What does it mean when a stream turns bright green? That DC Water is doing some investigative work.

People walking along part of Broad Branch Stream on Sunday were startled to find it was colored a rather unnatural shade of green.

A Twitter user asked the question: “Is this where Ninja Turtles come from?”

Just got these pics from a friend’s run this morning? Is this where Ninja Turtles come from? @McBorowik pic.twitter.com/BqbhUFm3Ry

— Christopher Naoum (@cnaoum) September 12, 2021

Other joking comparisons on Twitter included Slimer from Ghostbusters, an early (or late) St. Patrick’s Day celebration, and…

Hey ⁦@NPSNewsDC⁩ what is going on with this tributary by Nevada Ave and Broad Branch? Looks like the Hulk was in the area. pic.twitter.com/eB2ox6VXtA

— Jason Ryan (@JasonARyan) September 11, 2021

Others wanted to know, more seriously, if it was toxic.

I reported it to Park Police just before 7 AM. At Broad Branch Rd NW at the base of the Soapstone Valley trail pic.twitter.com/bkjw4ZSiVb

— Lisa Kralovic (@LKralovic) September 12, 2021

NBC4 reports DC Fire and EMS got some phone calls, too. So did DC Water, which told provided the definitive answer: It’s a harmless dye. And while it won’t produce any mutant turtles, it is used for science.

DCist dived into the story on Monday and reported the dye test was performed for a local family with massive stormwater problem. ABC7 reported a month ago that their backyard turns into a lake after downpours.

Our #backyard is #flooding again in #NWDC @chevy_dc @patcollins4 @7NewsDC @wusa9 @fox5dc @nbcwashington @marycheh @MayorBowser @DCGovernment @DOEE_DC @DCDPW @311DCgov @dcra @councilofdc @DDOTDC @dcwater #help us find a #solution pic.twitter.com/NhLEpgtE9Y

— Colleen Quinn (@cmquinn22) August 20, 2021

DC Water tells DCist that the dye test was done to confirm drains behind the house are connected to the storm sewer system that, in turn, drains into Broad Branch Stream. The next step will be to check for and clear any blockages that may be causing the drains to back up.

Dye tests are also used to check for leaks from sanitary sewer systems. We can be especially thankful that wasn’t the case here. (DC Water used the dye to confirm a 2015 sanitary sewer leak in the Soapstone Valley.)

So the next time you see a local stream turn fluorescent green, know that it’s not a precursor to a leprechaun invasion. It’s DC Water investigating an issue.