Why do ice cubes formed from water of a glacier freeze at a higher temperature than ice cubes formed from water of an underground aquifer?

Water of an underground aquifer is likely to have more minerals dissolved in it. 
for each mole of ions & molecules in 1 kilogram of water, the water freezes 1.86 degrees Celsius colder than zero C. The water form glacial melt should have less minerals and should freeze closer to that of pure water at zero celsius. 

668900 atoms in total