Mike,
In dealing with circulating currents in large motors it is very common for this current flow to cause pitting and pin holes on the bearing surfaces as the current moves across the air or oil gap in the bearings. It is possible the current flow itself is not causing his pin hole problem but the presence of a parallel path of sufficient low impedance that must push through some higher impedance medium ( wet earth ) to continue to flow, Ky soil is very rocky with iron ore clay mixed in. Good conductor for a parallel path. I'd try a good quality Ammeter and measure the currents at different points along the buried pipe.
FYI - EC&M Sept 2006 had a great article on EMF and childhood cancers ( pg 36 - 40 ) is relating new study focus on children experiencing this ground current flow for extended times as a possible cancer causing agent. His home would have plastic drain lines so it may not be quite his issue but defintely and eyeopener for us macho tough guys who work with out the proper PPE (rubber gloves) while around energized circuits. Also had a good article on "stray currents" to continue the industry discussion.
Keep up the good work Mike.
God Bless. Bob LeRoy
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