Copper has been named as a hazard when present in drinking water at above 1.3 ppm. The good thing is that yeast sequester virtually copper during a ferment, so beer is generally considered copper-free...even when produced in copper kettles. Copper's capability to produce a Fenton-reaction does increase oxidation in wort and limiting its presence does appear to be a somewhat good idea. However, having a copper-free brewery can result in higher prevalence of sulfury compounds in finished beer.