Acid Rain: Its Effects on Aquatic Environments

Acid Rain: Its Effects on Aquatic Environments

What is Acid Rain?:

Acid rain is rain, snow or fog that is polluted by acid in the atmosphere and damages the environment. Two common air pollutants acidify rain: sulfur dioxide (SO2) and nitrogen oxide (NOx). When these substances are released into the atmosphere they are transformed into sulfuric acid (H2SO4) and nitric acid (HNO3) and can be carried over long distances by prevailing winds and return to earth as acidic rain, snow, fog or dust. When the environment cannot neutralize the acid being deposited, damage occurs.

Causes of SO2 and NOx:

* Sulfur dioxide (SO2) is generally a byproduct of industrial processes and burning of fossil fuels. Ore smelting, coal-fired power generators, and natural gas processing are the main contributors.

* The main source of NOx emissions is the combustion of fuels in motor vehicles, residential and commercial furnaces, industrial and electrical-utility boilers and engines, and other equipment

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Acidification of Freshwater:

General Info:

Freshwater acidification is not an entirely new problem. First linked to the decline of salmon and other fish stocks in Norwegian rivers some sixty years ago, freshwater acidification was later identified as a problem in Scandinavia during the 1970’s. Since then, thousands of rivers and lakes have been proven acidified.

Areas most susceptible to acidification are those with an unreactive catchment such as granite and a base, nutrient-deficient soil. Areas of high acidification incidence include the United States, Canada, Scotland, Central Europe, and Scandinavia.

Any lake below pH7.0 is, by strict definition, an acid lake, but it is generally argued that acid waters are those below pH5.0 where sulfate concentrations exceed those of carbonate or the sum of calcium and magnesium content.

Process:

Acid rain enters water by two routes: directly and through the catchment. The vast majority enters through the catchment while a relatively small percentage enters directly. Acid water passes easily to the lake through catchment consisting of shallow soil cover and alkaline-deficient bedrock such as granite, which does not contain the carbonates necessary to neutralize the acid.

In areas where a continual supply of base cations is not assured then the gradual depleting of the bicarbonate in the lake means that the once stable pH will drop rapidly resulting in an acidified lake. Acidification can also occur in surges after snowmelt or drought; the first 30% of snowmelt can contain 50 - 80% of the total acids in the snow.