Calcium carbonate, [Ca][CO3] is a very common mineral. Limestone is one familiar form of calcium carbonate. Acids in acid rain promote the dissolution of calcium carbonate by reacting with the carbonate anion.
This produces a solution of bicarbonate. Because surface waters are in equilibrium with atmospheric carbon dioxide there is a constant concentration of carbonic acid, H2CO3, in the water.
The presence of limestone and other calcium carbonate rock in lakes and streams helps to maintain a constant pH because the minerals react with the excess acid. However, acid rain eventually can overcome the buffering capacity of the surface water. The acid neutralizing capacity (ANC) of any solution of natural water is equal to the concentration of proton acceptors minus the concentration of proton donors.