Aluminum is one of the most abundant elements in the earth's crust. Its mineral forms are not very soluble at "normal" pH levels, but solubility increases as pH decreases. In acidic water, aluminum
minerals dissolve to form Al3+, AlO+ and other aluminum species. The dissolved aluminum enters the gills of fish where it precipitates due to the alkaline environment
of the fish tissue. The precipitate causes irritation, stress and may result in mortality. The direct benefit of limestone treatment is that the stream pH is raised by the added bicarbonate ion (HCO3-) neutralizing the acid (H+).
Once the pH is increased, aluminum species are no longer stable in the stream water, are precipitated and returned to the mineral form.
Not only is the bicarbonate ion important in the restoration of an acid degraded stream as described above, but calcium ion (Ca2+) also plays an important role. Calcium ion is beneficial to aquatic life as
it is an essential nutrient and offsets the effect of toxic aluminum on fish and other aquatic life. The calcium from the limestone helps prevent aluminum adsorption by competing for cation exchange sites in fish