Buffering ... the pretty short, not too
A stream is buffered when it can resist changes in pH, say from the addition of acid mine water. A stream gains its buffering capacity by having limestone (or similar mineral) dissolved in it. The limestone adds carbonate and bicarbonate ions that have the ability to neutralize acid. A buffered stream will maintain a pH close to neutral (7). The more dissolved limestone, the better the buffering capacity. It's a really good thing to have a highly buffered stream. It allows the stream to take environmental hits without decimating aquatic life. Buffering makes the aquatic system more robust. A buffered stream will likely have high alkalinity.
Buffering for the more technically inclined
A buffered solution is one that resists a change in its pH when hydrogen ions (H+) or hydroxide ions (OH-) are added. Water that is not buffered is subject to drastic changes in pH by addition of an acid or base. Pure water is an example. In nature, the healthiest, most biologically diverse streams and waterways have a pH close to 7 (neutral) and do not vary by more than a pH unit. The ability to resist changes in pH comes about by addition of certain chemicals that can consume hydrogen ions (H+) or hydroxide ions (OH-).
For AMD issues we need be concerned on assaults on the pH only from the acid side, i.e. the unwanted addition of acidity in the form of H+ from pyrite reactions. An increase in the number of H+ ions in solution will lower the pH of an unbuffered solution, sometimes drastically. However, if a chemical species is present in the water that can consume or tie up H+ ions, drastic changes in the pH may be avoided. This is the function of a buffer.
By far, the most important buffer for water systems in nature is the carbonate (CO32-) system. Here a variety of chemical species and reactions occur that are highly interactive. The source of the chemicals is carbon dioxide from the atmosphere and carbonate containing minerals in the earth, limestone being the most common. For those not interested in the details, here is the short description. If sufficient limestone (CaCO3) has dissolved into a stream, it will contain adequate numbers of carbonate (CO32-) and bicarbonate (HCO3-) ions that are in turn capable of consuming hydrogen ions and maintaining a fairly constant and nearly neutral pH. This carbonate based buffer protects the stream from too low of a pH and can mitigate the ill effects of AMD. However, if more H+ ions enter the stream than there are carbonate and bicarbonate ions to counteract them, the buffer becomes overwhelmed and is no longer effective. Water that has been fortunate enough to be in contact with limestone enjoys a measure of protection (from the acidity anyway) from AMD.
The sum of all acidity (H+) consuming ion concentrations hydroxide [OH-] , carbonate [CO32-], and bicarbonate [HCO3-] is known as alkalinity. Alkalinity is the ability to consume or neutralize acidity.
Alkalinity = [OH-] + 2[CO32-] + [HCO3-] - [H+]
The primary mechanism of limestone dissolving in the presence of acidity (H+) is as follows
CaCO3 (s) + H+ (aq) à Ca2+(aq) + HCO3-(aq)
forming alkalinity in the form of bicarbonate (HCO3-). When bicarbonate is the predominate carbonate species, the water is buffered at a pH close to 7. The term bicarbonate alkalinity is often used.
The other acid consuming reactions of the carbonate system are
CO32-(aq) + H+ (aq) HCO3-(aq) (carbonate to bicarbonate)
HCO3-(aq) + H+ (aq) H2CO3(aq) (bicarbonate to carbonic acid)
OH-(aq) + H+ (aq) H2O (hydroxide to water)
Note that carbonate can effectively consume 2 hydrogen ions (in 2 steps), and is why 2 is a factor in the alkalinity equation.
Also note that all of these chemical equations can happen in either direction (reversable). A complex equilibrium is thus established among all the chemical species and is ultimately responsible for the resulting pH. The calculations, however, are rather tedious and are best left to a more rigorous treatment of the subject.
The terms buffering and alkalinity, when referring to streams, are often used interchangeably.
Limestone alkalinity may occur naturally if
the water passes through geologic strata containing limestone. AMD that
has had the fortune of having sufficient alkalinity is generally the easiest to
treat. On the other hand, if AMD has not been buffered naturally, a common
treatment strategy is to
add limestone alkalinity by design.
See St. Vincent College Environmental Education Center module on pH/Alkalinity/Acidity/Buffering.