Water has an incredible ability to trickle and drip far away from the original source so once you're sure where the leak is use a chain cutter to remove the section of cast iron pipe. They are usually available at tool rental places for around $50.00. It will be a snap ( pun ) to use ... 10 minutes you're done.

A reciprocating saw will take a life time to cut through the pipe besides what you will spend on blades will almost be the cost as buying a chain cutter.