Why tap water have low temporary hardness? why tap water have more calcium hardness than magnesium hardness?

Question

Determination of water hardness

Answer 1

Temporary hardness is caused by the presence of the hydrogencarbonate ion (as well as Ca2+ or Mg2+). This is much less likely to be found in rocks that the rain-water passes over than other anions, such as sulphate. Look for the ratio of calcium: magnesium in rocks and I bet you have the answer to your second question!