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In the natural movement of ground water, discharge occurs in numerous places, especially into streams. As much as 40% of Maine's streamflow is thought to be derived from ground water discharge (Wagner, 1977). Besides discharge into streams and other bodies of water, ground water comes to the surface at many other points. Wet areas of diffuse ground water discharge are termed seeps, while areas of more obvious flow are termed springs.
Spring water has had a certain mystique in the minds of people from ancient times to the present. Water that "magically" flows at the earth's surface and sometimes bubbles or boils out of the ground is reason enough for feelings of wonderment. Bottled spring water brings a premium price because of its presumed special qualities. Warm springs and mineral springs, some very high in odoriferous sulfur compounds, have been bathed in for therapeutic effects for centuries. All of these kinds of ground water could be and are obtained by wells, yet bottled well water, if it were marketed as such would find few buyers, and a household bath in sulfurous well water would find few takers.
Regardless of whether springs are magical and wells mundane, the source of both spring and well water is ground water. A spring is a natural point of noticeable ground water discharge; whereas a well is a man-made opening into the subsurface from which ground water can be extracted. Springs and wells occur in bedrock, as well as in unconsolidated sediments, and are influenced by the same differences in permeablility, flow gradients, flow divides, and other hydrologic aspects. The flow rate of springs in Maine ranges up to several hundred gallons per minute.
The typical spring in unconsolidated sediments results from the intersection of the water table and the ground surface where there is a significant contrast in permeabilities of strata. Figure 17 shows a water table in a gravel deposit overlying a less permeable till layer. Ground water cannot pass readily downward through the poorly permeable layer, but tends to spread laterally until discharge occurs at the gravel-till contact on the hillside. This is one example of a contact spring, where water is discharged at the contact between an overlying water-bearing layer and an underlying aquiclude.
Another common kind of spring is found where the water table meets the ground surface because of a cut or depression (Figure 18). A depression spring does not need a contrast in permeabilities, but only a gully or pit entrenched to a depth below the local water table.
There are many variations of these kinds of springs that are associated with either perched or permanent water tables. Although these water tables occur most typically in unconsolidated sediments, they can also occur in bedrock where the fractures are numerous and open to the immediate ground surface. In such a situation a plumbing-system spring may occur in the bedrock (Figure 19).
Artesian springs occur where confined ground water leaks to the surface through the overlying less permeable layer. They are found in sand and gravel and in bedrock.
A typical artesian spring occurs where clay overlies gravel. The clay layer confines the ground water under pressure, but where this confining layer is breached by an erosion channel or depression, is penetrated by roots that have decayed, or is otherwise absent, ground water will flow out to the ground surface (Figure 20).
An artesian spring in bedrock occurs where ground water, in a series of interconnected fractures recharged at higher elevation, finds its way to the surface through some fracture at a lower elevation than the recharge area. Some apparent water-table springs derived from unconsolidated sediments may be the result of springs in the underlying bedrock. Bedrock artesian springs probably are common, but go unrecognized.
Last updated on March 25, 2009
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