Dave,

{{Standard disclaimer that local Codes and laws vary, and should be verified prior to starting any work}}

The ground wire in your sheathed electrical cable should run (with the sheathed electrical cable) straight back to your electrical panel, and should terminate on the ground bus in the panel. If this panel is the service entrance panel for your house (with a main breaker), then the neutral bus should be bonded to the panel enclosure and the ground and neutral wires are typically both terminated on this same bus. If not, then there should be separate neutral and ground buses.

Your system grounding normally occurs at the location of your service disconnect (whether that's a main breaker in the panel or a separate disconnect upstream of the panel). This is where the neutral and ground are bonded (connected), and where connections should be made to the metal water service pipe and other grounding electrodes (if present and required by Code).

If you are in doubt as to whether your service entrance is properly grounded and bonded, please get a licensed Electrician in there to look at it. There are life safety and property protection issues here.

Jarrod

Is the reason that the cast iron stack i just that - cast and not as dense as cold-formed or drawn wire?

The reason is that at some point a plumber will replace a section of metal pipe with plastic thereby eliminating the continuity of your ground.

Run the sheathed electrical cable back to the panel, connect the ground as Jarrod suggests. Be safe.