The pH is a measure of the level of acidity or alkalinity of water or another solution. A pH of 7.0 is neutral. Values less than 7 are acidic, and those more than 7 are basic. Weak organic acids lower pH slightly. pH is affected by carbon dioxide (CO2) because CO2 in water forms a weak organic acid called carbonic acid. Strong mineral acids (e.g. sulfuric, nitric and hydrochloric acid) can lower the pH to lethal limits for aquatic life.
The pH of water can change throughout a season or even within a day. The optimal pH range for aquatic life is 6.5 to 8.5 or 9.0. pH less than 4.0 or more than 11.0 is usually lethal to fish and other organisms. Natural waters reflect the pH of the soils that have come into contact with the water. Industrial, municipal or agricultural waters may have significantly higher or lower pH than natural waters. Decreases in pH could indicate acid rain, runoff from acid soils (e.g. acid mine drainage) or contamination from livestock waste.