Nearly 4 million Americans drink water that contains unsafe levels of nitrates, a naturally occurring chemical frequently found in pesticide residue, an Environmental Protection Agency study shows. The study also found high levels of two pesticides in a significant percentage of groundwater supplies tested. Groundwater provides 40 percent of drinking water in the United States.

Overall, however, the study concluded that the ``vast majority of drinking water wells'' in the country contain insignificant levels of dangerous substances, EPA officials say.

``There is no current cause for alarm, but there is enough evidence of contamination that is cause for concern,'' said EPA Deputy Administrator Henry Habicht at a news conference. Mr. Habicht cautioned: ``If we don't nip it in the bud, we could have a serious problem.''

The study showed that 52.1 percent of community water system wells surveyed throughout the country contained safe traces of nitrates, while 56.8 percent of rural wells yielded safe levels, the EPA said.

But the five-year study found that 1.2 percent of 564 community wells surveyed and 2.4 percent of the 783 rural wells tested contained unsafe levels of nitrates. The sites of the wells were not disclosed.

Nitrates are formed naturally and are a byproduct of pesticides, human waste, and other substances. In low levels they are not a threat to humans.

But at unsafe levels, which the EPA sets at above 10 parts per million, nitrates can be harmmful to infants.

The EPA has informed well owners whose groundwater contains dangerous levels of contaminants and offered advice on cleanup of the wells.

The EPA study represents the government's first effort at documenting national levels of the frequency and concentration of pesticides and nitrates.

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