Ground water is a vital natural resource in the United States, supplying about 25% of all fresh water used. Over 50% of the American public, 117 million people, obtain all or part of their drinking water from ground water, and 95% of rural Americans depend on it. As recently as 10 years ago, ground water was generally considered a pristine resource, pure and ever-available. It was used, and sometimes abused, without being fully understood. In the 1970s, synthetic organic chemicals were discovered in ground-water-supplied drinking water sources in several states. Currently, 40 states have documented instances of serious ground-water contamination.