All of my reservoirs are made of granite because it can endure more water pressure than other material in early game.
Polluted water reservoir accidentally got a thin layer of water as a surface in the reservoir (and I don't know why the water can be here but never mind...).
This thin layer of water presses the polluted water cause the polluted water quantity per tile got more than usual (~800 kg per tile as usual) and create enormous water pressure to the bottom tiles.
I don't know this is intended by devs or not but just a thin layer of water can presses the whole polluted water and make this high pressure? looks unreasonable to me.
Make a thin layer of water on the surface (not mix) of polluted water by some method (I don't know how to do this too). You will see the enormous water pressure from pressed polluted water.