Jack Lengemann, age 12, of Omaha, Nebraska, for his question:
How do springs differ from artesian wells?
When we chart the visible ups and downs of surface geographical features, we are amazed at the multitude of different rocks and their jumbled formations. A similar wish mash of jumbled strata continues twenty miles or so down through the continental crust. Springs and artesian wells originate in different types of rocky formations be¬low the surface.
It is estimated that only about three per cent of the liquid fresh water is held in surface streams and lakes. The rest is out of sight at various levels dawn in the earth's crust. Hydrologists estimate that about two million cubic miles of this ground water is distributed below the total area of dry land. In a few places, holes can be drilled to release fountains of artesian water.
Rains and melting snows percolate down through certain types of porous rock. Vast reservoirs of this ground water are contained in basins of dense rock. Hydrologists chart majestic motions of underground water that usually teed to match the major surface features of the continents. Springs occur where layers of water logged porous rocks reach the surface, often along the slopes of limestone and sandstone hills.
An artesian well rarely if ever fountains forth by itself. Its supplies are held in porous rocks trapped in an impermeable basin. But it happens to be capped with a thick roof of more dense, impermeable rock. The trapped water in the bottom of the basin is under great pressure primarily from the water above. This type of formation usually is quite deep and extends over a large subterranian region called an artesian basin.
If this buried reservoir had an outlet through weak rocks it would not be capped under pressure. Some could escape to the surface in ordianry springs. But it must be reached by drilling through its roof of dense rock, which may be 1,000 or even 2,000 feet thick. The drill hole acts somewhat like a cap coming off a pop bottle. Under¬ground pressures on all sides push up a jet of water, perhaps 15 feet above the surface.
Many wells are drilled down into the artesian basin and the water may supply a whole city. The tough drilling job is worth while because artesian wells provide same of the world's purest drinking water.
In the past, the city of London drew artesian water from a soggy layer of chalk, trapped 300 to 500 feet below the surface. In the 1850s, the city of Paris drilled almost 2,000 feet to reach artesian water from a basin that still produces. It is said that ages ago the Chinese used bamboo sticks to drill for deep artesian water. They called them grandfather wells because the patient job tools a couple of generations.