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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.
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.
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,