What can cause water to become contaminated, and what can be done about it?

Sources of water pollution can be grouped into two categories: 1) naturally occurring; and 2) those caused by human activities. The first category includes microorganisms from wildlife and soil, nitrates and nitrites in soil, and heavy metals—such as arsenic, lead, and fluoride—in underground rocks. The effects of floods, earthquakes, and other natural disasters can also cause water pollution. The second category includes bacteria from human waste, nitrates and nitrites from pesticides, industrial waste, lead and copper in plumbing, lead, paint, and much more.
 
In addition, water that has not been properly treated or disinfected and/or water that travels through an improperly maintained distribution system can become a health risk. Under the Safe Drinking Water Act of 1974, municipal water supplies are typically tested for chemicals 4 times a year.
 
While disinfection is not required for bottled water, most of it is disinfected because it is derived from municipal water sources. All water COULD contain pathogens—that’s why it is tested. Once it is opened, additional pathogens could be introduced, but they would not grow because the water contains no nutrients. Keeping it refrigerated won't kill most pathogens, but they die during storage (whether refrigerated or not) because of lack of nutrients. Most pathogens are “vegetative” (living) cells, so they need nutrients for energy.
 
The EPA offers this following reassurance: “Actual events of drinking water contamination are rare and typically do not occur at levels likely to pose health concerns.” If water becomes contaminated after a disaster, boiling water for one minute kills the microorganisms that cause disease. However, the EPA warns people not to attempt to rid water of high levels of lead or nitrites by boiling. That will only increase the concentration of these chemicals.
 
Source(s):
United States Environmental Protection Agency (EPA) “Water on Tap: What You Need to Know”
The Washington Post, “Herbicide Found in Water May Pose Greater Danger,” by Kari Lydersen,
Aug. 25, 2009
Susan Brewer, Ph.D, University of Illinois, Department of Food Science and Human Nutrition