The acidity of tap water is determined by its pH level, which measures the concentration of hydrogen ions in the water. The pH scale ranges from 0 to 14, where 7.0 is considered neutral. Values below 7.0 are acidic, while values above 7.0 are basic or alkaline. Pure, distilled water has a neutral pH of exactly 7.0, but water found in nature and in municipal systems rarely matches this point. Understanding your tap water’s pH is important for both the longevity of your home’s plumbing and your health.
Understanding the pH Scale and Tap Water Standards
The acidity or alkalinity of drinking water is measured on the logarithmic pH scale, meaning each whole number change represents a tenfold difference in hydrogen ion concentration. Water with a pH of 6.0, for instance, is ten times more acidic than water with a pH of 7.0. This sensitivity explains why small shifts in pH can significantly impact water quality and corrosiveness.
The optimal range for public drinking water is between 6.5 and 8.5 pH units. This range is recommended by the U.S. Environmental Protection Agency (EPA) to ensure water palatability and protect the network of pipes that deliver water to homes. The EPA classifies pH as a secondary drinking water standard, meaning it is regulated based on aesthetic or technical effects rather than immediate health risks.
Maintaining the pH within this band helps prevent water from tasting metallic, which is a common complaint with overly acidic water. It also plays a significant role in minimizing the corrosive action of water on pipes, a technical concern that can indirectly lead to serious health issues. Water that is too far outside of the recommended range can cause problems, even if the pH itself is not considered a primary health contaminant.
Factors Causing pH Variation in Tap Water
The natural source of the water is a primary determinant of its initial pH level before treatment. Groundwater filtering through mineral-rich rock formations often picks up carbonates and bicarbonates, increasing the pH and resulting in alkaline water. Conversely, surface water from rivers or reservoirs is often more acidic because it absorbs atmospheric carbon dioxide (\(CO_2\)), which forms a weak carbonic acid.
The presence of dissolved carbon dioxide (\(CO_2\)) is a common factor causing pH variation. While this process naturally occurs in the environment, it can also result from certain water treatment processes or microbial activity, further lowering the water’s overall pH level.
Finally, the chemicals used in the water treatment process can temporarily alter the pH. For example, disinfectants or coagulants used to remove impurities may slightly lower the pH. However, treatment plants actively monitor and adjust these levels before the water enters the distribution system to ensure the final product meets the recommended standards.
Practical Effects of Acidic Water on Plumbing and Health
Water that is overly acidic (pH below 6.5) is known as corrosive water. The primary concern is its destructive interaction with the metal pipes and fixtures in a home’s plumbing system, accelerating the degradation of materials like copper, galvanized steel, and lead solder.
As the water corrodes the pipes, it causes the leaching of heavy metals into the drinking supply, which poses the most significant health risk. Low pH water can dissolve lead from old pipes or solder and copper from modern plumbing, introducing these toxic substances. Lead exposure is linked to developmental and neurological problems, especially in children, making the indirect effect of acidic water a public health concern.
The corrosion also produces noticeable aesthetic issues that serve as warning signs. The leaching of copper commonly results in blue-green staining on porcelain fixtures. Dissolution of iron from galvanized pipes can leave rust-colored or reddish-brown stains. Acidic water may also impart an unpleasant metallic or sour taste.
How Water Treatment Plants Control pH Levels
Public water systems view pH adjustment as a primary method for corrosion control within their extensive network of pipes. To counteract the corrosive tendencies of acidic water, utilities raise the pH by increasing the water’s alkalinity before distribution. This practice helps form a protective layer on the inside of metal pipes, minimizing the potential for metal leaching.
Alkaline substances are commonly added to the water to achieve this adjustment. These neutralizing agents include lime (calcium hydroxide), soda ash (sodium carbonate), and caustic soda (sodium hydroxide). The addition of these compounds helps neutralize the acidity and maintain the water’s pH within the target range of 6.5 to 8.5.
The precise control of pH is also important for optimizing other steps in the water purification process. Processes like coagulation, flocculation, and disinfection rely on the water being within a specific pH range to work effectively. By adjusting the pH, treatment plants ensure the safety and quality of the water while simultaneously protecting the infrastructure that delivers it.