The city of Melbourne, Florida, has a surface water treatment plant that produces 20 MGD of potable drinking water. The water source has hardness measured as 93 mg/L as CaCO3, and after treatment, the hardness is reduced to 78 mg/L as CaCO3. Assuming all the hardness is derived from calcium ion, what would the concentration of calcium be in the treated water (mg Ca2+/L)