>Using an average calcium concentration in public water supplies of 26 mg/liter and a maximum of 145 mg/liter (Durfor and Becker, 1964) and assuming that the average adult drinks 2 liters of this water daily, then the drinking water could contribute an average of 52 mg/day and a maximum of 290 mg/day. On an average basis this would represent 5% to 10% of the usual daily intake or approximately 6.5% of the adult RDA.
>Therefore, typical drinking water in the United States, Canada, or Europe provides approximately 3% to 7% of the RDA for magnesium intake by a healthy human.
https://www.ncbi.nlm.nih.gov/books/NBK216589/
I don't think 5% of the RDA really moves the needle either way and those are the major minerals in tap water.
I feel better going back on tap water, fewer stomach issues. I don't have a proper theory that can explain why this is the case, but it's certainly the case.