Alkalinity is a measure of water quality often confused with the more familiar concept of pH. It is defined as the water’s capacity to neutralize acids, acting as a natural buffer against pH changes. Water systems commonly test for alkalinity because it influences the stability and treatability of the water supply and infrastructure. Alkalinity is primarily considered a secondary measure rather than a primary health concern.
Alkalinity vs. pH: Defining the Difference
The terms alkalinity and pH describe different properties of water chemistry. The pH value quantifies the activity of hydrogen ions within the water, indicating how acidic or basic the water currently is on a logarithmic scale from 0 to 14. A pH of 7 is neutral, with values below 7 being acidic and values above 7 being basic or alkaline.
Alkalinity, by contrast, measures the water’s capacity to resist a change in pH when an acid is introduced, often called its buffering capacity. This capacity is determined by the total concentration of alkaline substances dissolved in the water, primarily bicarbonate, carbonate, and hydroxide ions. Alkalinity is expressed as a concentration, typically in milligrams per liter (mg/L) as calcium carbonate equivalent (\(\text{CaCO}_3\)). A water sample can have a high pH but low alkalinity, meaning its pH could shift easily.
Regulatory Guidelines for Alkalinity in Drinking Water
The Environmental Protection Agency (EPA) does not set a mandatory maximum contaminant level for alkalinity in public drinking water supplies. This is because alkalinity does not pose a direct threat to human health at concentrations typically found in natural water sources. Instead, it is managed as an operational and aesthetic parameter.
The EPA does establish Secondary Maximum Contaminant Levels (SMCLs) for related properties like pH, recommending a range between 6.5 and 8.5 units. Water system operators target an alkalinity range to maintain this recommended pH and minimize corrosion or scaling issues within the distribution network. A range of 50 to 180 \(\text{mg/L}\) as \(\text{CaCO}_3\) is considered desirable for finished drinking water. Levels exceeding 200 \(\text{mg/L}\) are common in areas with specific geological features but can lead to aesthetic issues.
Health and Aesthetic Effects of High Alkalinity
High alkalinity primarily affects the aesthetic quality and the utility of the water supply. A highly alkaline supply can sometimes impart a distinct, bitter, or “soda-like” taste, making it unpalatable for some consumers. The presence of high alkalinity can also interfere with common household activities, such as properly brewing coffee or tea.
High alkalinity is strongly associated with the formation of scale, which involves the deposition of mineral solids inside pipes and appliances. This scale buildup, often calcium carbonate, can reduce the efficiency of water heaters, decrease water pressure, and shorten the lifespan of household plumbing and appliances. Excessively high alkalinity may also be linked to an elevated pH, which can sometimes cause minor digestive upset.
Natural Sources and Water Treatment Processes
The primary origin of alkalinity in natural water supplies is the geological composition of the area through which the water flows. Water passing over or through soil and bedrock dissolves minerals that contribute to its buffering capacity. The dissolution of carbonate rocks, such as limestone and dolomite, is the most significant natural source of bicarbonate and carbonate ions.
Water treatment facilities frequently adjust alkalinity levels to achieve optimal water quality and protect infrastructure. Processes like lime softening, used to reduce water hardness, can significantly impact alkalinity. Conversely, treatment plants may intentionally add alkaline chemicals to finished water for corrosion control, ensuring the water does not become too acidic. Water with very low natural alkalinity may be treated with substances like sodium bicarbonate to increase its buffering capacity.