Concerned pilots who fly single-engine planes say it's easy to glide over a reservoir and dump hundreds of pounds of hazardous chemicals into the drinking-water system.
"I was surprised and scared at the same time," said Charles Sibirsky, 55, an amateur pilot from Brooklyn. "The reservoirs are so vulnerable, it's ridiculous."
Last week, NYPD helicopters grounded a single-engine plane after it was spotted flying over three city-owned reservoirs, Cannonsville, Pepacton and Ashokan. That came a day after witnesses said something fell or was thrown from a similar Cessna flying about 150 feet above a reservoir in Connecticut.
The incidents turned out to be false alarms, but pilots said they highlight a lack of air security over the city's 2,000-mile network of reservoirs.
Sibirsky said he and his stepson Jacob flew out of New Jersey last week for a lesson on a four-seat Cessna and were shocked at the ease with which they were able to fly over reservoirs.
"I could have just opened the window and dumped anything into the water," said Sibirsky.
Experts said it would require massive amounts of a deadly substance to contaminate the nearly 1 billion gallons of water that pours into the city.
"It's a very large watershed, and policing it is a huge endeavor," said Charles Sturcken, chief of staff for the city Department of Environmental Protection.
"Restricting the airspace over reservoirs is something we need to look at."
In October, The Post obtained a confidential city report that called reservoirs "vulnerable to a wide array of threats."