Comment: 09:37 - 11:24 (01:47)
Source: Annenberg/CPB Resources - Earth Revealed - 21. Groundwater
Keywords: "artesian well", groundwater, gravity, "open aquifer", "closed aquifer", "water table", "hydrostatic pressure", permeability, "saturated zone", aquiclude, "recharge area"
Our transcription: Getting water out of the ground requires working against gravity and pumping the water up.
But in some wells the groundwater will move upward of its own accord in apparent defiance of gravity.
These are called "artesian wells."
To understand how these valuable wells work, a classification of aquifers into two types, "opened" and "closed" is useful.
In an "open aquifer," a stratum of permeable rock overlies the water table, so that the pressure of the water, in other words the "hydrostatic pressure," at the top of the zone of saturation is quite low, too low for the water to rise up unassisted to the land surface.
The other main type of aquifer, the "closed aquifer," is overlain by a layer of impermeable rock, an "aquiclude."
Where such an aquifer is inclined with its highest level exposed at the surface, water can enter directly into it in what is known as the "recharge area."
The weight of the water entering the inclined aquifer via the recharge area combined with the confining effect of the layer of impermeable rock above causes hydrostatic pressure to build up.
If a well is dug in the appropriate place, this pressure is often strong enough to drive water up to the surface against the pull of gravity.
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