What is pH?
The pH value of a water source is a measure of its acidity or alkalinity. It is a measurement of the activity of the hydrogen atom which is a good representation of the acidity or alkalinity of the water. The pH scale, as shown below, ranges from 0 to 14, with 7.0 being neutral. Water with a low pH (< 7.0) is said to be acidic, and water with a high pH (> 7.0) is alkaline. Pure water would have a pH of 7.0, but water sources and precipitation tend to be slightly acidic due to contaminants.
The pH scale is logarithmic, which means that each step on the pH scale represents a ten-fold change in acidity. For example, a water body with a pH of 5.0 is ten times more acidic than water with a pH of 6.0. And water with a pH of 4.0 is 100 times more acidic than water with a pH of 6.0.
How does the pH of a water source change?
Surface water typically has a pH value between 6.5 and 8.5 and groundwater tends to have a pH between 6.0 and 8.5. The pH of a water source can vary naturally. Some types of rock and soil, such as limestone, can neutralize acid more effectively than other types of rock and soil, such as granite. Or, when there are a large number of plants growing in a lake or river, they release carbon dioxide when they die and decompose. When the carbon dioxide mixes with the water, a weak carbonic acid is formed; this can then cause the pH of the water body to decrease.
Many human activities have a harmful effect on the pH of nearby water sources. When sulphur dioxide and nitrogen oxides are emitted, through industrial operations and vehicles, acid rain can be produced. For more information about acid rain, see the Acid Rain fact sheet.
Chemical pollution, from industrial operations, individuals and communities, can cause a water body to become acidic. These chemicals can enter the water through illegal discharges or after inadequate wastewater treatment. For more information about chemical pollution, including ways in which you can minimize pollution, see the Water Pollution fact sheet.
What happens when the pH of the water changes?
A change in the pH of water can have a number of consequences. In the environment, many plants and animals are harmed, or even killed, as a result of acidification. Many varieties of fish and aquatic life are extremely sensitive to changes in water temperature and composition. The below diagram illustrates the pH that is required for a number of aquatic species. Notice that when the pH is around 6.0 to 7.0 (which is natural for many lakes and streams), the biodiversity within the ecosystem is vast. As the pH decreases and the acidity increases, fewer and fewer organisms can survive.
Acidic water is synergistic. It means that a combination of a low pH and an increased concentration of certain substances is far more harmful than the sum of the parts. For example, aluminium, lead and mercury are potentially dangerous substances. But when the pH of the water source is already low, these substances can have extremely detrimental consequences for aquatic life.
Acidic water can also cause problems for human consumption. While slightly acidic water is not dangerous, on its own, it can be quite dangerous when combined with other compounds. Water with a pH that is less than 6.5 can leach metal ions, including iron, manganese, copper, lead and zinc from plumbing fixtures and pipes. This, in return, can be quite dangerous. On the other end of the pH scale, water that has a pH greater than 8.0 can be difficult to disinfect. The World Health Organization recommends that the pH of the water be less than 8.0 because basic water does not allow for effective chlorination.
How do water treatment facilities change the pH of water?
Several methods can increase the pH of water, before disinfection. The pH is commonly increased using sodium carbonate and sodium hydroxide. However, a better way of dealing with low pH is to use calcium and magnesium carbonate, which not only will increase pH levels but will also make the water less corrosive. Also, calcium and magnesium are of health benefits as opposed to sodium.
To access more information, visit the Safe Drinking Water Foundation website at www.safewater.org