Question

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?

Answer 1

if they are underground they are closer to the earths core where it is hotter

Answer 2

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.