Mainly Calcium Sulphate plus additives plus whatever the plastere/his mate add to speed up or retard the rate of cure.
You don't dump any effluent into the stormwater drains. Well not till Brexit anyway.
I'm conscious about stuff like my motorbike fluids (which go to the dump) but for paint resedue and the small amount of paster waste water I've produced I've never thought about it.
From now on I guess I'll keep it to go along with my oil etc.
It's basically lime and sand surely. What sort of damage is a small amount diluted going to do to a river?
Increase the acidity? By how much?
Obviously the larger quantity you'll leave to dry out and then take to the dump. But I'm surprised there's an issue with the water.