About Water Quality

In many supermarkets, bottled water now has its own aisle. There are large containers for home use, and smaller bottles for people on the go; there's water drawn from local sources and water that's been transported from as far away as Fiji and France. Why are so many Los Angeles residents turning off their faucets and resorting to these bottled substitutes? The answer is simple: they're concerned that tap water isn't safe.

All water - regardless of its source - contains some contaminants. A mountain stream, for example, might contain significant amounts of animal waste; a remote underground spring may have high levels of radon (a radioactive material present in certain types of rocks and soil that can seep into ground water).

Water - particularly water drawn from sources near populated or industrialized areas - also frequently contains a variety of chemical impurities. Hazardous household waste like paint thinner or antifreeze that is improperly disposed of can migrate from disposal sites and contaminate water sources; pesticides applied to crops can be carried to lakes and streams after a heavy rain.

Some water impurities cause significant health problems. High levels of the bacteria E. coli have been linked to gastrointestinal illness; high levels of arsenic have been tied to cancer.

To ensure that the public water supply is safe to drink, strict water quality standards established by the U.S. Environmental Protection Agency limit the amount of various impurities in drinking water by setting "maximum contaminant levels", or "MCLs". Drinking water may not, for example, contain more than 0.002 mg/liter of mercury or more than 0.005 mg/liter of the chemical benzene. Currently, MCLs have been set for more than 80 different contaminants.

When water does not meet established standards, it's treated to reduce contaminant levels. The type of treatment methods used depends on the quality of the water and the specific contaminants that need to be removed. Filtration removes particulate material like sand and silt, while disinfection kills dangerous bacteria and viruses. Contaminant levels can also be reduced through a process called blending; blending involves mixing water drawn from different sources together.

Although highly effective, treating and blending water are not intended to totally eliminate impurities from drinking water. The cost of reducing contaminant levels to zero would be prohibitively expensive. And, perhaps more importantly, would be unlikely to provide any additional health benefits.

Public water suppliers are required to routinely test their water. The Los Angeles Department of Water and Power, for example, monitors its water supply for over 200 different contaminants throughout the year. The water is sampled and tested at various locations around the city.

Water suppliers must also provide an annual report to its customers. The report provides information on local drinking water quality, including the levels of various contaminants found in the water. Water quality reports can be obtained by contacting the supplier directly, or through the U.S. EPA's website on Local Drinking Water Information.

Unfortunately, it's impossible to test for all potential contaminants. There are, for example, thousands of chemicals used in industry that wind up at least in small quantities in the water supply, and it simply isn't feasible to check levels for each of them.

That's not to say, however, that bottled water is any safer than what comes out of the tap. In fact, regulations governing bottled water in the United States are nearly identical to those imposed on public drinking water. The bottom line: Spending more on water doesn't necessarily mean that it's any safer to drink.

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Email: tox@ph.lacounty.gov

County of Los Angeles Public Health
Environmental Health
Toxicology & Environmental Assessment Branch