Calcium and Magnesium
High levels of calcium and magnesium in water for whisky production can lead to flocculation in whisky. Harmless, but unpleasant.
Lead
Lead found in whisky may come from lead crystal decanters.
Other Heavy Metals
Analysis of mercury, cadmium and copper may be carried out to meet certification requirements or for due dilligence.
Arsenic
Very unlikely to be found in spirit drinks in modern times but testing may be required by some importers.