Researchers at the Harvard School of Public Health issued a report this month recommending major improvements to the nation’s water supply systems, including storage reservoirs, water treatment facilities and water distribution infrastructure.

Although overall water quality in the United States has improved greatly over the past decades, the report said, surface water is generally undrinkable without treatment.

Researchers argued the nation’s water utilities need to implement major improvements to ensure continued safety of the water supply.

Water-supply problems demand immediate attention from the federal government, said Paul R. Epstein, associate director of Harvard’s Center for Health and the Global Environment.

“Like all nations, water [in the U.S.] is under stress,” Epstein said. “This will be an increasingly big issue. The U.S. is not immune.”

Researchers warned that addressing water supply problems will require significant expenditures from water utilities and possibly from consumers and local governments. The report cites a figure, determined by the Environmental Protection Agency, that puts the cost at approximately $151 billion.

Regionally, the Massachusetts Water Resources Authority (MWRA), which supplies water to Boston and serves as a backup supplier for Cambridge, has recently invested about $1.7 billion dollars in revamping the water distribution system, according to MWRA spokesperson Jonathan L. Yeo.

As a result of the heightened state of security due to terrorist threats, the MWRA has also increased security throughout its systems, Yeo said.

Such expenses are justified, the Harvard report says, because without improvements the quality of drinking water will suffer. The decrease in water quality could affect public health, especially at-risk groups such as the elderly and pregnant women.

The report suggests that consumers will need to bear a part of the cost—a worthwhile expense, according to the researchers.

“We dramatically undervalue drinking water,” said Tim E. Ford, associate professor of environmental microbiology and one of the report’s authors.

—Staff writer Zachary Z Norman can be reached at znorman@fas.harvard.edu.