Tap water can be a significant source of both magnesium and calcium. One recent study found that 2 liters of some municipal tap waters could provide up to 30% of the magnesium DRI (Daily Recommended Intake) for adults. But many tap waters are quite low in magnesium, providing no significant magnesium source. How can you tell?

We have had a geologist, Dennis Hibbert, compile recent values for magnesium and calcium from 20 U.S. cities, and they are listed below. These values are imprecise and of variable accuracy. They depend upon averages and then averages of averages, and there is no uniformity in methods of either analysis or calculation. Beyond this, there’s a story for each of these numbers. For example, in Honolulu municipal water is drawn from several different wells. The wells close to the mountains have, in general, more magnesium than calcium while those closer to the ocean have slightly more calcium than magnesium.

All these Honolulu well sources are interconnected in the municipal water system so that one could get mountain source water one day and seacoast source water the next. Your city or town undoubtedly has its own story. To get any real idea of your local water supply’s magnesium, you should call the water company and interview someone in the chemistry or analytical laboratory. There are some general trends. For the most part, the recent values Dennis compiled fall into the same order, generally, as values from a large survey of USA cities’ water conducted in 1962 and published in the Geologic Survey Water-Supply paper #1812. This lends credibility to both sets of numbers. The other pertinent fact is that in both 1962 and 2000, in all cities except Honolulu, the calcium is much higher than the magnesium. It is interesting to note that Hawaii has traditionally had one of the lowest rates of heart disease death of all 50 United States.

City State Magnesium ppm Calcium ppm Hardness ppm
Atlanta GA 0.3* 7.3 30
Seattle WA 0.5 20 23
Portland OR 0.64 1.8 6.6
Oakland CA 2.8 9 35
San Francisco CA 3.4 8.9 37
Dallas TX 3.8 51 134
Memphis TN 4.5 7.9 45.4
Denver CO 6.2 25 80
New York NY 6.4 16 67.4
Washington DC 8 40 135
Cleveland OH 8.8 32.5 116
Chicago IL 11.1 35.2 143.7
Los Angeles CA 12.2 16.3 137.3
St. Louis MO 12.6 26.4 124.5
Austin TX 15 15 104
San Diego CA 17.7 44.9 202.3
Lincoln NE 19 64 220
Honolulu HI 6 – 23 9 – 26 – – –
Salt Lake City UT 30.2 78.9 311.9
New Orleans LA 41 109 150

These values, ppm, are parts per million or milligrams per liter of water. You can assume that you drink 2 to 2 ½ liters of tap water per day, and calculate the milligrams of magnesium you daily drink with your tap water. You can also calculate the amount of calcium your tap water provides you daily.

If your city is not on this list, you can call your water department and ask for the latest values for magnesium and calcium. They may have only hardness, which is a combination of both the calcium and magnesium. If you have a value for calcium and hardness, you can use a calculation to determine the magnesium, which sometimes works, but not always. Here is the calculation.

Calculation: Multiply the calcium concentration in ppm by 2.5. Subtract this number from “hardness.” Divide the result by 4.1. This should give you a rough estimate of the magnesium in that water. But it assumes a lot of chemistry which may not hold for your situation, so do not be too surprised if you come up with a crazy number such as a negative magnesium value.

Some municipalities soften their water. Some households purchase systems that soften the water at point of use, or POU, as it flows into the home. The water softening process allows soap to form suds. Calcium and magnesium bind with soap so that it will not make suds, but instead produce a scum that can stay on clothes and dishes, etc., leaving them grungy. When water is softened, magnesium and calcium are removed and sodium is added. Sodium does not impede suds formation.

Bottled Water Sources

The range of magnesium, calcium and sodium levels in bottled waters is enormous.

To determine the magnesium, calcium and sodium content of a bottled water, either European or North American, use the following websites:

https://www.pmgeiser.ch/cgi-bin/mineral?sort=mg

https://www.mgwater.com/waters.shtml

You may also get this information from The Good Water Guide by Maureen & Timothy Green (available via Amazon.com for $7.42) which details magnesium, calcium and sodium levels for 73 European bottled waters. Another source is The Pocket Guide to Bottled Water by Arthur von Wiesenberger, 1991, also available via Amazon.com.

Many bottled waters in North America are distilled or deionized water, and as such they have almost zero magnesium. Often a bit of sodium is added for taste. There are products designed to “fortify” such deionized or distilled water with magnesium and sometimes other essential elements often available in health food stores.

References:

Azoulay A, Garzon P, and Eisenberg MJ, Comparison of the Mineral Content of Tap Water and Bottled Waters, J Gen Intern Med 2001;16:168-175. https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=11318912

Hibbert, D, Mineral Levels of 20 Municipal Water Supplies, 2002, Personal Communication.

Mason, Paul, Magnesium@IX.netcome.com, personal communication.

Public Water Supplies of the 100 Largest Cities in the United States, 1962. Geologic Survey Water-Supply paper #1812.

________, Water (Industrial Treatment) in Kirk-Othmer Encycl Chem, 1984 Ed3, Vol 24, John Wiley & Sons, Inc. pp 375-399.

Faust SD and Aly OM, Chemistry of Water Treatment, 1983, Butterworths – Boston, London, Sydney, Wellington, Durban, Toronto, and Ann Arbor Science Book.