that is dumped on the surface can dissolve and migrate into the subsurface, causing contamination. Sources of pollution are considered:

Microbial biological activity can break things down, not everything! Certain toxins accumulate (i.e. metals) in the groundwater.

2. NON-RENEWABLE RESOURCES: urbanization in desert regions result in the extraction of ground water faster than natural recharge by infiltration. This results in the MINING of water. For example: The Okalala Aquifer, a aquifer that is a primary source of groundwater in the Great Plains region of the U.S. Water mining has drawn down the water table almost a mile in places; huge regions of the west will be out of ground water almost perminently.

3. LAND SUBSIDENCE - taking water out of the ground causes the porous spaces in the sediments to collapse, causing land to sink. This is particularly problematic in coastal regions.

4. SALT WATER INTRUSION ALONG COASTAL AREAS - pulling too much freshwater out of the ground cause sea water to infiltrate into areas where fresh water was once available: a real problem in coastal urban areas. (Salt water is denser than fresh water.)

GEOTHERMAL ENERGY - renewable energy in igneous provinces where heat from deep in the earth heats water for steam generation or heating.

HOT SPRING: any spring where the average temperate is 10 degrees C above the average air temperature.

GEYSER: intermittent spring that spouts steam when a hole in the ground fills with water, it boils, erupts, then the hole refills with water.

REGIONS that have a lot of igneous activity can be tapped for geothermal energy if there is: enough HEAT FLOW and enough POROSITY, PERMEABILITY and WATER SUPPLY. (California: 3-6% of state's electrical needs, but their "heat fields" were over- developed and are in trouble in the long run.)

Iceland and New Zealand have vast reserves of geothermal energy, Hawaii has potential great potential, but not enough people...

Yellowstone debate: Energy or Geysers? Industry has put a lot of pressure on the NPS to allow "mining" of heat near the park's borders.


A "CAVE" is any overhang (including caverns). A "CAVERN" is a passage underground carved in limestone by water moving underground. (Not all caves are caverns!)

United States has about 17,000 known caverns (source of data: National Speleological Society).

Two most visited American caves:

A "canyon" cave is carved much the same way a canyon is carved on the surface, by fast, running water along cracks and fissures undergound. A "solution cave" is carved by slow moving water dissolving rock under the water table over long periods of time. Most cave display evidence of both processes.

Ingredients to make a cavern:

KARST TOPOGRAPHY: a name applied to regions drained by caverns. Karst areas are underlain by LIMESTONE, lack surface streams, and are covered with abundant SINKHOLES.

SINKHOLES are collapse structures where the limestone beneath the surface has dissolved away and the roof of the "hollow" falls in.

Droughts in karst areas lower the water table, this sometimes causes the sudden collapse of caverns forming new sinkholes. This is quite common in central Florida in areas where new neighborhoods are being built in karst regions; sinkholes commonly "eat" houses and cars.

SPELEOTHEMS - TRAVERTINE (limestone) deposits in caverns. Types of speleothems include:

Other notable deposit features in caverns include spectacular crystals of gypsum, calcite, and other salt minerals.

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