I've been noticing green stains in the lavatory basin at the top of my gravity recirc loop and no others.

This has only occurred since a new water heater was placed in service nine months ago. This heater has a cupronickel heat exchanger, and stainless tank. The old heater was glass-lined steel with a magnesium anode rod.

The pipes are Type L and M copper tubing. The city water (Source: Lake Michigan treated with ozone, alum, chlorine, phosphorus, chloramine) is softened with a conventional sodium-chloride softener. The salt dial is set to 9 lbs. for about 460 cu ft of water, and if my calculations are reasonable, that means about 300 mg/l NaCl, or 150 mg/l of sodium remaining in the treated water. (Edit: Water hardness is 8 gpg. One site says 8 gpg * 30 mg per grain = 240 mg sodium per gallon meaning roughly 63 mg/l, so I think my figures are suspect.)

The heater condensate also leaves a blue-green residue.

I am concerned about the heat exchanger deteriorating. But, I realize there is probably a burn-in period, and this sure beats rust flakes as I've heard happens with some cheaper heat exchangers. Not sure if a patina will form on the outside of the heat exchanger, or if constant expansion and contraction means this stuff will end up in solution. Should I be worried? Anything I can do to mitigate?

This article discusses the symptoms but that's about it:
[www.copper.org]

City water treatment:
[milwaukee.gov]

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Links to the State of Wisconsin Plumbing Code:
[docs.legis.wisconsin.gov]
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I am not a plumber.
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Edited 1 times.

Without knowing anything about your system and seeing your title 'green stain in sink' I instantly thought that A) you have a water softener it is set too high
Having a softener set too high can cause severe damage to your system's metallic components. Personally, I would have your water checked pre and post softener to make sure that it is not over softened. Even if your calculations are correct and the incoming water is exactly what you think, it is possible that the softener settings aren't accurate.

I will have to re-assess the softener settings. I took the numbers from my scrawlings when installed just over ten years ago and I'm not sure I'm interpreting properly.

My glass-lined heater lasted over 13 years, though.

*******************************
Links to the State of Wisconsin Plumbing Code:
[docs.legis.wisconsin.gov]
*******************************
I am not a plumber.
*******************************

Water is 'soft' when a reading of 0 gpg/ppm is obtained. There are no degrees of softness, i.e. 'negative' soft. Applying excessive salt does not result in softer water; it merely wastes salt.

and eats copper - just like sea water will

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Soft water and the minute amount of sodium in it will not eat copper. If there is a corrosion factor between copper and soft water, it is due to the absence of minerals in soft water which makes it more conducive to electrolysis.

The addition of extra salt in the water speeds ion exchange in the electrolysis process though.