A great place to get started with profiling is the chapter in Real-World Haskell:

For a problem like this I would look at general heap profiling (-hc), retainer profiling (-hr), and also type profiling (-hy) to see if any of them provide new insight.  For example, -hc might tell you which functions are problematic, but -hr is more likely to help you there.  Sometimes the specific type that is leaking is not what you think and that's why -hy is nice.

I hope that helps,

Jason