It seems it isn’t easy being green, especially if you are the Chicago River. For the last forty years they have been dyeing the water the brightest shade of emerald green to mark St Patrick’s Day. It isn’t your monitor playing up. The water really is that colour!

You would think as a fan of all things riverine I might object to pumping an artificial colour into a watercourse but legend has it that the tradition was actually a result of work to improve water quality in the city of Chicago. Tracer dyes are used to locate the sources of pollution and temporarily colour the water. The use of the bright green tracer dye that was used by the city prompted the somewhat lateral leap towards colouring every year. The dye that was originally used contained the now banned chemical fluorescein and the mix now used is a secret formula.

There is a great animation of the process here (with music)

The fountains at the White House were dyed green in 2009 in honour of Chicago born Michelle Obama.

Tracer systems have now progressed and it is even possible to use nanotechnology to determine the exact form of the pollutant but we don’t usually aim for such a drastic result!

Happy St Patrick’s Day!