Effects of Acid Deposition

There has been considerable controversy on the effects of acid deposition on vegetation. It is reasonably well agreed that over large areas there has been relatively little agricultural or horticulture damage due to acid precipitation. But I emphasize that this is on the large scale. There are numerous examples of severe damage to crops or forests near sources of acidic emission. But widespread damage is much more difficult to quantify. Forests, for example, may be affected both by direct deposition on vegetation and through chemical processes in the soil. Agricultural soils are generally well buffered, but forest soils may not be as resistant to effects of acid deposition.

Waterways present another area that can be very vulnerable to consequences of acid deposition. We can have input of acidic substances directly from the atmosphere or through runoff or through generation of H+ ions within the watersheds. A lake has a natural buffering capacity particularly if it is in a region that has limestone that combines with the H ions to form H₂O and CO₂. The alkalinity of the lake decreases with only a small change in pH as acid rain and runoff enter. However if the bicarbonate ions become depleted, then the pH will drop and acidic conditions prevail. Surges of acidic material may occur in the spring with snowmelt or during the summer with heavy rains. These episodes may be particularly detrimental to marine organisms if they occur at vulnerable spawning times.

The pH may ultimately stabilize around 4.0 as humus and aluminum react to absorb the free hydrogen ions. Increase in concentration of aluminum ions can lead to aluminum toxicity which may make the water become clear. In this regard, you might say that a clear lake is a nice place to visit but you might not want to live there, particularly if you were a sensitive marine organism. Acid tolerance of various fish is shown in an EPA educational website. Concepts of sensitivity and critical load provide quantitative estimates of impacts of acid deposition on various ecosystems.

Figure 4 shows the pH levels in rainfall over the U.S. from the 1950s, 60s, and 70s. The area of West Virginia, Pennsylvania and New York experienced a drop in pH from 4.52 to 4.3 over a period of twenty years. Figure 5, a more recent map shows that the eastern US continues to have low pH levels despite significant reductions in emissions. In a later lecture, we will discuss another aspect of acidic materials in the atmosphere, namely their contribution to global cooling.

The National Acid Deposition Program provides annual nationwide plots of deposition of numerous chemical compounds for the last several years.

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