The USGS Water Science School
The buffering capacity of a water body; a measure of the ability of the water body to neutralize acids and bases and thus maintain a fairly stable pH level
Alkalinity is not a chemical in water, but, rather, it is a property of water that is dependent on the presence of certain chemicals in the water, such as bicarbonates, carbonates, and hydroxides. A definition of alkalinity would then be "the buffering capacity of a water body; a measure of the ability of the water body to neutralize acids and bases and thus maintain a fairly stable pH level". In more simple terms, water with a high alkalinity will experience less of a change in its own acidity, for instance, when acidic water, such as acid rain or a acid spill, is introduced into the water body.
You might use this analogy—you and your friend are in separate small rowboats in a lake. Both rowboats develop a leak, which doesn't worry you, since both of you were wise enough to bring a bailing bucket onboard just for this contingency. Except your friend grabbed a gallon-sized bucket while all you have is a smaller pint-sized bucket. You both start bailing water out of your boat, but after a while your friend's feet are barely wet while the inside of your boat has water up to your knees. Your friend can bail incoming water out as fast as it comes in, but you can't keep up. Your friend has better "buffering capacity" than you have and can neutralize the incoming water to a greater extent. In this analogy, his boat would have a higher alkalinity than your boat has.
Why alkalinity is important
Although you don't often hear about the alkalinity of your favorite lake in the news, alkalinity can be important to the health and welfare of a lake. The ecosystem and organisms that live in the lake evolved in water bodies that didn't change quickly. Before humans came along water bodies were not subjected to chemical spills and acid rain. Likely the pH and aquatic characteristics of a lake did not change much over the short term, which suited the fish in the lake just fine.
In modern times, water bodies can be subjected to sudden inputs of chemicals, such as those contained in acid rain and wastewater, that can cause rapid changes in the acid/base balance of a lake—lowering the pH of the lake water, for instance. A sudden shift in pH is not healthy for the fish and organisms living in the water. Aquatic organisms benefit from stable pH values, and waters with a high alkalinity are better able to maintain a fairly constant pH.
What affects alkalinity?
In a surface water body, such as a lake, the alkalinity in the water comes mostly from the rocks and land surrounding the lake. Precipitation falls in the watershed surrounding the lake and most of the water entering the lake comes from runoff over the landscape. If the landscape is in an area containing rocks such as limestone then the runoff picks up chemicals such as calcium carbonate (CaCO3), which raises the pH and alkalinity of the water. In areas where the geology contains large amounts of granite, for instance, lakes will have a lower alkalinity. But, a pond in an suburban area, even in a granite-heavy area, could have a high alkalinity due to runoff from home lawns where limestone have been applied (used to raise the soil's pH to better grow lawns).
One common method the U.S. Geological Survey (USGS) uses for measuring alkalinity is to use take a water sample and to add acid to it while checking the pH of the water as the acid is added. An initial pH reading of the water is taken and then small amounts of acid are added in increments, the water is stirred, and the pH is taken. This is done many times. In the beginning, the acid added will be neutralized by compounds in the water, such as bicarbonates. As more acid is added, the bicarbonates get "used up", as it is also being neutralized by the acid. Eventually all the acid-neutralizing compounds are used up. After this point, any acid added to the water will lower the pH in a linear fashion, and the scientist will be able to see this reflection point by viewing a line chart of the amount of acid added to the water and the resulting pH. The point at which the change in pH line becomes linear is used to determine the water's alkalinity.
In this picture, a USGS scientist is performing an alkalinity titration to determine the alkalinity of a water sample. Notice the pH meter in the background (reading 5.477 at the moment). The larger grey tube going into the beaker is the pH probe. The black device on the right side pushes a fixed amount of acid through the white tube, which goes into the right side of the beaker of water. The black box on the bottom has a magnet that twirls, which then twirls a small magnet placed in the bottom of the beaker, which keeps the water sample stirred.
Map of alkalinity in surface waters in the U.S.
Here is a map made by the EPA that shows alkalinity values for surface waters the Untied States. According to the EPA, this map provides a general illustration of the national patterns of surface-water alkalinity in the conterminous United States. Alkalinity is the most readily available measure of the acid-neutralizing capacity of surface waters and provides a reasonable estimate of the relative potential sensitivity of lakes and streams to acidic deposition. Although the actual sensitivity of a water body depends on many watershed characteristics and processes, the low-alkalinity areas on the map indicate where sensitive surface waters are most likely to be found.
The map is based on alkalinity data from approximately 39,000 lake and stream sites and the associations of the data values with factors such as land use, physiography, geology, and soils. Data were acquired from a variety of sources including federal and state agencies, university researchers. and private corporations. In many of the areas represented by a specific alkalinity range, an even greater range was observed in the water-quality data. The shading on the map indicates the range of alkalinity within which the mean annual values of most of the surface waters of the area fall.
Credit: James M. Omernik Glenn E. Griffith, Jeffrey T. Irish, and Colleen B. Johnson
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