Water used to be simple. It came out of a tap, you drank it and it was good. Now, with concerns about safety and taste, water is complicated. Here are answers to the top water questions.

Q. Where do we get tap water?

From two sources — ground (underground) and surface (reservoirs, lakes, rivers, etc.). Larger cities use surface water, while smaller municipalities use ground. Also, according to James M. Symons, aka Dr. Water, about 80 million people in the United States use water from sources such as wells, which are not regulated.

Q. Is tap water safe?

Tap water's risk relative to other risks in the world is low, says Mark Wiesner, a professor of environmental engineering at Duke University. The concern for municipal water facilities is to eliminate pathogens or microbes — the things that can make you sick right away.

The Environmental Protection Agency has two sets of water-quality goals. The first is the EPA maximum contaminant levels, which municipalities can't exceed. Second are the maximum contaminant level goals. Here the objective is to determine what the goals would be if cost was not an issue.

Environmental groups are concerned about the long-term effects of tap water. Can you get cancer or lead poisoning from water? We don't know — and that's what's frightening.

The Environmental Working Group analyzed the nation's municipal water supply and found more than 90- percent compliance with the EPA's enforceable health standards. The problem, according to the group, is the EPA's failure to establish enforceable health standards or to monitor other sources of tap-water contaminants. Of the 260 contaminants detected in tap water from 42 states, the EPA has set enforceable maximum contaminant levels for only 114 and unenforceable goals (called secondary standards) for five others.

Of the 141 unregulated contaminants detected in public water supplies from 1998 to 2003, 52 are linked to cancer, 41 to reproductive toxicity, 36 to "developmental toxicity" and 16 to immune-system damage, according to government and industry toxicity references. Despite the risks, any concentration of these chemicals in tap water is legal, no matter how high.

Cities may also be concerned about nitrates and nitrites, too much fluoride and pesticides. "There are so many that it is ludicrous to list what the consumer should avoid, since the consumer can't actually identify any of them," says Tim Ford, a professor of microbiology at Montana State University.

The Environmental Working Group lists the contaminants you should be concerned about, regulated and unregulated. Read more at www.ewg.org.

The other tap-water issue is the potential problems created by chemicals used to clean and disinfect water. "Long-term effects of disinfection byproducts are hotly debated. Yes, many have been shown to be carcinogenic at high concentrations, but if we didn't have disinfection, we'd die of infectious disease long before the effects of disinfection byproducts kill us from cancer," says Ford.

Q. Why is tap water contaminated?

Rivers and reservoirs often are polluted with industrial chemicals, farm waste, sewage, pesticides, fertilizer and sediment. So municipal water supplies start with a contaminated supply, which they must clean up. "We are paying for a lot of current and past sins," says Symons.

Q. How is tap water cleaned?

By disinfection (primarily using chlorine) and filtration. It's best if the water is clarified (filtered) to remove particles, because germs hide in them. But some municipalities, such as New York City, only chlorinate, Symons adds.

Why don't municipalities take additional water-purification steps? There's no point in sterilizing water that is going to be sent through an aging, leaking distribution system (which is almost all of them), says Ford. But most tap water is treated to overcome most of the distribution issues that could occur.

Q. If I drink tap water, should I use a filter?

"It's not an unreasonable investment to have some sort of carbon filter to remove lead, bacteria and chlorine. It can be a bit safer and also improve the taste," says Wiesner.

Q. How can I find out the quality of the water where I live?

Ask your water company for a copy of its water-quality report (sometimes called a right-to-know report). Also get a copy of "Making Sense of Your Right to Know Report" (www.safe-drinking-water.org/rtk.html or www.epa.gov/safewater/dwinfo/index.html to help you understand it.

The Environmental Working Group has compiled a great source to check the contamination of drinking water of more than 39,751 water utilities in 42 states. See how your tap water stacks up at www.ewg.org/tapwater/yourwater/.

Note: This is the first of a two-part series on water quality. Next week: bottled water.

Charles Stuart Platkin is a nutrition and public-health advocate, and author of "Breaking the FAT Pattern" (Plume, 2006). Sign up for the free Diet Detective newsletter at www.dietdetective.com.