The median values for all 4 groups ranged from 0.3 to 12.3 µg I L -1 (Table ). Distinctively, group C demonstrated higher concentrations of Mg and Ca than the other groups (Fig. , Supplementary material, Appendix B), and as a result, this group exhibited the highest water hardness (Table ). Therefore, group C was treated separately in further correlation analysis. ). However, an observation made in 2008 revealed that in seven of the locations tested, the tap water had a magnesium level higher than 10 mg L -1 .