Alkalinity Information Sheet

Background

Alkalinity is a measure of the ability of a water system to resist changes in pH** when acid is added to water. A stream that has a high alkalinity is well buffered so that large inputs of acid (from acid rain for instance) can be made with little affect on the stream pH. A stream that has a low alkalinity is poorly buffered and may undergo large, sudden drops in pH in response to acid inputs.

The amount of carbonate (CO3^-2) and bicarbonate (HCO3^-) in water helps to determine its alkalinity. The more of these present, the better chance the water has to resist a change in pH (alkalinity). Carbonate (CO3^-2) will react with a free hydrogen ion (H+) to form bicarbonate (HCO3^-). Bicarbonate will react with a free hydrogen ions to create carbonic acid (H2CO3), which then can dissociate (break down further) into water (H2O) and carbon dioxide (CO2). During this process, free hydrogen ions have been locked up, thus keeping the pH from lowering (keep in mind, a low pH has lots of extra hydrogen ions present). The formula for the above reactions follows: CO3^-2 + H+ > HCO3- (reactions can also reverse) HCO3- + H+ >H2CO3 H2CO3 >H2O + CO2

This reaction process can also reverse itself. In other words, water and carbon dioxide can combine to form carbonic acid. Carbonic acid can dissociate (break down) into bicarbonate and hydrogen, and the bicarbonate can dissociate into carbonate and hydrogen. The reaction is balanced and is able to deal with the free hydrogen ions that are present before they make the pH level drop. A problem occurs when additional free hydrogen ions are added to this balanced system. Acids such as sulfuric acid (H2SO4) and nitric acid (HNO3) (components of acid rain) provide extra hydrogen ions when they dissociate. For instance, sulfuric acid will eventually break down yielding 2 hydrogen ions (H2SO4 à 2H+ + SO4-2).

To combat these additional hydrogen ions, which would lower the pH if left alone, additional bicarbonate and carbonate need to be added to the water. Carbonic acid (H2CO3) will do this for us. Carbonic acid (H2CO3) does not have to dissociate into water and carbon dioxide; instead it can react with carbonate based rocks such as sandstone, limestone, and dolomite as part of the rock's weathering process. Calcium carbonate (CaCO3) makes up limestone and the cement that holds sandstone together, while magnesium carbonate (MgCO3) makes up dolomite. Both can react with carbonic acid yielding either calcium bicarbonate Ca(HCO3-)2 or magnesium bicarbonate Mg(HCO3-)2 [H2CO3 + CaCO3 à Ca(HCO3-)2 ]. The calcium (Ca+2) and magnesium (Mg+2) drop off as a solid to the stream bottom while 2 bicarbonates (HCO3-) remain, each able to react with one free hydrogen (thus maintaining the pH). This reaction yields carbonic acid again (HCO3- + H+ à H2CO3).

Watersheds with high alkalinity have the sandstone, limestone, and dolomites and the corresponding calcium carbonates/magnesium carbonates needed to help buffer a stream. They are able to handle additions of extra hydrogen ions. These rock types exist in Western Pennsylvania. Watersheds where the bedrock does not consist of sandstone and limestone, but instead have igneous rocks like granite and basalt, are unable to provide the needed calcium/magnesium carbonate that rid acidity. Streams in those areas have low alkalinity and a pH below 5.4 (extra hydrogen ions present). An artificial source of alkalinity is lime (calcium carbonate), used to neutralize a stream or even treat acid mine drainage (with lots of extra hydrogen ions). Lime is also used as a soil amendment to rid acidity in cropland, gardens, and lawns.

Because of its contact with bedrock and soils containing calcium/magnesium carbonate, groundwater usually has a higher alkalinity than surface water. Acid rain does not directly come in contact with groundwater, but instead, surface stream water has to use some of its alkalinity to buffer the acidic storm runoff (that never made into the groundwater table). Space for information footer on pH

Plants can also contribute to lowering alkalinity in a stream. Plants utilize carbon dioxide in water during photosynthesis to produce oxygen. Less carbon dioxide reduces the capability for the reaction between water and carbon dioxide to produce carbonic acid. Less carbonic acid means less dissociation to bicarbonate and less reaction with calcium carbonate. The reduced alkalinity of the stream leaves the stream more susceptible to sudden additions of hydrogen and resulting changes in pH.

Human Impact

Alkalinity is an important measure of a stream ability to absorb inputs of acid. Acid rain and acid mine drainage from coal mining causes a considerable drop in pH of stream water. A decrease in pH of a stream can disturb the natural equilibrium and destroy many habitats for aquatic life, especially for species intolerant of pH changes.

Rapid seasonal changes in pH often occur in the spring and fall. Increased organic matter in the fall can cause greater inputs of organic acids from decaying organic matter (remember organic acids can dissociate forming extra hydrogen ions). To address these increased hydrogen ions from the organic acids, bicarbonate and carbonate must be used, removing their availability to react with hydrogen supplied by acid rain. During the spring, heavy rains and melting snow can result in a large, sudden input of acid into hydrologic systems, too much to buffer, causing a rapid drop in pH. In some cases, such an "acid spike" results in fish kills as the pH drops below acceptable levels for supporting aquatic life.

Water Quality Criteria

The Environmental Protection Agency has suggested a minimum of 20mg/L of CaCO3 for freshwater aquatic life except where natural concentrations are less. Although this criteria has been established, many problems exist as streams that are acidic or streams that suffer changes in alkalinity through the year.