Dec. 23 (Bloomberg) — New York, the largest U.S. city, called on state officials to ban drilling for natural gas in the watershed that supplies drinking water for its population of more than 9 million people.

Natural-gas production using hydraulic fracturing and horizontal drilling, techniques that crack thousands of feet of rock so that the fuel can escape presents “unacceptable threats” to drinking water, the city’s acting environmental commissioner, Steven Lawitts, said today in an e-mailed statement.

The decision joins the administration of Mayor Michael Bloomberg with members of the city council and environmental groups including the Natural Resources Defense Council. The city’s formal opposition is based on an engineering study completed this month, Lawitts said. The watershed lies atop a portion of the gas-rich Marcellus Shale.

“New York City has invested $1.5 billion to protect these pristine waters,” Lawitts said in today’s statement. “The known and unknown impacts associated with drilling simply cannot be justified.”

New York Mayor Bloomberg is owner of Bloomberg L.P., the parent of Bloomberg News.

The city stands to lose an exemption from drinking-water filtration requirements by the U.S. Environmental Protection Agency should its water be fouled by chemicals or silt from drilling, Lawitts said. Without the exemption, the city would need to build a $10 billion filtration plant, he said.

Development of the Marcellus Shale, a formation that stretches under Pennsylvania to West Virginia and may contain 50 trillion cubic feet of recoverable gas, stopped in New York while the state revised permit conditions for horizontal drilling adopted in 1992. Comments will be taken through Dec. 31, and the state’s environmental protection unit must respond to them before adopting of final rules.

Rules proposed by the state Department of Environmental Protection in September would allow horizontal drilling and hydraulic fracturing in the city watershed when certain conditions are met.

An Oct. 28 no-drill pledge by Chesapeake Energy Corp., which called itself the only natural-gas producer with leases in the watershed, isn’t sufficient protection, Lawitts told state officials in a Nov. 10 letter posted on his department’s Web site.

To contact the reporter on this story: Jim Polson in New York at jpolson@bloomberg.net.

To contact the editor responsible for this story: Kim Jordan at kjordan2@bloomberg.net.