What is it?
- Elemental sodium is a soft, silver-gray metal, with the atomic number 11 and atomic weight 22.99 Daltons. Sodium’s elemental symbol, Na, stands for natrium.
- The free element is never found in nature because it reacts rapidly with water to produce sodium hydroxide and hydrogen, so it is only found in salts.
- Its abundance in the earth’s crust is about 2.3 percent as ocean salinity and terrestrial salt deposits. It ranks sixth among elements in nature.
- Typical open ocean sodium ion concentrations are in the range of 10,500 parts per million (ppm), but some confined seas (such as the Persian Gulf) reach almost 16,000 ppm. The Great Salt Lake is about seven times more saline than seawater.
- About 280,000,000 tons of sodium chloride are mined each year.
- Most sodium salts are very soluble in water, for example, sodium chloride’s (NaCl) water solubility is 357 grams per liter (g/l) at 0oC.
Occurrence in drinking water
- Typical drinking waters contain less than 20 milligrams per liter (mg/l) of sodium, but some groundwater can be substantially greater, usually reflected in total dissolved solids (TDS) values.
- The National Inorganics and Radionuclides Survey reported in 1988 that about ¾ of 989 groundwater drinking water systems had concentrations of sodium less than 50 mg/l.
- Runoff from road salt use can significantly increase sodium in surface waters.
- Sodium was listed in the U.S. Environmental Protection Agency’s 2012 Contaminant Candidate List, not with likelihood that it would be regulated, but that the guidance should be reconsidered.
- Sodium is an essential nutrient and has numerous functions including maintaining ionic balance and interacting with potassium, calcium and magnesium for cardiovascular function.
- Sodium is excreted in urine and perspiration along with other salts.
- Inadequate sodium intake or excessive losses from perspiration without replacement can result in hyponatremia, which can be fatal.
- The National Research Council recommends that most healthy adults consume at least 500 mg per day, but no more than 2,400 mg per day. That level is usually exceeded in the typical American diet.
- Excess salinity in water results in adverse taste. Thresholds are about 150 mg/l for sodium chloride, 220 mg/l for sodium sulfate and 420 mg/l for sodium bicarbonate.
- Atomic absorption spectrophotometry and flame photometry measure sodium in water. Low-cost test kits are also available.
- Sodium imparts a characteristic yellow color in flame tests and fireworks, as well is in low-pressure sodium vapor lamps.
- Sodium ions and TDS in drinking water are most readily removed by desalination processes such as reverse osmosis membranes.
- Distillation units can reduce all salts including sodium.
- Cationic water softeners replace the calcium or magnesium ions in hard water with sodium, or sometimes potassium if potassium salts are part of the regenerations.
- No health- or aesthetics-based drinking water standard for sodium exists in the U.S. The guidance of 20 mg/l for persons on a 500 mg per day sodium-restricted diet is essentially a suggestion since the diet is the dominant source of sodium intake, and people on sodium-restricted diets or with hypertension are treated with diuretics. The guidance level will likely be raised.
- The World Health Organization does not have a recommended level for drinking water because it finds no firm conclusion that relates sodium in drinking water to hypertension. The taste-based guideline is 200 mg/l.
Dr. Joe Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the EPA Drinking Water Standards Division.