Definition: Measure of ability to "resist change" in pH or the number of hydrogen ions (H⁺). Has acid neutralizing capacity and the ability to buffer.

Background:

• Without this acid-neutralizing capacity, acid added to a stream would cause an immediate change in pH - amount of free hydrogen ions (H⁺) in water. Extra hydrogen ions make water acidic.
• Amounts of carbonates (CO₃^-2) and bicarbonates (HCO₃^-) help determine alkalinity. They react with the free hydrogen ions, maintaining pH levels.
• Extra carbonate and bicarbonate provided by interaction between calcium carbonate (CaCO₃ - a component in limestone and sandstone) and carbonic acid (H₂CO₃ - a natural acid that forms from water and carbon dioxide) [H₂O + CO₂ = H₂CO₃]

Alkalinity is influenced by:

• Rocks and soils - if an area's geology contains calcium carbonate, the stream will have higher alkalinity.

• Addition of lime (CaCO₃) - soil amendment (correction) often used to decrease acidity.

• Groundwater is well buffered after coming in contact with calcium carbonate rocks.

• Stormwater runoff does not have the chance to react with rocks to become buffered.

• Photosynthesis - removes CO₂, thus lowering the chance to form carbonic acid, which reacts with calcium carbonate.

• Decomposition / respiration - adds CO₂, increasing the chance to form carbonic acid, which reacts with calcium carbonate.

Environmental Impacts:

• Water with low alkalinity is at risk of being affected byincreased acidity (hydrogen ions).
• Acid rain can increase acidity if a stream has low alkalinity (New York's Adirondacks & Eastern Canada Lakes).
• Aquatic life cannot tolerate large changes in pH (level of acidity).
• Acid rain, if not buffered, can cause fish kills.