On a helicopter trip up the Potomac on Monday with local and federal officials, Emminizer said the sheen was spotted just south of the Trump National Golf Club, about 10 miles downriver of the Dickerson Plant in Sterling, Va. It did not originate at the club, he said, and probably came from somewhere north of the power plant.

Although the substance forming the sheen has not been identified, Emminizer said it is “definitely a hydrocarbon” — a chemical compound that “runs the gamut from jet fuel to wax” — and poses no threat to the public or natural resources.

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“It makes a pretty rainbow color in the water, but if you put out a boom, it’s so diluted that it won’t stick to the stuff designed to pick it up,” Emminizer said, adding that the substance is “nonrecoverable.”

The Washington Suburban Sanitary Commission, which provides drinking water to Montgomery and Prince George’s counties, said it was monitoring what it called a “plume” expected to reach its Potomac Water Filtration Plant on Tuesday.

“At this point, WSSC does not expect any impact to its ability to meet customer demand, nor does it expect any adverse impact to its drinking water quality,” it said in a statement.

According to a statement from the Metropolitan Washington Council of Governments, officials from Fairfax Water, Frederick County, the town of Leesburg and the city of Rockville were among the authorities monitoring the sheen. Some of these municipalities were ready to deploy booms around their water intakes in the Potomac, as well, a council spokeswoman said.

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A spokeswoman for Fairfax Water said the utility switched to an underwater intake valve to avoid introducing the substance, which is floating on the river’s surface, to the water supply.

Emminizer said the sheen, up to 30 feet wide and 60 feet long in some places, was caused by a discharge of more than five gallons but not more than 1,000 gallons of the substance into the river.

“This is a large quantity but not a significant quantity,” he said, pointing out that a teaspoon of motor oil could cause a sheen on a “football field’s worth” of river.