Although ground water flows through underground rock formations, that rate of flow can vary dramatically. In more permeable material, ground water may flow several yards daily, although this is exceptional. In impermeable materials, such as clay and shale, ground water may flow very slowly. In some cases, ground water may move only a few inches in a century. Water percolating into the earth may spend days, weeks, or thousands of years or more underground.
Ground water is cleaned by a natural filter as it flows through underground soils.
Even so, man's activities can put ground water at risk from chemical or biological pollutants. Leaking underground pipes or storage tanks, industrial spills, seepage from waste disposal sites or faulty septic systems may be significant sources of pollution. Most pollutants have the potential to contaminate large amounts of ground water, and as ground water moves it can spread the effects of spills far beyond the original site. One gallon of gasoline, for example, can contaminate millions of gallons of ground water. In many cases, the problem is only noticed long after contamination occurs.