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Acidity as applied to natural water and wastewater is the capacity of the water to neutralize hydroxyl ions (OH^-). It is analogous to alkalinity, the capacity to neutralize the hydrogen ion (H⁺). Acidity is the quantitative expression of the capacity of the water to neutralize a strong base to a designated pH and an indicator of how corrosive water is.

Acidity can be caused by weak organic acids, such as acetic and tannic acids, and strong mineral acids including sulfuric and hydrochloric acids. However, the most common source of acidity in unpolluted water is carbon dioxide in the form of carbonic acid (H₂CO₃). On the other hand, The alkalinity of water refers to the capability of water to neutralize acid in which the water has a buffering capacity – a buffer is a solution to which an acid can be added without changing the concentration of available hydrogen (H⁺) ions (without changing the pH) appreciably.

A surface water body, such as a lake, the alkalinity in the water comes mostly from the rocks and land surrounding the lake. Precipitation falls in the watershed surrounding the lake and most of the water entering the lake comes from runoff over the landscape. If the landscape is in an area containing rocks such as limestone then the runoff picks up chemicals such as calcium carbonate (CaCO₃), which raises the pH and alkalinity of the water. In areas where the geology contains large amounts of granite, for instance, lakes or ponds may have a lower alkalinity.

The acidity and/or the alkalinity of a surface water system can be influenced by the production of the products of the use of non-renewable fuels, such as those carbonaceous fuels that produce carbon dioxide during use or conversion to other forms of energy. However, switching from non-renewable (fossil fuels) to renewable fuels such as biomass is not always the answer. When biomass is directly used as a fuel or when biomass is used as a process feedstock the products can be sufficiently acidic, and it causes a change in the acidity or alkalinity of surface water systems.

See also: Acid Number.