Natural buffering materials in water slow the decline of pH to around 6. A rapid pH drop follows this gradual decline as the bicarbonate buffering capacity is used up. At pH 5.5, only very weak buffering ability remains and the pH drops further with additional acid. The ability of an aqueous solution to carry an electric current is called conductivity. The current is conducted in the solution by the movement of ions.
Conductivity increases with increasing amounts and mobility of ions.
In natural water, the dissociation of inorganic compounds is the main source of ions in the solution. Therefore, measuring conductivity reveals the concentration of dissolved salts in water. Conductivity is also affected by heavy metal ions released into water by acid mine drainage. The quantity of dissolved material is mainly determined by the solubility of rocks and soils that the water contacts. Water that flows through limestone and gypsum dissolves calcium, carbonates, and sulfates, resulting in high total dissolved solid levels.