Look at the potential (mature) spread of any tree - and that's the normal spread of the roots in general. It would be unfortunate to say the least to plant a tree too close to a cesspit! I'm sure you don't want it damaged, let alone the foundations or drains to your house.

If it starts growing particularly vigorously one year, you will know that the roots have broken into the cesspit!!!

I shouldn't laugh, Fractal - but I can't help it....lol.

Lol