Plumbing woes

For some inexplicable reason when we shower on the second floor the plumbing leaks in the basement. At this point in the game I've come to expect these things however this one has me nervous.

It's difficult to see in this picture but the plumbing from the upstairs shower comes into the basement by the blue arrow to the right side of the stack. The leak is occurring on the left side of the stack under the downstairs tub drain where we have rigged up the bucket. I'm thinking that no matter what it ends up being the remaining galvanized waste pipes will probably be getting the heave ho.