Water Hardness and Alkalinity
Hardness and Alkalinity of water are often expressed "as CaCO3". Hardness-as referring to the cation concentration, and alkalinity-as refers to the anions i.e. bicarbonate. If your local water analysis does not list the bicarbonate ion concentration (ppm), nor "Alkalinity as CaCO3", to give you an idea of the water's buffering power to pH, you will need to call the water department and ask to speak to one of the engineers. They will have that information.
Calcium, and to lesser extent magnesium, combine with bicarbonate to form chalk which is only slightly soluble in neutral pH (7.0) water. The total concentration of these two ions in water is termed "hardness" and is most noticeable as carbonate scale on plumbing. Water Hardness is often listed on municipal water data sheets as "Hardness as CaCO3" and is equal to the sum of the Ca and Mg concentrations in milliequivalents per liter (mEq/l) multiplied by 50 (the Equivalent Weight of CaCO3). An Equivalent is a mole of an ion with a charge, + or -, of 1. The Equivalent Weight of Ca+2 is half of its atomic weight of 40, i.e. 20. Therefore if you divide the concentration in ppm or mg/l of Ca+2 by 20, you have the number of milliequivalents per liter of Ca+2. Adding the number of milliequivalents of Calcium and Magnesium together and multiplying by 50 gives the hardness as milliequivalents per liter of CaCO3.
(Ca (ppm)/20 + Mg (ppm)/12.1) x 50 = Total Hardness as CaCO3