Plumbers have isolated a minor leak beneath our toilet, evidently where the PVC riser and "lead flange" are joined (somewhere near base of slab ~6" below grade?). They propose tunneling and repairing from below slab at huge expense.

Trying to understand the nature of this leak, what caused it, and potentially if something that I can fix myself (from above / not below!). Any thoughts?

There are no " lead flanges". Could you be referring to a lead closet bend under the slab? If this is the case all you can do is replace it. This can be accomplished by either taking up the slab or if the toilet's on a outside wall you can tunnel to it. Want to try it? Regards, tom

Thanks Tom. Guess it shows my ignorance of basic plumbing and what I thought I heard the plumber say. I've attached the actual hand-drawn schematic that the plumber drew describing the leak where lead meets PVC. Does this look like or mean anything to you?