[Mordahl]

Hi guys,

There is something I have been encountering for a while that I just can't figure out. When reading anything with furigana I occasionally see ZU written as づ instead of ず.
Unfortunately the only example I can remember off the top of my head is the reading of the name おにづか （鬼塚）from GTO.
I know that 塚 is normally つか　is this the reason?

Any help with this would be most appreciated.
I am a beginner but I can read Kana, so please feel free to use it in your responses where possible. :-)