I have a 2002 Chevy trailblazer and whenever I crank the motor there is water pouring from the rear of the motor what could that be

 

You're going to need to be more specific. Is it water or is it coolant? Your eye are going to have to look at it and see where it's coming from.

 

An Achilles Heal for our Blazers is the bad lower intake manifold (LIM) gaskets used in the late 90s to perhaps early 00s.

But as Walleye Hunter said, is it water or coolant and try to pin-point the leak best you can and get back to us. Off the top of my head if not the LIM gaskets, the only other possibility would be heater hoses.