Skip Maine state header navigation
Skip First Level Navigation | Skip All Navigation
|Home | Contact Us | Publications|
Wells: Water Table and Artesian
One kind of well is the water-table well. Drilled or dug down through the saturated zone, this well fills with water to the level of the surrounding water table. As the water table changes position due to pumping or seasonal variation, the water level in the well changes accordingly. The static (non-pumping) water level in such a well can be considered equivalent to the local water-table level. A water-table well is essentially nothing more than an enlarged pore in the porous medium from which water can be withdrawn.
When water is pumped out of a water-table well, the water table immediately surrounding the well begins to slope inward as the water is removed. In a homogeneous and isotropic, or "ideal," material, pumping causes a true cone of depression to form as illustrated in Figure 21. Flow potential, as indicated by equipotential lines inside this cone, is lower than in the surrounding aquifer; thus, ground water flows into the well by gravity. The water-table gradient is steepest near the base of the cone, where ground water flow concentrates and moves rapidly into the well.
As the well is pumped, the water level in the well lowers, and the cone of depression enlarges. Initially, the level drops quickly, and the cone spreads rapidly. But as the cone increases in volume, more and more water flows into the well. At some point, the discharge from the well is nearly equal to the release of water from storage within the interstices of the aquifer, and the size of the cone becomes relatively stabilized.
Water released from the aquifer flows into the pumping well leaving the interconnected voids within the cone of depression drained to approximately the specific yield. In the vicinity of the pumping well the unsaturated zone has expanded at the expense of the water table. The form of this change is ideally an inverted cone, but is typically elongated in a down-gradient direction because of a sloping regional water table. In fractured bedrock, the cone of depression is often considerably elongated, as pictured in Figure 22. In all water-table situations, however, a "cone" of depression of whatever form defines a region of the aquifer that has been drained of water. When pumping of the well is ended, the drained region is refilled by water moving from other parts of the aquifer. If there is no additional recharge, the regional water table is slightly lowered. The water level in the well returns to the same elevation as the surrounding water table.
Artesian wells are a second kind of well. When such a well is dug or drilled through the confining layer of an artesian aquifer, water rises up the well to the level of the local potentiometric surface. In some cases this level is higher than the ground surface, and the well flows naturally, without pumping. Otherwise, the water rises to some point higher that the lower surface of the confining layer.
Pumping an artesian well lowers the pressure in the well relative to that in the surrounding aquifer. The potentiometric surface around the pumping artesian well is lowered under ideal conditions in the form of an inverted cone. The cone of pressure relief is similar to the cone of depression, except that it represents a decrease in water pressure rather than a lowering of a water table (Figure 23). Whereas in a water-table aquifer the materials surrounding a pumping well are drained, in an artesian aquifer only the pressure on the water is decreased. Water is released from storage by expansion of the compressed water and by compaction and settling of the aquifer materials. This process releases only a very small amount of water per cubic foot of aquifer.
The volume of water released from storage in an artesian aquifer is indicated by its storativity, which in simplified mathematical terms is the total volume of water pumped, divided by the volume of the cone of pressure relief. Storativity is a dimensionless value. In water-table aquifers the storativity is the same as the specific yield and is usually in the range of 0.100 to 0.400. For artesian aquifers, storativity is much lower, commonly in the range of 0.00005 to 0.005.
In artesian aquifers, because so little water is released from storage per cubic foot of aquifer, the cone of pressure relief is far larger than a corresponding cone of depression would be under unconfined conditions. The result is that an artesian well affects a much larger area than does a water-table well when both are being pumped at the same rate of discharge. The size of a cone of depression in unconsolidated gravel might be in the range of 20 to 500 feet for a domestic well, and 200 to 3000 feet for a municipal/commercial well. Cones of pressure relief in a similar but confined gravel aquifer might be 5 to 10 times the above figures. Within the crystalline bedrock of Maine, the area of influence of domestic wells is in the range of 100 to 1000 feet, while that of the higher yielding municipal and industrial wells is a mile or more.
In some cases, aquifers are neither water-table nor artesian, but are a mixture of the two. These semi-artesian, or leaky-artesian, aquifers may be in overburden or bedrock. In bedrock semi-artesian aquifers, wells yield water from unconfined fractures or fractures in the upper part of the wells which drain by gravity flow as the water level in the well is pumped down. In overburden semi-artesian aquifers, the confining layer leaks a significant volume of water into the aquifer as the well is pumped.
Last updated on March 25, 2009
|Copyright © 2005 All rights reserved.|