It’s a beacon that’s supposed to shine red, if heavy rain sends untreated waste into the Potomac River. Except this weekend, there was no warning light from the lone beacon, standing in the shadow of the Watergate.

The warning system alerts boaters when polluted sewage streams into the river. The device looks like a lamppost, glowing red when sewage mixes with rain water. The beacon then turns yellow, illuminated 24 hours after an overflow event.

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“We're looking at DC's only public notification system telling people when untreated sewage is being dumped, millions of gallons, into the Potomac River,” said Phillip Musegaas, vice-president of programs and litigation at the Potomac Riverkeeper Network.

“We have a problem here because anyone who happens to be on the river right now does not know that there's untreated sewage pouring out into the river, polluting it, and threatening public health.”

The D.C. Water and Sewer Authority maintains the single beacon on the banks of the Potomac, along with a second warning light on the Anacostia. The $850,000 system also includes sewer flow monitoring equipment, tracking when heavy rainfall overflows old sewer pipes.

In an email Monday, a spokesperson for D.C. Water said the authority is aware of the malfunction, and is in the process of repairing the Potomac beacon. A timetable could not be immediately provided for when the warning light will be online again.

Substantial rain events that send untreated water into the Potomac or Anacostia Rivers are expected up to 77 times a year. A $2.7 billion project is expected to capture an estimated 96 percent of untreated waste water, with construction completed in 2030.