Water alkalinity is a measure of a water body’s capacity to resist changes in its acidity, known as buffering capacity. It is distinct from pH, which measures the water’s current level of acidity or basicity on a scale of 0 to 14. Alkalinity quantifies the amount of dissolved alkaline compounds that can neutralize incoming acids, thereby stabilizing the pH level. The primary cause of high alkalinity is the dissolution of specific alkaline compounds into the water as it moves through the environment. The concentration of these dissolved components dictates how much acid a water body can absorb.
The Chemical Composition of Alkalinity
Alkalinity is an aggregate property determined by the concentration of several dissolved ions, not a single chemical substance. The most significant contributors in natural water systems are bicarbonate (HCO3-) and carbonate (CO3 2-) ions. Hydroxide ions (OH-) also contribute, though usually in lower concentrations than bicarbonates.
The total amount of these acid-neutralizing ions determines the water’s alkalinity level. These ions exist in a dynamic equilibrium, shifting their forms in response to the water’s pH and dissolved carbon dioxide (CO2). For example, when an acid is introduced, bicarbonate ions react with the hydrogen ions, neutralizing the acid and maintaining the original pH. Alkalinity is typically measured and reported as milligrams of calcium carbonate (CaCO3) per liter.
Geological and Mineral Sources
The most widespread source of high alkalinity in water is the natural interaction between water and certain types of rock and soil, a process called chemical weathering. This occurs when water flows over or through the Earth’s crust. Water that contains dissolved carbon dioxide forms carbonic acid, a weak acid able to dissolve alkaline minerals.
When this slightly acidic water encounters carbonate bedrock, such as limestone (calcium carbonate, CaCO3) or dolomite, the chemical reaction releases alkaline ions. The carbonic acid reacts with these solid minerals, dissolving them and producing soluble bicarbonate ions that are carried away by the water flow.
Groundwater is particularly susceptible to this process, as it spends extended periods in contact with subsurface rock layers. Water that emerges in springs or feeds into rivers and lakes will have naturally elevated levels of alkalinity. Watersheds situated on these carbonate-rich geological formations are strong predictors of high-alkalinity water.
Human Activity and Biological Processes
While geology is the main driver, human activities and natural biological cycles also contribute to elevated water alkalinity.
Human Activities
Certain industrial processes, such as concrete production and mining, can discharge effluent containing high levels of alkaline compounds into surface waters. Agricultural runoff can also carry lime or high-pH fertilizers from treated fields into waterways, increasing the concentration of alkaline ions.
Water treatment facilities intentionally increase alkalinity to achieve specific water quality goals. This is often done by adding chemicals like lime or soda ash to the water supply to control corrosion in distribution pipes. The added alkalinity helps form a protective layer on the inside of the pipes, minimizing the leaching of metals.
Biological Processes
A significant biological process occurs in surface water bodies with abundant plant life and algae. During the day, aquatic plants consume dissolved carbon dioxide (CO2) for photosynthesis. The removal of CO2 shifts the carbonate equilibrium, reducing the amount of carbonic acid present. This shift results in a higher concentration of carbonate and hydroxide ions, which consequently raises the water’s alkalinity and pH in a daily cycle.
Practical Effects of Elevated Alkalinity
Elevated alkalinity levels have several practical consequences that affect both infrastructure and the environment. One common issue is the formation of scale, the mineral buildup of calcium and magnesium carbonate on surfaces. This scale can clog pipes, reduce the efficiency of water heaters, and shorten the lifespan of household appliances.
Alkalinity can also affect the aesthetic qualities of drinking water, with very high levels sometimes imparting a “soapy” or “bitter” taste. However, the buffering capacity is beneficial for aquatic ecosystems, as it prevents sudden, harmful shifts in pH caused by acid rain or chemical spills. The buffering action protects sensitive aquatic life, like fish, which require a stable pH range to survive.