While we should bear in mind that ground water contamination is a problem, and that serious efforts are needed to safeguard our ground water resources, it should be noted that only a small percentage of the potentially available ground water is polluted. Using approximate figures for total available ground water 
and the known extent of
ground water contamination, in 1984 the U.S. Office of Technology Assessment
(OTA) estimated that somewhere between 1 to 2 percent of the nation's ground
water was contaminated. This report conceded that this may be an underestimate
because monitoring for contamination has focused on public water supplies,
which are generally afforded some level of protection, while the release of
substances known to contaminate ground water is undoubtedly more widespread.
Nevertheless, this recognized contamination is significant because it is often
near heavily populated areas where potable water supplies are needed and the
demand for ground water continues to increase.
Currently, the ground water contaminants that are of greatest concern are synthetic compounds. These man-made contaminants are usually divided into organic substances, i.e., compounds based on carbon, and inorganic substances, which are not based on carbon. However, many other contaminants not synthesized by man are also of concern. These include various naturally occurring elements, e.g., arsenic and radionuclides, and microbiological contaminants. Microbiological contaminants include bacteria, viruses, and parasites such as Cryptosporidium. Six well-documented outbreaks of Cryptosporidiosis attributed to drinking water have been recognized in the United States, including an outbreak in Milwaukee in 1993 that affected over 400,000 people.
While these breakouts occurred in surface water,
Cryptosporidium has also been known to occur in ground water.
Because of its widespread occurrence, the inorganic compound that is perhaps of greatest concern in ground water is nitrate. U.S. EPA estimates that 52 percent of the community water wells and 57 percent of the domestic water wells in the country contain nitrate. Nitrate in ground water has few natural sources, but at levels above 10 mg/l it can be an acute health problem in infants. Fertilizer application, inadequate design and maintenance of septic systems, unlined waste water holding ponds, leaking sewer lines, and improper sludge and manure application are the major contributors to nitrate contamination.
At least 65,000 synthetic organic chemicals are in common use in the U.S. today, and this number continues to grow each year. Organic chemicals have become a more frequently detected contaminant in ground water supplies. The 1984 OTA report listed 175 different organic compounds that have been found in ground water, and EPA ground water surveys conducted over the last decade confirm the widespread occurrence of organic contaminants. The increased detection of these compounds is due largely to the fact they are so much a part of everyday life. Solvents, pesticides, paints, inks, dyes, varnishes, and gasoline are just a few of the ubiquitous products that contain organic chemicals.
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