I doubt it is due to ac current as Mike mentions. However, it can still be due to electric current from galvanic action. Are there any dissimilar metals anywhere in the piping? That is do they change from copper to galvanized or black pipe? Something as simple as using a galvanized or black pipe fitting in place of a copper or brass can trigger this.
Other causes, well as mentioned the type of copper used could be at fault. Yes there are different types of copper pipe for different applications. Using the wrong type burried will result in the problem the writer is having. Something to consider since the homes appear to be of about the same age. Were they built by the same contractor and plumber? If they were trying to do the job cheaply, the cheap copper pipe will result in the problem the writter is having. Someone should be able to look at a sample of the removed pipe to determine if it was the wrong pipe or not.
Water quality can be another source of the problem. You would be surprised at how corrosive water can be. Do you know what the city is doing to treat the water? Are they doing some softening of the water? Soft water can be very agressive. This one will be difficult to figure out as trying to find and professional in this area will be expensive. The local home water softener person is not the one to contact.
Last issue the soil itself can be the source of the problem. Some soils are highly alkaline which can attack metals when burried in the soil. Unfortunately to find out on this you need a soil sample down where the pipe is burried not at the surface. Vernon LIppert
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