Some residents of a five-county area of West Virginia are now able to drink and bathe again after state health authorities decided Monday to lift some water bans that affected more than 300,000 people since a chemical spill last week.

But the accident illustrates some of the vulnerabilities of the nation’s water supply and the fact that toxic chemicals are sometimes stored right next to water treatment plants. Experts say that it’s much easier to prevent threats to sources of water rather than try to clean them up afterward.

“We really need to get a handle on things before they start,” said Lynn Thorp, water program director at Clean Water Action, an advocacy group based in Washington. “If we wait to solve the problem in the drinking water plant, we’ve waited too long.”

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Each year there are more than 10,000 spills of oil and hazardous substances, according to federal estimates, many that get into water supplies. From raw sewage to rocket fuel, sometimes these spills evaporate or dissipate into the air or water. Other times, as in Charleston, W.Va., the results are disastrous to human health and wildlife.

The nation’s worst municipal water contamination occurred in Milwaukee in 1993, when an outbreak of the Cryptosporidium parasite sickened 400,000 people and killed 69. A malfunctioning filter at the city water plant allowed the organism to spread throughout the city’s entire public water system.

Thorp and others say the accident on the Elk River in Charleston was the result of lax regulation by both state and federal officials. Even though Charleston’s water treatment plant has advanced carbon filtration systems, it wasn’t enough to remove the 7,500 gallons of a chemical used to wash coal before shipment.

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Across the country, communities have battled harmful runoff from animal feeding operations (North Carolina) naturally-occurring arsenic (New Hampshire, Ohio, Michigan and parts of California’s Central Valley), acidic mine drainage from coal mines (Appalachia) and perchlorate from fireworks displays (Lake Tahoe, Calif.), former missile factories (Los Angeles), or seepage into the Colorado River.

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Some of these sources can’t be handled by chlorination, which is the method used by the majority of municipal water systems.

“We have a huge number of contamination sources that are poorly regulated if controlled at all,” said Erik Olson, water expert at the Natural Resources Defense Council. “The water utilities say: ‘We don’t know what to do with them,’ and throw up their hands.”

Building plants that use carbon filters (like the one in your fish tank), ozone treatment or special membranes has proved too expensive for most cities. So Olson says the key is protecting the original source of a town’s drinking water.

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Officials in New York City, Seattle, Boston and Manchester, N.H., have aggressively purchased land to keep industrial, mining and farming operations away from their water supplies.

In Cincinnati, city leaders decided instead to build a state-of-the-art treatment system, Olson said.

While the United States has strong environmental regulations, there are some gaps.

It turns out that although the federal Environmental Protection Agency is tasked with keeping drinking water safe, the actual implementation of the law comes down to individual city, county and local municipal authorities.

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While the Safe Drinking Water Act requires local officials to identify potential hazards -- like the chemical plant on the riverbank in Charleston -- the law doesn’t give those same officials the power to actually go after potential polluters.

“A lot of cities are doing nothing about their source-water protection, so they are at the mercy of polluters upstream,” he said.

The EPA does monitor test results from municipal water systems, and you can check out how your local system is doing at this website.