If this were 10,000 B.C., you'd get your drinking water from streams, rivers, and lakes, all rich in mineral content. Humans became reliant on obtaining a considerable proportion of daily mineral needs from natural water sources.
21st century: We obtain drinking water from a spigot or plastic bottle. Pesticides and other chemicals seep into the water supply. Municipal water purification facilities have intensified water purification in most communities to remove contaminants like lead, pesticide residues, and nitrates. (For a really neat listing of the water quality of various cities , the University of Cincinnati makes this data available.)
But intensive water treatment also removes minerals like calcium and magnesium.
Many people have added water filters or purifiers to their homes,, like reverse osmosis and distillation, that are efficient at extracting any remaining minerals, converting “hard” into “soft” water. In fact, manufacturers of such devices boast of their power to yield pure water free of any “contaminant,” minerals like magnesium included. The magnesium content of water after passing through most commercial filters is zero .
Modern enthusiasm for bottled water has compounded the problem. Americans consumed a lot of bottled water, nearly 8 billion gallons last year. In the U.S., nearly all bottled water has little or no magnesium .
The result is that we can no longer rely on drinking water to provide magnesium. The Recommended Daily Allowance (RDA)—the amount required to prevent severe deficiency—for magnesium is 420 mg per day for men, 320 mg/day for women. In cities with the highest magnesium water content, only 30% of the RDA can be obtained by drinking two liters of tap water per day. In most cities, only a meager 10–20% of the daily requirement can be obtained. That leaves between 70–90% that needs to come from other sources. As a result, the average American ingests substantially less than the RDA.