Barbara Cooke, age 12, of Sandersville, Ga., for her question:
What makes a well run dry?
We do not know who first found that water could be hauled up from a hole in the ground. But he invented the well, and the event took place before the dawn of history. For ages, many people have depended upon well water and learned that even the best of wells may run dry.
Most modern cities get their boundless water supplies from reservoirs, which are man made lakes to collect the run off from surrounding streams. This surface water is filtered of its debris and purified of its harmful bacteria by elaborate man made equipment. Many rural populations and a number of sizable settlements get their water supplies from wells. Most of these deep holes in the ground gather water that has been filtered by the porous rocks of the Earth's crust. Water from wells in layers of limestone is rarely suitable for human use, and water from wells near sewage and other systems of waste disposal are definitely dangerous.
Well water rich in chemicals dissolved from surrounding bedrock is unsuitable for everyday use, but the cool water from deep layers of shales and clays is excellent, and the many people who use it have no complaints about its quality.
However, like almost every well in the world since time began, the purest and the deepest and the wettest well may run dry. As a rule, the shut down does not come without warning. The level of water drops and the dwindling supply may become murky. A trusty well may not dry up for years or even centuries, but when it does the shut down comes some time after a season or two of unusually dry weather.
A well is filled by ground water, the buried reservoir of rains and melted snows that have seeped down into the rocky crust of the Earth. This buried water is trapped and held in the pores and pockets of porous rocks. It may rest on solid layers of dense rock hundreds of feet down, and its upper level may be a few feet or 200 to 300 feet below the surface. Since ground water is supplied by rains and melting snows, it increases and decreases with the seasons. After a wet season the upper level rises, and after a long seige of arid weather the level of ground water sinks.
A well reaches down into the buried reservoir, and water from the surrounding rocks seep through the porous bricks that line its walls. The level of water in the well remains even with the level of ground water. But when the ground water sinks lower than its floor, the faithful well can collect no more water. It runs dry.
The upper level of ground water is called the water table. In dry places and on high slopes, the water table may be so far down that well makers must drill deep holes through layers of hard rock. Pumps may be needed to extract the buried water for everyday use, just as pumps may be used to coax extra supplies from normally wet wells that tend to run dry from time to time.