Not sure why you guys would say that an MB dealer might use poor quality water for coolant dilution. Do you know that based on actual knowledge of how a dealer dilutes antifreeze?

If they do use city water, I am sure that they would first make sure it complies with their own specifications. (See section 4 of pdf below). Mercedes are usually very particular about how their dealers maintain standards.

https://bevo.mercedes-benz.com/bevolisten/310.1_en.html

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5