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Groundwater |
Below this layer is the saturated zone, where all of the pores, cracks, and spaces between rock particles are saturated with water. The term ground water is used to describe this area.
Another term for groundwater is "aquifer," although this term is usually used to describe water-bearing formations capable of yielding enough water human use. Aquifers are a huge storehouse of Earth's water and people all over the world depend on ground water in their daily lives.
To Find Water, Look Under the Table ...
the Water Table
Here's an illustration: if you dig a hole in the sand at the beach, you will end up with a small pool of water. At a specific depth, the ground - if it is permeable enough to hold water - is saturated with water.
The surface of this pool is the water table.
The breaking waves of the ocean are just to the right of this hole, and the water level in the hole is the same as the level of the ocean.
Of course, the water level here changes by the minute due to the movement of the tides, and as the tide goes up and down, the water level in the hole moves, too.
In a way, this hole is like a dug well used to access ground water. You know that at the beach if you took a bucket and tried to empty this hole, it would refill immediately because the sand is so permeable that water flows easily through it, meaning our "well" is very "high-yielding" (too bad the water is saline).
To access freshwater, you have to drill wells deep enough to tap into an aquifer. The well might have to be dozens or even thousands of feet deep.
But the concept is the same as our well at the beach—access the water in the saturated zone where the voids in the rock are full of water.
Groundwater flows Underground
Once in the ground, some of this water travels close to the land surface and emerges very quickly as discharge into streambeds.
But because of gravity, much of it continues to sink deeper into the ground. If the water meets the water table (below which the soil is saturated), it can move both vertically and horizontally.
Water moving downward can also meet more dense and water-resistant non-porous rock and soil.This causes it to flow in a more horizontal fashion, generally towards streams, the ocean, or deeper into the ground.
The direction and speed of groundwater movement is determined by the various characteristics of aquifers and confining layers (which water has a difficult time penetrating) in the ground.
Water moving below ground depends on the permeability (how easy or difficult it is for water to move) and on the porosity (the amount of open space in the material) of the subsurface rock.
If the rock has characteristics that allow water to move relatively freely through it, then ground water can move significant distances in a number of days.
But ground water can also sink into deep aquifers where it takes thousands of years to move back into the environment, or even go into deep ground-water storage, where it might stay for much longer periods.
Most wells in the Blanco area are in the Trinity aquifer, and run from about 300 or 350 feet to over 900 feet deep (drilled into a hill).
Depending on how quickly the water is moving through the aquifer, the well may produce anywhere from 1/2 gallon to 60 gallons of water per minute.
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