As a consequence of burning tremendous amounts of fossil fuels such as coal and oil, the United States discharges nearly 40 millions metric tons of sulfur dioxides and nitrogen oxides into the atmosphere each year. Through a series of complex chemical reactions some of these pollutants are converted into acids, which then fall to the earth’s surface as rain or snow. Another portion is deposited directly on surfaces and converted into acid after coming in contact with rain, dew, or fog.
Before discussing acid rain, we must first become acquainted with the pH scale. The pH scale is used to measure the degree of acidity or alkalinity of a solution. The scale ranges from 0 to 14, with the value of 7 representing that a solution is neutral. Values below 7 indicate greater acidity and numbers above 7 indicate greater alkalinity. The pH scale is logarithmic, meaning that each whole number indicates a ten-fold difference. Until recently it was believed that unpolluted rain naturally has a pH of about 5.6. However, studies in uncontaminated remote areas have shown that precipitation usually has a pH closer to 5. Unfortunately, in most areas within several hundred kilometers of large centers of human activity, precipitation has much lower pH values. Rain, or precipitation with these values is termed acid rain.
The best known effect of acid rain is the lowering of pH in thousands of lakes in Scandinavia, Canada, and the northeastern United States. As the pH drops, the organisms that live in or near lakes and streams are affected. Accompanying the lower pH have been a substantial increase in dissolved aluminum, which is leached from the soil by the acidic waters. This increase of dissolved aluminum is toxic to fish and wildlife. As a consequence, some lakes are devoid of fish while others are approaching this condition.
The effects of acid rain on lakes varies from one lake to another. These variations is related to the nature of the soil and rock materials surrounding the lake. Minerals such as calcite in some rocks and soils can neutralize acid solutions which would keep the lakes from becoming acidic. However, over a certain period of time, the pH of lakes that have not yet been acidified may drop as the buffering material in the surrounding rocks and soil become depleted. Lakes that lack the buffering materials such as calcite and limestone are more likely to be or become acidic.
Lakes at high altitudes are particularly sensitive to acid rain. Only about 5 or 10% of the water in a lake comes from precipitation that has directly fallen on lakes; the vast majority of water is from rain that has fallen on the surrounding watershed and run off into the lakes. Meaning that most of the water in a lake has had an opportunity to be neutralized by soil. However, lakes at a higher altitudes are often located in rocky outcropping, and comprise a large fraction of their whole watershed. In some cases the rainwater runs uninterrupted over a few bare rocks, directly into the lake. The problem is multiplied during the winter and spring when a carpet of snow or ice prevents rain and melted water from contacting the ground at all before running into a lake. In this case the lake receives a surge of acidic waters which can kill fish and other aquatic life outright.
There has been recent evidence to show that acid rain may be responsible for the widespread decline in the growth of evergreen forests in the eastern United States and Western Europe during the past decades. Acid rain has caused such severe damage to the forest in Germany that they have taken major action to reduce sulfur dioxide emissions.
Laboratory and field studies have shown that acid deposition can have the following effects on forest: (1) damage leaves, roots and microorganisms that form beneficial symbiotic association with roots; (2) decrease a plant’s resistance to other forms of stress, including climate, insects, and pathogens; (3) leach nutrients such as calcium and magnesium from soils; (4) dissolve metals in the soil such as aluminum at levels potentially toxic to plants; and (5) impair reproduction and the survival of seedlings.