This past Saturday in observance of St. Patrick’s Day, hoards of drunk bros, wide-eyed tourists and everyone claiming to be Irish for the day flocked downtown at 9:30 a.m. to gawk at the annual dyeing of the Chicago River. While staring at a massive accumulation of neon green liquid may be a good time for some—I’ve never felt compelled to spend my Saturday sleep-in time watching drunkards cheer for colored water—the science behind the method is more than a little unsettling.

The ritual began in 1961 when a plumber approached Stephen Bailey, business manager of the Chicago Journeyman Plumbers Local Union #110. The plumber’s overalls were dyed the perfect shade of Irish green after he was splashed with river water that had been treated with a special powder that detects sewage leaks. The men determined that a large amount of the chemical would dye the entire river, and thus the tradition was born.

It’s a cute story and a nice ritual, but did you catch the part in that last paragraph that says the chemical detects sewage leaks? That means the water in the river is so polluted that the entire thing turns phosphorescent green with the addition of only 40 pounds of the sewage-detecting chemical.

Conservationist group American Rivers estimates that an average 1.2 billion gallons of untreated human and industrial waste are dumped in the river every day, and it’s pretty disgusting that Chicago revels in dyeing it green.

Reflect on that as you nurse your hangover that’s been going strong for the last four days—I will.