And main power should be off when working on ground wires. Also protective rubber gloves should be used. Pipes can be energized by faulty equipment at a neighbor's house. I would use large #10 copper ground wire (get out your checkbook!). (Best to check with your local electrical inspector's office for bonding requirements and ground wire sizes.)
And all the other grounds on the plumbing should be checked. Best is if there are two ground rods at the main electrical service placed 6 ft. apart.