Alkalinity is a measure of the amount of dissolved weak bases, such as carbonates, bicarbonates and hydroxides.
Freshwater usually has alkalinity levels between 20-200 mg/L (milligrams per liter of calcium carbonate) depending on the composition of surrounding soils and bedrock. Alkalinity levels in the 100-200 mg/L range will restabilize pH changes in a stream.
The abundance or lack of alkalinity in a stream determines how well it can neutralize acidic pollution and resist changes in pH, referred to as the buffering capacity of a stream. Increases in acidic substances will decrease the amount of available buffers, or weak bases.
Over time surrounding alkaline soils and rocks, such as limestone, can restore the alkalinity/buffering capacity of a stream by naturally adding weak bases, such as carbonates, to the water. However, even momentary decreases in pH are harmful to macroinvertebrates, especially the pollution intolerant macroinvertebrates.