Groundwater

I. Origin and Distribution of Groundwater

II. Springs and Wells

III. Groundwater Mining

IV. Geysers

V. Formation of Caves

VI. Implications for Man

VII. Possible Essay Questions

VIII. Practice Questions

I. Origin and distribution of groundwater

• Rainfall percolates downward through pore spaces in rocks.
• High pressure at great depth in the crust of the earth compacts the rocks and closes all the pore spaces, so that the rock becomes impermeable to the flow of water.
• Downward-percolating water sinks until it comes to an impermeable layer and cannot go any lower. There it piles up and saturates (totally fills in) the remaining overlying pore spaces.
• The top surface of this zone of saturation is called the water table (the boundary between saturated and unsaturated
• Groundwater moves downhill under the influence of gravity; hence, the water table, like the surface of a lake, constantly moves toward flatness.
• Groundwater moves fastest through porous and permeable rocks (like sandstone) called aquifers.
• Groundwater moves slowest or not at all through nonporous and impermeable rocks (like mudstone) called aquicludes.

• Relative location of the water table in humid climates -

• The water table is relatively high under the hills and it slopes downward toward the streams.
• Groundwater movement is in the direction of the water table slope - toward and into the streams.

• Relative location of the water table in slightly arid climates -

• The water table is relatively high under the streams and slopes downward under the hills.
• Groundwater movement is in the direction of the water table slope - away form the stream beds and down into the depths below the hilltops.

• Relative location of the water table in very arid climates -

• The water table is relatively high under the streams and slopes downward under the hills.
• Groundwater movement is in the direction of the water table slope - stream bed is dry because the water table is below the level of the stream.

II. Springs and Wells

• Spring - a spring occurs wherever an aquiclude is present at the ground surface so that water is forced to flow out of the ground onto the surface.

• Well - a hole drilled or dug below the water table so that water can be extracted.

• An artesian well or spring flows freely, without pumping, where high water pressure develops in groundwater trapped below an aquiclude.

• Rate of withdrawal must equal the rate of recharge in order to keep the level of the water table constant.

• Withdrawing water too fast produces a cone of depression in the water table, which takes a long time to eliminate greater the velocity will be.

• Hot Spring - A hot spring occurs where hot groundwater rises from great depth along a crack or fracture in the ground at a place where a spring would exist even if there where no crack.

III. Groundwater Mining

• The intentional overpumping (no recharge time) of groundwater causes:

1. A regional lowering of the water table,
2. Compaction and loss of porosity in the aquifer, and
3. Subsidence and cracking of the ground surface.

• This mining philosophy treats groundwater as a nonrenewable resource and is only justified if the mined groundwater produces enough financial gain to allow later import of water.

IV. Geyser

• Geysers occur where the water table is near the surface, where magma is present below the surface, and where a deep, open, water-filled crack or cracks exist.

• How a geyser operates -

1. The boiling point of water at great depth is several degrees higher than normal because of the high pressure at that depth.
2. When water at the bottom of the crack boils, water vapor bubbles form and start to move upward, pushing some of the water out of the crack ahead of them.
3. The loss of water in the crack lowers the pressure at the bottom of the crack and hence lowers the boiling point of the water.
4. Water at the bottom of the crack, which is still at the previous boiling-point temperature, is now several degrees above the boiling point, so it flashes into steam and shoots out of the crack, pushing the water ahead of it.

V. Formation of a cave

• Groundwater dissolves limestone or marble along underground cracks and in time these cracks enlarge into water-filled caves.

• Formation of cave deposits -

• Caves will dry out and fill with air when the water table is lowered; dissolving ceases and deposition begins.
• Water dripping into the dry cave leaves calcite deposits behind when it evaporates.

• Stalactite - hangs from the ceiling downward.
• Stalagmite - grows from the floor upward.

Source for Diagram: Carlsbad Caverns on CD-ROM, Finley-Holiday Film Corp.

Source for Diagram: Carlsbad Caverns on CD-ROM, Finley-Holiday Film Corp.

Source for Diagram: Carlsbad Caverns on CD-ROM, Finley-Holiday Film Corp.

Source for Diagram: Carlsbad Caverns on CD-ROM, Finley-Holiday Film Corp.

Source for Diagram: Carlsbad Caverns on CD-ROM, Finley-Holiday Film Corp.

Source for Diagram: Carlsbad Caverns on CD-ROM, Finley-Holiday Film Corp.

• Sinkhole -

• A hole in the ground where the roof of a cave has collapsed.

VI. Implications for Man

• Implications for man of groundwater overpumping -

• The water table is lowered until the groundwater is no long economically available.
• The ground surface subsides and cracks due to the collapse of the pore spaces in the rock when the water is withdrawn an no longer serves to support the pore spaces.
• Salt water contaminates wells near the coastline where fresh groundwater is floating on salty groundwater.

• Implications for man of groundwater pollution -

• Typical pollutants include sewage, gasoline, oil, toxic chemicals, and nuclear waste.
• Bacterial contaminants such as sewage normally will be cleaned as they move slowly through porous rocks. If the groundwater moves too fast, such as along a crack, it will not be purified by the normal processes.
• Other pollutants can only be removed by thorough pumping and flushing of the contaminated groundwater aquifer. Cleaning up the source of the contamination will prevent future pollution, but will not make the groundwater safe.
• Knowing the direction and rate of flow of the groundwater is important to an analysis and correction of the situation.

VII. Possible essay questions -

• What is groundwater, where does it exist, and what are its lower and upper boundaries?
• How and in what direction does groundwater move?
• Describe and illustrate the relative position and topographic shape of the water table with respect to surface topography in an humid climate.
• Describe and illustrate the relative position and topographic shape of the water table with respect to surface topography in a semiarid climate.
• Describe and illustrate the relative position and topographic shape of the water table with respect to surface topography in a very arid climate.
• Describe and illustrate the geologic conditions associated with an artesian well.
• Describe and illustrate the geologic conditions associated with a cold spring.
• Describe and illustrate the geologic conditions associated with a hot spring.
• Describe and illustrate the geologic conditions associated with a geyser, and explain how and why a geyser works as it does.
• Describe and illustrate how a cave forms and the circumstances associated with its later filling with cave deposits.
• Describe and illustrate the conditions responsible for salt water contamination of coastal water wells.

VIII. Practice Questions

Back to Index