Gerry Sparrow, age 12, of Don Mills, Ont., Canada for his question:
What is an artesian basin?
Geological basins are scooped formations in the crust of the earth. The largest of them is thousands of miles, and its floor dips more than 3 miles below sea level. It is, of course, the enormous basin that holds the waters of the Pacific Ocean.
Most of our planet's geological basins are water traps. River basins, the drainage basins around lakes and the abyssal ocean basins are scoops in the crustal surface of the earth. There also are basins between layers of crustal rock below the surface. An artesian basin is one of these buried water traps. If it can be reached by drills or wells, it may supply an enormous amount of the world's purest drinking water.
The continents are provided with an amazing system of underground reservoirs for storing surplus rains and melting snows. Without it, both plant and animal life on the dry land would be very scanty. This buried ground water is trapped and held in layers of various porous rocks. Its top level or water table may be near or far below the surface, and it often follows the hilly ups and downs of the ground above it.
There are porous and nonporous crustal layers, and both types of rock are needed to form an artesian basin. Its base may be a waterproof layer of nonporous clay, perhaps 2,000 feet below ground level. Above it will be a thick layer of sand or some other porous rock. This is topped with one or more layers of nonporous and perhaps porous rock. The entire crusty sandwich must be bent to form a sagging syncline formation, lower in the center and higher toward the edges. This is the basic structure of all artesian basins.
The underground reservoir is trapped in porous rocks and sealed inside a sandwich of nonporous rocks above and below it. This water is under tremendous pressure from surrounding ground water. When a hole or well is drilled down to the trapped water, the pressure forces it to gush up to the surface and spurt forth in a fountain sometimes many feet high. Dozens of these wells may be drilled in the area, and a sizable artesian basin may provide enough fresh water for a whole city.
The Cities of Paris and London both sit upon artesian basins. Paris still draws water from its buried reservoir: But the London basin was over drawn. Its pressure and water level sank, and no new wells are allowed. Some of the lakes and fountains in the London parks, however, are still fed with waters from the city's artesian basin.