[Guzba]

From another study:

"Perchlorate is an oxidizer used in fireworks. Though fireworks displays are held over Lake Biwa, which supplies water to Kyoto, the city’s water treatment process lacks the ability to remove perchlorate. This study investigated perchlorate contamination in source and tap water resulting from a fireworks display. During 2016, the perchlorate concentration in the source water increased to 22.3 μg/L during the 19 hours following the display and then decreased to ,0.5 μg/L during the 43 hours following the display. The perchlorate concentration in the tap water increased to 13.6 μg/L during the 35 hours following the display, and then gradually decreased... it took 11–29 days to decrease its concentration in tap water in Kyoto to <0.5 μg/L."

Flow behavior of perchlorate from a source lake to a water supply tap following fireworks displays https://iwaponline.com/wpt/article/14/3/695/69039/Flow-behav...