This was wonderfully hard to find out. ChatGPT tells me that NASA employees a detention pond to treat water after a deluge. Found this french post about the construction of it all: http://www.capcomespace.net/dossiers/espace_US/shuttle/ksc/S...

Interestingly, ~70% of the water is evaporated during a launch.

SLS's main engine is hydrolox fueled, so it's byproducts should be water and hydrogen. But the SRBs have a lot of nasty ingredients like perchlorates.