The pH of drinking water poses few direct health risks. However, the pH can strongly influence the concentrations of other contaminants in the water. Many metals, for example, tend to dissolve in acidic water, and many common metals are toxic at high concentrations.
The pH of any given water supply depends primarily on local geology. Regions with large deposits of limestone and other alkaline minerals will tend to produce groundwater with a slightly basic pH. Other regions that lack limestone will tend to produce slightly acidic groundwater.
pH
The pH of water-based solutions operates on a 0-to-14 scale where a pH of 7 represents neutral, meaning neither acidic nor basic. A pH greater than 7 indicates a basic solution, and a pH less than 7 indicates an acidic solution. The pH scale is logarithmic, which means that water with a pH of 6 contains 10 times the acid as water with a pH of 7, and water with a pH of 5 contains 100 times the acid as water with a pH of 7.
pH and Drinking Water
Pure water exhibits a pH of exactly 7. Pure water, however, behaves as an aggressive solvent. Pure water therefore tends to not remain pure when it is exposed to any source of contamination. Virtually any water that is exposed to air, for example, will absorb carbon dioxide. The carbon dioxide molecules combine with water molecules to form carbonic acid, which results in a decrease in pH. Rain absorbs enough carbon dioxide to lower its pH to about 5.6. Similarly, water that contacts limestone will dissolve minerals, particularly calcium carbonate, that raise pH.
The pH of drinking water also depends on the disinfection method used at the municipal water supply facility. Some chemical disinfection methods only work effectively within certain pH ranges, and the treatment facility may have to adjust pH prior to treatment. Chlorine, for example, loses its effectiveness against bacteria if the water pH exceeds 8.5. Furthermore, the treatment process itself may alter the water's pH.
EPA Guidelines
The U.S. Environmental Protection Agency recommends a pH level between 6.5 and 8.5 for drinking water. The EPA, however, only stipulates pH levels as secondary standards, which means the agency does not enforce the standards, but rather issues them for guidance only. Most states have adopted enforced drinking water standards for municipal water supplies, but the allowed pH range is typically much broader than the EPA's recommendation. Connecticut, for example, allows pH levels of 6.4 to 10.0.
Effects of High pH
Water with high pH typically contains large amounts of dissolved minerals, particularly calcium and magnesium. These represent the principal components of "hard" water. The minerals do not pose a direct health threat, but they cause other problems, such as scale buildup inside water pipes, soap scum, and decreased effectiveness of soaps and detergents.
Effects of Low pH
Acid water with a pH less than 6.5 poses no direct health threat. However, acidic water tends to dissolve metals, including lead, copper, zinc, iron, manganese and other metals commonly found in household water-supply pipes. This process also results in corrosion of the pipes.
References
- Utah State University Extension: pH
- University of Kentucky: Study Shows Regional Variations in the pH of Kentucky Groundwater
- Connecticut Department of Public Health: Private Drinking Water in Connecticut
- Ohio State University: Chlorine
- U.S. Environmental Protection Agency: Drinking Water Contaminants
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