Initially the dye, which used to be made of fluorescein, was used to test leaky pipes, and it would last for a week. But environmentalists have since identified environmental issues behind using oil-based fluorescein in a natural body of water, so in 1966, the plumbers union transitioned to using what’s used currently: a low-impact orange powder that’s made from vegetables. The mixture, which they call “Leprechaun Dust,” turns green as soon as it hits the water, and lasts for 48 hours.
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