Chloride is a naturally occurring ion present in virtually all tap water supplies. This negatively charged ion, represented chemically as Cl⁻, is a form of salt found dissolved in water, most commonly as sodium chloride, which is ordinary table salt. Because of its high solubility and natural abundance, its presence in drinking water is expected and generally harmless at the typical concentrations found in municipal systems. Chloride is not considered a direct health risk at normal levels, but it is routinely monitored for its effects on the taste and physical properties of the water.
Chloride vs. Chlorine: Clarifying the Difference
The similar-sounding terms “chloride” and “chlorine” describe substances with fundamentally different chemical properties and roles in water treatment. Chlorine is a highly reactive chemical element, which in its elemental form is a toxic, greenish-yellow gas rarely found freely in nature. It is purposefully introduced into water supplies for disinfection, where it acts as a powerful oxidizing agent to kill bacteria and viruses.
In contrast, chloride is the stable, negatively charged ion that forms when a chlorine atom gains an extra electron, achieving a balanced and stable configuration. This makes chloride mostly non-reactive in water and unable to disinfect. The chloride ion is an electrolyte and a component of neutral salts, such as sodium chloride or potassium chloride, which are abundant in the earth’s crust.
The disinfection process itself can also contribute to the chloride concentration in water. When chlorine or a chlorine-containing compound like sodium hypochlorite is added to water, it eventually forms chloride ions as it reacts and breaks down. However, the vast majority of chloride in tap water is not directly from the disinfectant, but rather from naturally dissolved salts. Understanding this distinction is important, as one is the stable byproduct of a salt, and the other is the active, reactive agent used for purification.
Natural and Treatment Sources of Chloride
The presence of chloride in water originates from a combination of natural geological processes and various human activities. Naturally, chloride is released into water as it moves over and through the earth, dissolving chloride-containing minerals in bedrock and soil. Groundwater in particular can acquire chloride from deep, salt-bearing geological formations through a process called mineral weathering. In coastal regions, the intrusion of salty ocean water into freshwater aquifers is a significant natural source of elevated chloride levels.
Anthropogenic sources introduce additional chloride into the water cycle from wastewater and industrial processes. Runoff from roads treated with de-icing salts, which are typically sodium chloride or calcium chloride, is a major contributor, especially in colder climates. Septic system effluent and sewage also contain high levels of chloride, which can be an indicator of contamination if concentrations are abnormally high. Furthermore, the regeneration cycle of water softeners, which use sodium chloride salt to remove hardness minerals, discharges brine containing high levels of chloride into the environment.
Health and Aesthetic Considerations
Chloride in drinking water is generally not a direct concern for human health at typical concentrations, as it is an essential nutrient obtained primarily from food. The United States Environmental Protection Agency (EPA) has not set a mandatory health-based limit for chloride. Instead, the EPA established a guideline called the Secondary Maximum Contaminant Level (SMCL) for chloride at 250 milligrams per liter (mg/L).
This SMCL is based on aesthetic and technical considerations, rather than a health risk. The most noticeable effect of high chloride levels is a salty taste in the water, which most individuals can detect when concentrations exceed approximately 250 mg/L. This salty taste can make the water unpalatable, leading people to avoid drinking water that is otherwise safe.
Elevated chloride levels also pose a technical problem by increasing the corrosiveness of the water. High chloride concentrations can accelerate the deterioration of metal pipes, plumbing, and appliances, potentially causing fixture damage and increasing the leaching of heavy metals like lead and copper into the drinking water. Water systems monitor chloride, aiming to keep levels well below the 250 mg/L aesthetic standard.