Regarding the corrosion issue, In addition to my first post, it may be the underground tubing is type M instead of L or K.
I copied and pasted the data below from Cerro http://www.cerrocu.com/hvacPlumbingTube.aspx website which is the first return that showed up doing a Google search.
I suspect the problem could be the type of tubing used in addition to the stress corosion I mentioned in previous post. Type M hard drawn would likely be very prone to stress corrosion/pinhole leaks when used in an acidic soil condition (this is speculation on my part as I have not had direct experience with pinhole leaks of the type in question. Builder may have been trying to save money by using the least expensive grade ---metalurgical testing would seem a good next step in addition to soil testing.
Bill
copied from Cerro Flow re. copper tubing, pasted within " " marks
"Typical uses include:
Type M – above ground residential and light commercial uses. (Sizes range from 3/8" – 8" diameter)
Type L – residential and commercial uses. (Sizes range from ¼" – 8" diameter)
Type K – underground residential, commercial and industrial uses. (Sizes range from 1-1/4" – 8" diameter)
Type DWV – ASTM B306: Used for drainage, waste and vents "
Bill
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