It's the head gasket leaking. If this subaru dealership is where you bought the car from, they should fix it for free, or you could invoke a 90-day return policy or some such thing...
Lesson learned: When buying a used impreza, always inquire as to whether the head gaskets, timing belt & waterpump have been replaced, etc.. if they don't have records about what engine work has been done, either don't buy it or assume you're going to be replacing head gaskets if you do.