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In the United States and other countries, there are many committed people who are doing a good job to ensure that the water delivered to your home is safe for drinking. These are scientists who are trying to figure out the effects of various chemicals and pathogens on the human body or discover better treatment technologies, government regulators who try to identify, monitor, and regulate the amount of certain contaminants allowed in water, and professionals in the water industry who strive to deliver safe, quality water to their customers.
You will also discover that 'the system' does not always work. Accidents happen, mistakes are made, systems wear out, toxic chemicals are released into the environment to find their way into the surface and ground water, regulators fail to regulate, population growth in a region puts more stress on a water system than it can handle, and any number of other weak links in the chain of processes that must function properly to deliver safe water to your faucet.
Water has been called the universal solvent because so many substances will dissolve in it. Water also can carry many materials in suspension. Unfortunately, water is not particularly selective in which compounds become dissolved or suspended. The water that dissolves your coffee or tea and sugar in the morning or that you use to reconstitute your orange juice or infant's formula might also have dissolved some atoms of lead from the pipes in your home or picked up a microgram of 2,4 D from the farm upstream from the filtration plant. If your water is chlorinated it almost certainly contains a few micrograms of chloroform (a byproduct of the disinfection process). The question you need to ask is not, "does my tap water contain contaminants" - all water outside of laboratory distilled, deionized water does.
The real questions are, "what are the contaminates in my water, what are their concentration levels, and do they pose short or long term health risks at those levels." Finding answers to these questions is not easy. The answers depend on where you live (country, city, surrounding land use, etc.), the primary source of your drinking water (confined or unconfined aquifer or surface water), your water supplier (private or community well, small or large municipal water system), and what is happening at any moment as your water travels from its source through the treatment/distribution system to your faucet. Water that is reasonably contaminant free (and safe) one moment can become dangerously contaminated the next because of accident, neglect, or some natural event.
One of the most notorious recent examples of water that was safe one day and dangerous the next was during the summer of 1993 in Milwaukee when one-fourth of the people living in the metro area (over 400,000) became ill with cryptosporidiosis (discussed below). According to the 1996 Houston Chronicle series, Tapwater at Risk , the contamination of Milwaukee's water supply was a combination of natural events (heavy rains) and accident (improperly installed monitoring equipment) that allowed the parasite to pass though the city's purification system and into the distribution system. In August of 1998 the citizens of Sydney Australia were told that Cryptosporidia had been detected in their municipal water supply. There are over a month of stories detailing the contamination of Sydney's water supply archived on The website of Professor Haas at Drexel University (Professor Haas also maintains a collection point for information on outbreaks of infectious disease in drinking water all over the world).
Fortunately, despite a two-day delay in informing the population, there were apparently no reported cases of death or illness linked to the contamination. Most government entities and water providers are extremely interested in supplying safe water to their citizens and/or customers. Consequently, barring accidents, the majority of dangerous contaminants that are liable to be in the drinking water of most Americans (and people from other developed countries) are typically present in minute amounts. They may contribute to health problems only after many years of exposure, making identification of the cause difficult, if not impossible. Examples of this type of contaminant are lead (that is usually dissolved out of distribution pipes or plumbing fixtures in the home) which causes intellectual deficits in children, and trihalomethanes (a byproducts of chlorine disinfection) that have been linked to a slight, but significant, increase in the chance of getting certain cancers after 20 - 50 years of drinking chlorinated water.
There is an ongoing and vigorous debate among the various groups interested in drinking water safety concerning the costs, the benefits, and the risks of every aspect of the water treatment and distribution business. Anything that is done to treat municipal water costs money, provides the benefit of water that has reduced levels of the targeted contaminants, and decreases risk of disease from the targeted contaminants. The treatment process may also add substances to the water that would increases risks of other disease for the people who drink the water.