Road salt is leaching into the reservoirs that hold New York City’s tap water and could make some of it unhealthy to drink by the turn of the century, according to a new study commissioned by city environmental officials.

The study, released last week by the city’s Department of Environmental Protection, found that while salt was edging upward throughout New York’s vast watershed, it was especially pronounced in the New Croton Reservoir, just north of the city.

In that supply, which provides about 10 percent of the city’s drinking water, levels of chloride — a chemical found in salt and an indicator of salinity — tripled over the last 30 years.

If the trend continues, drinking water from the New Croton Reservoir may not meet current safety standards by 2108, according to the report.

While road salt is a main driver of salinity levels in drinking water throughout the United States, other contributing factors include wastewater treatment plant discharges and agriculture, according to the report. Elevated salt levels in fresh water can contribute to health issues like high blood pressure and can also damage the ecosystem, the study said.

Reducing salinity levels throughout New York City’s water system should start with more prudent use of road salt, said Rohit T. Aggarwala, the head of the environmental protection department, which manages the water supply and commissioned the report.

“We just need people who operate roads to start realizing that this is a chemical that we are adding to our environment, and we have to take that seriously,” Mr. Aggarwala said.

Road salt is cheap and plentiful, but it is also dangerous for the environment and corrosive for infrastructure.

In general, local municipalities, and often the state’s Department of Transportation, make the decision to use salt on roadways — a crucial safety measure that melts ice.

“We understand that there is a delicate balance between protecting the environment and maintaining safe highways for motorists,” said Joseph Morrissey, a spokesman for the Department of Transportation. The department minimizes salt usage, Mr. Morrissey said, with methods that include adhering to prescribed application rates, calibrating equipment throughout the winter, training drivers on best practices and using brine, a liquid version of salt that is less concentrated but more expensive.

Officials are focused on curbing the use of salt, while continuing to explore other affordable alternatives, like beet juice — which lessens salt’s corrosive qualities when mixed with brine — and sand, which does not melt ice but provides traction.

Recently, Pete Harckham, a Democratic state senator who represents parts of Westchester County and the Hudson Valley, introduced legislation to create a city and state task force that would explore the issue. “We’ve got to use this as a teachable moment and rethink how we do things,” Mr. Harckham said.

New York City’s pristine tap water is a source of pride among residents and local leaders. Most of it, about 90 percent, comes from rural areas in the Catskill Mountains, a range that extends more than 125 miles north of New York City. It represents the largest unfiltered water supply in the United States.

The remaining 10 percent from the New Croton Reservoir, a collection of 11 smaller reservoirs and three lakes, is filtered, but not for chloride. New Croton is in Westchester County, a relatively dense suburban area, which offers less of an opportunity for the natural environment to absorb runoff from salt on the road. In the less populated Catskills, more vacant land surrounds the water supply, and there have been only marginal increases of chloride levels, Mr. Aggarwala explained.

Last fall, the city temporarily relied on the Croton reservoir for more of its drinking water when half the Catskills supply went offline for repairs to a major aqueduct. An unexpected drought halted the repairs.

Should salinity levels continue to rise in the Croton reservoir, the city could lose a valuable resource, Mr. Aggarwala said.

“One of New York’s greatest strengths is the fact that we have so many different sources of water, 19 reservoirs,” he said. “If we lose the Croton system, that just makes New York City’s water supply much less resilient, and that makes it much less reliable.”

For several suburban towns that draw water directly from smaller reservoirs that feed into the New Croton, the salinity levels are more of an immediate concern. Somers, Yorktown and the City of Peekskill, all in Westchester, draw water from one of those smaller reservoirs, the Amawalk, where chloride levels could exceed safety standards in about 30 years, the study said.

“While I was alarmed by the report, I was not surprised in any way, because we’ve been dealing with this for some years now,” said Mr. Harckham, who represents many of these towns.

Several private wells in the area have had to be taken offline because of salinity levels, he said.

Methods that can help with over-salting roads, Mr. Harckham said, include thermal devices that can take a road’s temperature, to avoid unnecessary applications.

The solution, Mr. Harckham said, “is an equation of knowledge, technology, best practices and money.”