Chlorides are common, naturally occurring inorganic compounds found universally in water sources. They are composed of the element chlorine bonded with another element, forming a salt that dissolves in water. The chloride ion, represented as Cl-, is a negatively charged ion, or anion. Monitoring chloride concentration is important for maintaining the quality and usability of drinking water systems. While not typically a direct human health concern at common levels, elevated concentrations can negatively affect water acceptance and plumbing infrastructure.
Defining the Chloride Ion
The chloride ion (Cl-) is formed when chlorine gains one electron, achieving a stable negative charge. This ion is always paired with a positively charged ion (cation) to form a neutral salt, such as sodium chloride (table salt) or potassium chloride. These salts are highly soluble and dissociate completely when they dissolve in water, releasing the chloride ions.
It is important to distinguish the chloride ion from elemental chlorine (Cl2) and chlorine-based disinfectants. Elemental chlorine is a highly reactive, yellow-green gas used in industrial applications and water treatment. Chlorine-based disinfectants, such as sodium hypochlorite (bleach), are strong oxidizing agents used to kill pathogens. Unlike the stable chloride ion, these disinfectants are chemically active and rapidly break down.
Where Chlorides Originate
Chlorides enter water systems through a combination of natural and human-related processes. Naturally, chloride ions are leached into water as it passes over and through salt-bearing geological formations and sedimentary rocks. Ancient marine deposits deep within the earth can also contribute high concentrations of chlorides to groundwater. Coastal areas frequently see elevated chloride levels due to the intrusion of saltwater into freshwater aquifers and the deposition of sea spray.
Human activities have become a significant source of chloride pollution in many municipal water supplies. The widespread use of road de-icing salts during winter is a major anthropogenic contributor, with runoff carrying sodium and calcium chlorides into surface water and groundwater. Wastewater effluent from sewage treatment plants also contains chlorides, primarily from human waste and the use of home water softeners. Water softeners operate by exchanging hardness ions with sodium or potassium, which are eventually discharged as a chloride-rich brine into septic systems or sewer lines.
How Chlorides Affect Water Quality
The most immediate and noticeable effect of high chloride levels is the taste of the water. For most people, a salty or metallic taste becomes detectable when the concentration exceeds the range of 200 to 300 milligrams per liter (mg/L). This taste threshold can vary depending on the type of positive ion (cation) paired with the chloride. While the taste is not a direct health threat, it often leads to consumer rejection of the water supply.
Beyond taste, chlorides are highly corrosive to metal plumbing and infrastructure. Chloride ions increase the electrical conductivity of water, which accelerates the electrochemical reactions that cause corrosion. This is particularly damaging to pipes made of galvanized iron, copper, and stainless steel. Increased chloride concentrations promote pitting corrosion, which creates localized holes in the pipe walls, and galvanic corrosion, which accelerates the deterioration of metal interfaces.
Increasing chloride levels can significantly increase the rate of pipe deterioration. High chloride levels can also indirectly compromise water quality by causing pipes to leach metals like iron, copper, or lead into the drinking water.
Regulatory Standards and Removal Methods
The concentration of chlorides in water is typically measured in milligrams per liter (mg/L). In many regulatory frameworks, chlorides are not considered a primary health concern at typical concentrations. Therefore, they are often regulated under the category of Secondary Maximum Contaminant Levels (SMCLs). SMCLs are non-enforceable guidelines set to address aesthetic qualities, such as taste, odor, and appearance, as well as the corrosive nature of the water.
The standard SMCL for chloride established by the U.S. Environmental Protection Agency (EPA) is 250 mg/L. This limit is primarily based on the aesthetic consideration of taste, as concentrations above this level are likely to impart a salty flavor. Public water systems exceeding this benchmark are advised to manage water quality to improve public acceptance, though no federal enforcement action is mandated.
For homeowners concerned about elevated chloride levels, there are effective methods for reduction. Reverse Osmosis (RO) systems are highly effective, using a semipermeable membrane to filter out dissolved salts and solids, including chlorides. Distillation is another viable option, as it involves boiling the water and collecting the purified steam, leaving the non-volatile chloride salts behind. Standard treatment methods like carbon filtration and traditional water softeners are generally ineffective for reducing chloride concentrations.