The quality of tap water is fundamentally determined by its pH level, which measures acidity or alkalinity based on the concentration of hydrogen ions. The pH scale ranges from 0 to 14, where 7.0 signifies a neutral state. Readings below 7.0 indicate increasing acidity, while values above 7.0 represent rising alkalinity. Monitoring and controlling the pH of tap water is a necessary operational practice for public water systems.
Defining Acceptable pH Ranges
The U.S. Environmental Protection Agency (EPA) recommends that public water systems maintain a pH level within the range of 6.5 to 8.5. This range is established as a Secondary Drinking Water Regulation, addressing aesthetic and technical effects rather than primary health standards. The secondary standard manages issues that could make water unappealing or cause damage to plumbing infrastructure.
A pH level within the 6.5–8.5 spectrum minimizes the corrosive nature of the water and prevents the excessive buildup of mineral deposits. While the EPA’s secondary standards are not federally enforceable, they provide a strong technical goal for water treatment facilities. International organizations, such as the World Health Organization (WHO), also cite a similar pH range, recognizing the operational benefits for water systems.
Consequences of Deviations in Water pH
When the pH of water deviates below the recommended 6.5 level, the water becomes more acidic and can cause significant damage to the plumbing infrastructure. This acidic water is corrosive, meaning it can slowly dissolve the metal from pipes made of copper, iron, or galvanized steel. The corrosion process can lead to pinhole leaks and cause blue-green staining on sinks and fixtures due to dissolved copper. A more serious concern is the leaching of toxic metals, such as lead, from older service lines and solder into the drinking water. Acidic water may also impart an unpleasant metallic or sour taste.
Conversely, when the pH rises above the 8.5 mark, the water becomes highly alkaline, leading to a different set of problems. The primary issue with high-pH water is the tendency for mineral deposits, known as scale, to form inside pipes and water-using appliances. This scaling is the result of calcium and magnesium carbonates precipitating out, which can eventually restrict the flow of water and reduce the efficiency of electric water heaters. Highly alkaline water can also reduce the effectiveness of chlorine disinfectants used by water treatment plants. Consumers may also notice a slippery feeling or a bitter, soda-like taste in the water.
Factors Influencing Tap Water pH
The natural pH of source water is the initial factor determining the eventual pH of tap water. Rainwater is naturally slightly acidic because it absorbs atmospheric carbon dioxide, forming a weak carbonic acid. Conversely, groundwater that flows through geological formations rich in alkaline minerals, such as limestone, will typically have a higher, more alkaline pH. The concentration of dissolved carbon dioxide in the source water plays a significant role, as higher concentrations push the pH downward.
Water treatment plants actively adjust the pH to meet the required standards and minimize the corrosive potential of the water. To raise a low pH, treatment facilities may add alkaline chemicals such as lime, soda ash, or caustic soda. This process helps establish a protective layer on the inside of distribution pipes, preventing metal leaching and corrosion. If the source water is too alkaline, acidic compounds may be introduced to bring the pH back into the acceptable range. Careful pH control is a continuous process used to ensure water remains stable throughout the entire distribution network.