The dam is up a little detour off the main drag on floor F going in the Oakeley direction from chamber 31 ("batcave")
Seems to be in vicinity of chamber 29, but chamber behind it not shown on plan. Don't think anyone cam get behind it to see what exactly is there.
So it's roughly in the Cwmorthin/Oakeley boundary area.
The plan is the colour coded Cwmorthin & Oakeley mine layout in the documents section under "Cwmorthin" on this site.
'I wonder how many breakfasts, and other meals we have missed inside that nasty clockless, timeless hole?'
'The Hobbit'
J R R Tolkien.