Gov. Andrew M. Cuomo has signed a 20-year extension of a landmark preservation program credited with helping halt the spread of dense suburbia to Long Island’s five East End towns.

Cuomo, in agreeing to extend the Community Preservation Fund law on Friday, also approved widening the program beyond land preservation by allowing expenditures on water quality projects.

The 2 percent tax on most land transactions that funds the program was scheduled to expire in 2030, but the extension would lengthen it to 2050. New language in the law would allow town officials to tap 20 percent of their town’s revenue for water-quality initiatives such as wastewater projects and habitat restoration.

For the changes to be finalized, referendums must pass in the towns of East Hampton, Southampton, Riverhead, Southold and Shelter Island. Each town would be required to outline plans for water-quality projects before residents vote on changing the law.

Southampton Town Supervisor Anna Throne-Holst said a referendum could be on the ballot in her town on Election Day 2016. She said the additional funding could help incentivize the tests of high-tech wastewater systems, which are being eyed as a remedy to nitrogen pollution.

“The problem has been recognized and identified and the solutions are now in the works, and the third and inevitable part of this is how to fund it,” Throne-Holst said.

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Community Preservation Funds in the five towns have raised more than $1 billion and preserved 10,000 acres of land since state lawmakers created the program in 1998, allowing town officials to buy farms, natural lands and historic buildings that comprise the scenery of eastern Long Island.

State Sen. Kenneth LaValle (R-Port Jefferson) and Assemb. Fred Thiele Jr. (I-Sag Harbor), the East End lawmakers who were central to creating the program, had pushed for its extension and expansion over the past year. Thiele said the program, which has been flush with money on the South Fork, could supply much-needed funding to combat nitrogen pollution and algal blooms in East End waters.

“The Community Preservation Fund was enacted more than 15 years ago, and land preservation was the single biggest issue for community character” at that time, Thiele said Tuesday. “We’ve had great success with the program. . . . There became a realization that if you really want to protect the community character of the East End, you needed to protect both the land and the water.”

LaValle said Monday in a news release that the program would “protect Long Island’s clean water for generations to come.”

The Community Preservation Fund program would yield an estimated $2.7 billion between 2016 and 2050, including up to $540 million for water-quality projects, the lawmakers said.

Jeremy Samuelson, executive director of Concerned Citizens of Montauk, said the environmental group supports the expansion of the program but also backs amendments “to help clarify exactly what this money can be spent on, down to the specific project level, and ensure transparency on all proposals and spending.”

“Water quality and land preservation go hand-in-hand,” Samuelson said. “One of the principal reasons we preserve land is to protect the quality of our drinking water and surface waters.”