This is a rather ridiculous myth, in the same tradition as moon-landing denial or Korean fan death. You get minerals from food, not water. You (presumably) have kidneys and other organs that maintain the mineral balance in your body. A few micrograms more or less in your water isn't going to make much difference.
I hardly think it's a myth. Human bodies aren't as simple as you're suggesting. It's a complex process but no water found in the natural world is going to be pure H2O so it makes since to be cautious of water that is going to be so different from nature.
From the WHO report[1] on water quality:
"Sufficient evidence is now available to confirm the health consequences from drinking water deficient in calcium or magnesium. Many studies show that higher water magnesium is related to decreased risks for CVD and especially for sudden death from CVD. This relationship has been independently described in epidemiological studies with different study designs, performed in different areas, different populations, and at different times. The consistent epidemiological observations are supported by the data from autopsy, clinical, and animal studies. Biological plausibility for a protective effect of magnesium is substantial, but the specificity is less evident due to the multifactorial aetiology of CVD."
There are two completely-unrelated assertions in that paragraph. I believe one of them -- that dietary Ca and Mg are important -- but not the part about water being an important source of either. The numbers simply make no sense.
