Sodium is present in water, not as the reactive elemental metal, but as a dissolved, positively charged ion (Na+). This sodium ion is a mineral, often combined with other elements like chloride to form highly soluble salts. The presence of sodium is a natural result of water’s journey through the environment and the processes used to make it safe for consumption.
Why Sodium is Naturally Present
Water is an excellent solvent, naturally picking up and dissolving minerals as it moves through the environment. Sodium is the sixth most abundant element in the Earth’s crust, existing in various rock formations and soils. As groundwater flows through deep geological deposits, it leaches sodium ions from minerals like feldspars and halite (rock salt) through mineral dissolution.
The natural concentration of sodium is heavily influenced by geographical location and local geology. For example, coastal areas experience a natural increase in sodium due to the intrusion of salty ocean water into freshwater aquifers or from wind-blown sea spray. In other regions, the dissolution of large evaporite deposits of salt can introduce significant quantities of sodium into the water supply.
Water Treatment and Infrastructure Contributions
Beyond natural sources, human activities and utility processes introduce additional sodium into the supply. Water purification often involves the use of sodium-containing chemicals for disinfection and pH adjustment. For instance, sodium hypochlorite is used to kill pathogens, and sodium hydroxide may be added to control the water’s acidity, both increasing the overall sodium concentration.
A significant contributor to localized high sodium levels is the use of home or municipal water softening systems. These systems use ion exchange to reduce water hardness by replacing hard minerals like calcium and magnesium with sodium ions. The amount of sodium added is directly proportional to the water’s original hardness.
Other human-made inputs include the runoff from road de-icing salts, which can infiltrate water sources, especially in colder climates. Leaching from aging infrastructure is a less common source compared to intentional chemical additions and softening processes.
Comparing Sodium Levels in Consumer Water
Sodium concentrations vary widely depending on the source and treatment method, typically measured in milligrams per liter (mg/L), which is equivalent to parts per million (ppm). Standard municipal tap water usually contains less than 50 mg/L of sodium, with many systems reporting concentrations below 20 mg/L. However, in specific regions with naturally salty water or heavy use of softeners, levels can sometimes exceed 250 mg/L.
The sodium content of bottled water is unpredictable and requires checking the label. Purified or distilled waters, which have undergone processes like reverse osmosis, often contain negligible amounts of sodium. In contrast, certain mineral waters with high mineralization can naturally contain much higher levels, sometimes ranging from 100 mg/L to over 1,000 mg/L. For example, water with moderate hardness run through a softener can add approximately 20 mg of sodium per eight-ounce glass.
Does Water Sodium Affect Your Diet?
For the majority of the population, the sodium consumed through drinking water contributes only a small fraction to the total daily intake. The average adult consumes roughly 3,400 milligrams of sodium per day, primarily from processed foods. Drinking two liters of water containing 20 mg/L of sodium contributes only 40 milligrams, which is generally not a health concern for healthy individuals.
Concerns about water sodium primarily arise for two specific groups: individuals on strict, physician-ordered low-sodium diets and those preparing infant formula. People advised to limit their daily intake to 500 milligrams or less may need to consider water if its concentration is high. For infants, mixing formula with water containing high sodium levels can be problematic; a physician should be consulted if the supply exceeds 20 mg/L. In cases of high water sodium, treatment methods like reverse osmosis filtration can effectively remove up to 99% of dissolved salts.