Nutrition mineral intake method
To absorb essential elements, body must get them in form of stable ions.
So for calcium and magnesium, our body absorbs them from the hard water in the same way it obtains from other sources and uses them identically.
For iron, we cannot obtain iron from iron fillings because iron reacts with water and other chemicals in our body which can produce harmful effects.
For further information on absorbtion of calcium from hard water visit http://goaskalice.columbia.edu/answered-questions/calcium-hard-water