New York City’s tap water — a source of pride for residents — might taste a bit different for a while.
This week, a pristine watershed in the Catskill Mountains that provides about 90 percent of the city’s drinking water will be reduced by about half, temporarily, and a lesser-used supply will make up the difference.
A chunk of the Delaware Aqueduct, which is responsible for delivering about half of the city’s water, is closing down for critical repairs for eight months, resulting in the loss of a core water source: the Delaware portion of the Catskill-Delaware watershed, which encompasses five rural counties in New York State. The watershed represents the largest unfiltered water supply in the United States.
But there is a contingency plan: Another watershed, which supplies the city with about 10 percent of its water but is typically reserved for backup, will make up some of the shortfall. Officials said this means that New York’s water, though still safe, might deviate from the taste residents are accustomed to, while the city completes the crucial repairs to its water infrastructure.
“Just like different brands of bottled water taste a bit different, so do our different reservoirs,” said Rohit T. Aggarwala, the commissioner of the Department of Environmental Protection, which oversees the city’s water supply, in a statement.
The nearly century-old Delaware Aqueduct is 85 miles long and extends from the Catskills region north of New York City to the city’s main reservoir in Yonkers.
In the 1990s, leaks were discovered in a 2.5-mile section of the aqueduct near Newburgh, N.Y., about 70 miles north of the city, and in another location farther north. The tunnel was losing up to 35 million gallons of water a day.
With the shutdown of this part of the Delaware Aqueduct, the goal is to bypass the leaking section by connecting the aqueduct to a new tunnel, the construction of which was completed three years ago. This final phase of the $2 billion aqueduct repair project is estimated to take eight months.
The city’s other water source, which will be making up the shortfall during the repairs, is known as the Croton watershed, and its water isn’t as clean as that of the Catskill-Delaware watershed. That’s because Croton is near more development in suburban counties north of New York City, which means its water contains more drainage and discharge. Croton’s water must be filtered, according to the Environmental Protection Agency.
In 2015, the Croton Water Filtration Plant opened in the Bronx. During the eight-month shutdown, the plant will increase its operations, but it will only be able to produce about a quarter of what the city needs on a daily basis.
To make up the difference, some unfiltered Croton water will be mixed with what is flowing in from the Catskills, at about a one-to-three ratio, and routed to the city’s main reservoir.
The technique, called “blending,” can be controversial, said Sarah Meyland, a water expert based in Huntington, N.Y. “If you are mixing high-quality water with less-than-the-best water, the concentrations will be diluted in the supply,” she said. “But that’s not to say you won’t have exposure to whatever is in the Croton water.”
It is likely that microorganisms commonly found in many watersheds could show up in the unfiltered blend of Croton and Catskill water, Ms. Meyland said. But Paul Rush, the deputy commissioner for the city’s Bureau of Water Supply, said that all water, filtered or unfiltered, was tested and treated for pathogens and other contaminants with chlorine and ultraviolet light.
A recent report on the repair project cited “episodic taste and odor issues” with the water in the Croton system that were being closely monitored. Although the plant in the Bronx will be able to treat the Croton water using a carbon filtration process, the Catskill-Croton blend will depend on simple dilution to do the job.
Mr. Rush said that the water from all of the watersheds and treatment plants serving New York City would ultimately combine in the pipelines supplying the city, further diluting any taste or smell changes.
There is also a technical issue with the Bronx filtration Plant, said Bob Cooney, a retired quality engineer: It has no backup power for its pumps.
Mr. Rush said that in the event of a power outage the system would continue to operate using gravity alone. “We’ve looked at every contingency,” he said.
Ms. Meyland, who followed the early planning stages of the Delaware Aqueduct shutdown, said that relying more on the Croton watershed was a sound and necessary approach, given that the demand for water in the city and its environs averages about 1 billion gallons a day.
But she acknowledged that the rejiggering of the city’s water system remained a significant technical challenge: “It’s kind of uncharted territory.”
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