Many people spotted vibrantly green water flowing along the Broad Branch section of Rock Creek earlier Sunday. And while the photos populating Twitter may have made it seem as if the water had turned into some kind of toxic sludge, D.C. Water said it was nothing to worry about.

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After a number of concerned calls to D.C. Fire and EMS, the water utility used Twitter to say the dyed-green water was related to a floodwater test. D.C. Water’s post said “the dye will dissipate and is harmless,” and it had used the dye to trace the water’s flow in the storm sewer system.

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D.C. Water noted its sewer operations team was working to see how the system worked after recent flooding in the 3900 block of Military Road in Northwest D.C. WTOP’s news partner NBC Washington confirmed the test with John Lisle, D.C. Water’s Vice President of Communications.