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Confined and Unconfined Aquifers with Pollution Sources.

A spring forms when the ground level drops to intercept either a confined or an unconfined aquifer (or water pressure in a confined aquifer forces water out through cracks in the confining layer), consequently, they are subject to the same contaminant risks as wells drilled into the aquifers themselves.

Unconfined Aquifer:
An unconfined aquifer is recharged from surface water directly overlying it.  It is apparent from the diagram above that, in general, an unconfined aquifer is much more prone to contamination than is a confined aquifer.    
This recharge area is frequently of a greater area and has more development on it than the recharge area for a confined aquifer.  
Although filtration takes place as the surface water percolates down through the pores of the soil and permeable rock, if pollutants are present in the recharge area, they will frequently make their way into the aquifer.  A good graphic showing sources for ground water pollution can be found here.  
Water tends to move more rapidly through unconfined aquifers than confined aquifers.
Unconfined aquifers are more prone to changes in water quality over time than are confined aquifers, because of the characteristics above. You can expect seasonal changes and changes after unusual precipitation amounts.

Confined Aquifer:
The Recharge area of a confined aquifer may be many miles from the developed areas where wells are drilled into it.  
The water is filtered as it moves through the rock pores as it is in unconfined aquifers,.
Often a relatively small area of permeable rock in the recharge area is exposed.  If the recharge area is less developed than the area where the water is used, the water in the confined aquifer will be more pure than the overlying unconfined aquifer or the surface water.  
On the other hand, water tends to move through confined aquifers more slowly than in unconfined aquifers.  For that reason, and because they have at least one layer of impermeable rock above them, if contaminants do enter a confined aquifer, it is much more difficult to remove them.

More information about aquifers in general can be found by clicking here.  The USGS maintains an on-line Ground Water Atlas of the United States with maps and descriptions of the important aquifers of the Nation.  A description of an extensive aquifer in Texas (the Edwards Aquifer) can be found by clicking here.

The USGS is a tremendous resource for learning about watersheds, streams, rivers, lakes, and aquifers in the United States. 

The Water Resources Division of the USGS has the principal responsibility within the Federal Government to provide the hydrologic information and understanding needed by others to achieve the best use and management of the Nation's water resources. To accomplish this mission, the Water Resources Division, in cooperation with State, local, and other Federal agencies,

  • Systematically collects and analyzes data to evaluate the quantity, quality, and use of the Nation's water resources and provides results of these investigations to the public.
  • Conducts water-resources appraisals describing the occurrence, availability, and physical, chemical, and biological characteristics of surface and ground water.
  • Conducts basic and problem-oriented hydrologic and related research that aids in alleviating water resources problems and provides an understanding of hydrologic systems sufficient to predict their response to natural or human-caused stress.
  • Coordinates the activities of Federal agencies in the acquisition of water resources data for streams, lakes, reservoirs, estuaries, and ground water.
  • Provides scientific and technical assistance in hydrologic fields to other Federal, State, and local agencies, to licensees of the Federal Energy Regulatory Commission, and to international agencies on behalf of the Department of State.
  • Administers the State Water Resources Research Institutes Program and the National Water Resources Research Grants Program.