In the past century, one of the greatest threats to North America's water system has been the widespread acid rain spread through thousands of waterways. Acid rain has affected plant and animal life within aquatic ecosystems, as well as microbiologic activity by affecting the rates of decomposition and the accumulation of organic matter. What causes this poisonous rain, and what can be done to improve North America's water quality and prevent future catastrophes? To answer these questions, we must first examine the cause and formation of acid rain, as well as understand ways to decrease or prevent its formation. Formation of acid rain. Acid deposition, more commonly known as acid rain, occurs when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) react in the atmosphere with water, oxygen, and oxidants to form acidic compounds. This mixture forms a mild solution of sulfuric and nitric acid which then falls to the earth in either wet (rain, snow, sleet or fog) or dry (gas and particles) form.
             Approximately one-half of the atmosphere's acidity falls back to earth through dry deposition in the form of particles and gases, and are then spread hundreds of miles by winds where they settle on surfaces of buildings, cars, homes, and trees. When does rain become acidic? Scientists determine whether rain or lake water is acidic by measuring its pH A value of 7 is considered neutral, whereas values less than 7 are acidic and values over 7 are alkaline or basic. A change of one unit on the pH scale represents a factor of ten in acidity; for example, a solution with a pH of five is ten times as acid as one with a pH of six (Somerville, 1996, p.174). Normal or clean rainfall--without pollutants--is slightly acidic due to carbon dioxide, a natural gas in the air that dissolves in water to form weak carbonic acid. But rain, snow, or other moisture is not called "acid rain" until it has a pH value below 5.6 (Gay, 1992, p.44). Rainfall in...