I am sure that you know this already but whatever the mold has touched and then some needs to be cut out!!! I am not really getting much of a visual in regards to the gravel thing....but is there anyway that you can clean all that our and cement that area? I would hate for you to fix this only to have the same issue.

I had mold in my first house in the bathroom....when we started digging into it....we were down to the studs. It was crazy...we ended up gutting the whole room and starting it from scratch. UGH... It was expensive...but just make sure you get a future full proof plan.