Christina Kobryn, age 14, of Philadelphia, Pennsylvania, for her question:
What are the sources of underground water?
On Old MacDonald's farm there is a well and if it goes deep enough it may dip down into groundwater. In New England, about 20 feet is usually deep enough. In the Southwestern deserts, it may be necessary to drill a well several hundred feet before reaching water if it is reached at all and install a pump. But in just about every area in the world there is bound to be groundwater at some depth below the surface. This is where about 97 per cent of the earth's useable water is reserved in storage.
Groundwater is a sort of depot in the global cycle that keeps the earth's supply of fresh water in constant motion. The enormous engine that runs it is powered by the sun and the earth's gravity, assisted by the seas and the ups and downs of the land's geography. The sun evaporates and hoists moisture from the surface into the atmosphere. The earth's gravity drags it down again as rain, snow and other forms of precipitation.
It is estimated that the planetary water cycle hoists and drops about 97,000 cubic miles of fresh water during each and every year. Slopes and rocky surfaces lead a portion of the surface precipitation back to the seas and some of it evaporates back into the water cycle. The rest percolates more slowly through the underground depot. About 16,000 cubic miles of water pause to moisten the world's soil. About a million cubic miles percolate within half a mile of the surface and an equal amount may seep through deeper levels.
A little brackish water may seep underground along the shores. But the major sources of groundwater are fresh rains and melting snows. As gravity drags it through the ground, it usually dissolves various chemicals from the rocks. The sturdy crust may seem too solid to hold all this water, but even its densest rocks contain tiny pores. Groundwater percolates fastest through coarse gravel and more slowly through fine grains of sand.
The thick crust is a jumble of different minerals, some arranged in neat layers like rocky sandwiches. Some are in fragments, some in slabs tilted at odd angles.
Layers of dense shale may delay the seeping groundwater or form barriers to trap it in pockets of more porous rocks. Here and there, rocky formations lead underground streams and springs back to the surface. Everywhere, the groundwater is guided by buried crustal formations. It moves much slower than surface runoff, but it moves ¬and eventually even its deepest supplies return to the surface and rejoin the never ¬ending global water cycle.
Meantime, all but about three per cent of our useable water is hidden underground. Here and there its reservoirs rise and fall with rainy and dry spells. A huge supply of good groundwater may be overdrawn, perhaps causing, the earth above it to sink. An irrigation project may draw up chemically saturated water and ruin large areas of farmland. This is why countless hydrologists are kept busy probing the hidden picture, tabbing the changing supplies available now, and estimating conservation precautions for the future.