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Does Rain Increase Alkalinity?

Alkalinity is a measurement of how well soil and water neutralize acids. Rain usually decreases alkalinity because it is acidic. However, minerals and other substances in water or soil can counteract the acids in rain. In order to neutralize acids from rain, soil and water must contain buffering agents, particularly calcium carbonate, to prevent acidification. Alkalinity is defined as the ability of soil or water to buffer, or neutralize, acids.

Alkalinity
- A number of factors other than precipitation contribute to alkalinity, including soil type, rock and mineral content, and the type and behavior of local plant life. Values of pH are measured in a range from 0 to 14, with a pH of 7 being neutral. Values of less than 7 mean that the sample is acidic, and greater than 7 is alkaline. However, it is important not to confuse high pH alkaline substances with alkalinity. Alkalinity describes how soil and water buffers added acid.
Alkalinity and Rain
- Most rainfall is slightly acidic. The pH value of unpolluted rain in most regions is 5.6, but many areas have rain with pH values below 4. Unless there are buffering agents in soil and water to counteract the acids in rainfall, rain can be expected to decrease alkalinity. In the eastern United States, summer rainfall is very acidic, ranging from 2.6 to 3.6, primarily due to pollution, and this has caused the soil and water there to be more acidic.
Acid Rain
- Acid rain is a problem because it decreases pH and adversely affects most life forms that depend upon this water. Acid rain is most often thought of as acidic rainfall or snowfall, but it can also consist of dry materials -- mainly dust, volcanic ash and combustion byproducts that contain nitric or sulfuric acids -- which will increase acidity where they drift to the ground. These substances, particularly pollution from the burning of fossil fuels, mix with water vapor in the atmosphere and fall back to earth in the form of acid rain. The primary method for alleviating the loss of alkalinity from acid rain is to reduce pollution from the combustion of fossil fuels. This will reduce the acidity of rainfall, and thus increase the pH value of soil and open bodies of water.
Mitigating Acids
- Since most rain is acidic, rain is unlikely to increase alkalinity. The capacity of soil and water to neutralize acids and maintain alkalinity varies greatly. An important buffering agent in both soil and streams is limestone. Dissolved limestone in soil creates a buffer of calcium carbonate that increases total alkalinity. Rain can play a role in this process by increasing the rate at which limestone weathers and dissolves. Soils in the western United States contain more limestone than those in the eastern part of the country, so most western soils do not suffer from excessive acidification.

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