Editing Alkalinity (section)

===Dissolution of carbonate rock===
Addition of CO<sub>2</sub> to a solution in contact with a solid can (over time) affect the alkalinity, especially for carbonate minerals in contact with groundwater or seawater. The dissolution (or precipitation) of carbonate rock has a strong influence on the alkalinity. This is because carbonate rock is composed of CaCO<sub>3</sub> and its dissociation will add Ca<sup>2+</sup> and {{chem|CO|3|2−}} into solution. Ca<sup>2+</sup> will not influence alkalinity, but {{chem|CO|3|2−}} will increase alkalinity by 2 units.  Increased dissolution of carbonate rock by acidification from acid rain and mining has contributed to increased alkalinity concentrations in some major rivers throughout the eastern U.S.<ref name="10.1021/es401046s" /> The following reaction shows how acid rain, containing sulfuric acid, can have the effect of increasing river alkalinity by increasing the amount of bicarbonate ion:

:2 CaCO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub> → 2 Ca<sup>2+</sup> + 2 {{chem|HCO|3|−}} + {{chem|SO|4|2−}}

Another way of writing this is:

:CaCO<sub>3</sub> + H<sup>+</sup> ⇌ Ca<sup>2+</sup> + {{chem|HCO|3|−}}

The lower the pH, the higher the concentration of bicarbonate will be. This shows how a lower pH can lead to higher alkalinity if the amount of bicarbonate produced is greater than the amount of H<sup>+</sup> remaining after the reaction. This is the case since the amount of acid in the rainwater is low. If this alkaline groundwater later comes into contact with the atmosphere, it can lose CO<sub>2</sub>, precipitate carbonate, and thereby become less alkaline again. When carbonate minerals, water, and the atmosphere are all in equilibrium, the reversible reaction

:CaCO<sub>3</sub> + 2 H<sup>+</sup> ⇌ Ca<sup>2+</sup> + CO<sub>2</sub> + H<sub>2</sub>O

shows that pH will be related to calcium ion concentration, with lower pH going with higher calcium ion concentration. In this case, the higher the pH, the more bicarbonate and carbonate ion there will be, in contrast to the paradoxical situation described above, where one does not have equilibrium with the atmosphere.