In Chapter 8 you will need to turn all the valves in order to drain the poisonous water. The first one is located behind the poisonous waterfall. There are huge pipes behind it that will allow you to find it easily.
The second valve can be found near the upper part of the waterfall. A small stone shelf that has a Summoning Stone is located behind the poison water pipes. Go there and jump down to proceed. Grab the points at the bottom and you should be able to move to the next location.
Your character might die in the process but you need not worry as he will respawn on the other side. This is where you can turn the third valve. The fourth valve is located behind a wall. You need to destroy it in order to get to it.
You can place bombs on the wall and light them up with a nearby bonfire in order to destroy the wall and gain access to the valve. That is all that you need to do in order to drain the poisonous water.