The term “pH” measures the concentration of electrically charged particles in a substance, indicating its acidity or alkalinity. This scale ranges from 0 to 14, where 7.0 is neutral. Values below 7.0 are acidic, and values above 7.0 are alkaline.
Tap water is generally not acidic when it reaches the consumer, but its pH is highly variable depending on the source and treatment. Most municipal systems deliver water that is neutral or slightly alkaline, typically falling within a range of pH 7.0 to 8.5. The presence of dissolved minerals, gases, and other compounds naturally shifts the pH, making the exact acidity or alkalinity a localized characteristic.
Defining Acidity: What is the Typical pH Range of Tap Water?
The pH scale is logarithmic, meaning each whole number change represents a tenfold change in the concentration of hydrogen ions, which determine acidity. For example, water with a pH of 6 is ten times more acidic than water with a pH of 7. Small numerical shifts, therefore, represent substantial chemical differences.
The U.S. Environmental Protection Agency (EPA) recommends that public water supplies maintain a pH between 6.5 and 8.5. This range is part of the National Secondary Drinking Water Regulations, which are non-enforceable guidelines focused on aesthetic qualities like taste and odor. The goal is to manage water quality for consumer acceptance and to protect the distribution infrastructure.
Water providers often actively adjust the pH of distributed water to be slightly alkaline, typically aiming for the upper part of the recommended range (7.5 to 8.5). This slight alkalinity helps minimize the corrosive potential of the water as it travels through the pipes. Maintaining this higher pH protects the entire water system.
Factors That Influence Local Water pH
The initial pH of water is dictated by the geology of its source, whether groundwater or a surface source. Water flowing through limestone bedrock (rich in calcium carbonate) tends to become naturally alkaline as it dissolves these buffering minerals. Conversely, water sourced from areas with granite bedrock or high organic matter content often has a lower, more acidic pH due to less natural buffering capacity.
Water treatment processes also influence the final pH before the water enters the distribution system. Utilities add various chemicals, such as coagulants and disinfectants, which can slightly alter the water’s acidity or alkalinity. Treatment plants monitor and adjust the pH by adding substances like lime or soda ash to stabilize the water and ensure it meets delivery standards.
The last significant factor influencing the pH is the infrastructure itself, particularly the piping inside buildings. Even if the water leaves the treatment plant with a balanced pH, the materials in the distribution lines can cause a shift. Older metal pipes, especially copper or galvanized steel, can react with the water, causing small fluctuations in pH right before the water reaches the tap.
The Practical Effects of Acidic or Alkaline Water
Water that is too acidic (pH below 6.5) is corrosive, meaning it can gradually dissolve the metals in plumbing systems. This corrosion leads to leaks and the leaching of heavy metals like copper, zinc, and lead into the drinking water. The presence of these metals can give the water an undesirable metallic or sour taste.
The indirect effect of acidic water is the main health concern because the consumption of heavy metals like lead can cause serious health problems. Signs of corrosion include blue-green stains in sinks (from copper pipes) or rust-colored stains (from iron pipes). The EPA’s Lead and Copper Rule focuses on corrosion control because low pH water facilitates the release of these toxic substances.
On the other side of the scale, alkaline water (pH above 8.5) can lead to scaling—the buildup of mineral deposits. This mineral buildup, primarily calcium carbonate, coats the inside of pipes, reducing water pressure and clogging fixtures. Scaling also decreases the efficiency of appliances like water heaters by insulating the heating elements.
Alkaline water is often associated with hard water, which is high in dissolved minerals, and may impart a slightly bitter or soda-like taste. While the direct consumption of high pH water is generally not a health hazard, scaling can cause cosmetic issues such as spots on dishes and dry skin or hair. The primary concern for both acidic and alkaline water relates to its effect on plumbing and the potential for indirect contamination or damage to home appliances.