Acid Rain

Background information:

Acid rain is formed from sulfur dioxide (SO2) and oxides of nitrogen (NOx). These are most commonly emitted from major industries, and combine with water vapor to form mild sulfuric and nitric acids that are returned to earth as rain or snow. Normal, clean rain is slightly acidic, with a pH level of around 5.6. Acid rain has pH levels of 4.0, and occasionally a pH of 3.0 in areas of heavy industry. Main concerns of acid rain are its effects on aquatic life, potential damages to crops and forests, and effects on building materials. Acidic rainwater can also dissolve metals.

Effects on Aquatic Systems:

The most important effect on aquatic systems is the decline in fish populations. Low pH in rainwater eventually results in acid lakes. Normal lakes have a pH of 6.5 to 9; lakes affected by acid rain have a pH ranging from 4.5 to 6.0. When the pH falls below 6 for a prolonged period of time, a reduction in certain species of zooplankton, algae and aquatic plants will occur (2). This in turn will disrupt the overall food chain in lakes. A major contributing factor to fish kills is high aluminum concentrations in acidifying waters. Aluminum is insoluble when the pH is neutral or basic. Insoluble aluminum is present in very high concentrations in rocks, soils, and river and lake sediments. In normal lakes aluminum concentrations are very low, however as the pH drops and becomes more acidic, aluminum begins to go into solution. Aluminum is highly toxic to many species of aquatic life, and thus will largely contribute to the decline in aquatic populations (1).

Effects on Forests:

Leaching of nutrients and acidification of soils are normal processes. The question of acid rain and forests is whether the increase in acidity will become to great for the forests to adapt to. In central Europe there has been dramatic death and dieback of trees. Over the last 15 years, fir forests in Czechoslovakia and eastern Germany have died by the thousands of hectares (1). What scientists believe happened is that the acidic rain and snow has caused leaching of calcium and magnesium from the soils, and has also increased aluminum concentrations in the soil. The fine absorbing roots of the trees uptake the toxic aluminum, causing the roots to die or have reduced vigor. This results in entry of disease-causing (pathogenic) bacteria and fungi, which infect the trees and eventually cause the populations to decline (1).