Questions to Consider 03

Biological Systems Theme

Acid Deposition

Normally, rainwater has a pH of about 5.6 because the carbon dioxide in the air combines with water to give a weak solution of carbonic acid. Acid deposition includes rain or snow that has a pH of less than 5, as well as dry acidic particles that fall to Earth from the atmosphere.

When fossil fuels such as coal, oil, and gasoline are burned, sulfur dioxide and nitrogen oxides combine with water to produce sulfuric and nitric acids. These pollutants are generally found eastward of where they originated because of wind patterns. The use of very tall smokestacks causes them to be carried even hundreds of miles away. For example, acid rain in southeastern Canada results from the burning of fossil fuels in factories and power plants in the midwestern United States.

Impact on Lakes

Acid rain adversely affects many aspects of biological systems. Aluminum may leach from the soil of lakes, particularly in areas where the soil is thin and lacks limestone (calcium carbonate, or CaCO₃) as a buffer. Acid rain may convert mercury in lake bottom sediments to toxic methyl mercury. Methyl mercury accumulates in fish, which wildlife and people eat. Over time, methyl mercury can accumulate in body tissues and cause serious sensory and muscular health problems. Acid rain in Canada and New England has caused hundreds of lakes to be devoid of fish, and in some cases, any life at all.

Impact on Forests

The leaves of plants damaged by acid rain can no longer carry on photosynthesis as before. When plants are under stress, they become susceptible to diseases and pests of all types. Forests on mountaintops receive more rain than those at lower levels; therefore, they are more affected by acid rain. Forests are also damaged when toxic chemicals such as aluminum are leached from the soil. These kill soil fungi that assist roots in acquiring the nutrients trees need. In New England and the southern Appalachians, millions of acres of high-elevation forests have been devastated. Sulfur dioxide and nitrogen oxides, the main precursors of acid rain, have been steadily decreasing in the United States due to clean air legislation and strict emission limits.

Impact on Humans and Structures

Humans may be affected by acid rain. Inhaling dry sulfate and nitrate particles appears to increase the occurrence of respiratory illnesses, such as asthma. Buildings and monuments made of limestone and marble break down when exposed to acid rain. The paint on homes and automobiles is likewise degraded. However, damage to natural systems and human structures due to acid rain is likely to decrease if we continue efforts to reduce chemicals that contribute to acid rain.

How might we prevent methyl mercury from entering biological systems and reduce the amount already present?