change in pH represents a large change in acidity: rain with a pH of 4.6 is 10 times more
acidic than normal rain (with a pH of 5.6). Rain with a pH of 3.6 is 100 times more acidic.
Regions that have a lot of coal-burning power plants have the most acidic rain. The acidity
of average rainwater in the northeastern United States has fallen to between 4.0 and 4.6.
Acid fog has even lower pH with an average of around 3.4. One fog in Southern California
in 1986 had a pH of 1.7, equal to toilet bowl cleaner. In arid climates, like in Southern
California, acids deposit on the ground dry. Acid precipitation ends up on the land surface
and in water bodies. Some forest soils in the northeast are 5 to 10 times more acidic than
they were two or three decades ago. Acid droplets move down through acidic soils to lower
the pH of streams and lakes even more. Acids strip soil of metals and nutrients, which collect
in streams and lakes. As a result, stripped soils may no longer provide the nutrients that
native plants need.
Acid rain takes a toll on ecosystems (Figure22.9). Plants that are exposed to acids become
weak and are more likely to be damaged by bad weather, insect pests, or disease. Snails die
in acid soils, so songbirds do not have as much food to eat. Young birds and mammals do
not build bones as well and may not be as strong. Eggshells may also be weak and break
more easily.
Figure 22.9: Acid rain has killed trees in this forest in the Czech Republic. ( 3 )The nitrates found in acid rain cause some plants to grow better. These nitrate-lovers can
drive out other plants, which may cause the ecosystem to change. Nitrates also fertilize
the oceans, which makes more algae grow. The algae use up all the oxygen in the water,
which can bring about disastrous ecological changes, including the deaths of many fish. As
lakes become acidic, organisms die off. If the pH drops below 4.5, all the fish die. Organic
material cannot decay, and mosses take over the lake. Wildlife that depend on the lake for