2. Pollution Moves Through Air, Water, and Soil

Pollution is not just "trash." It is any substance or energy added to the environment at a harmful level. Air pollution can come from burning fuels. Water pollution can come from sewage, industrial discharge, oil spills, and agricultural runoff. Soil pollution can come from pesticides, heavy metals, and waste.

One important GED® example is eutrophication. Fertilizers contain nutrients such as nitrogen and phosphorus. When too much fertilizer washes into a lake or coastal water, algae may grow rapidly. When the algae die, decomposer bacteria break them down and use dissolved oxygen. Low oxygen, called hypoxia, can kill fish and other aquatic organisms.

The key is the sequence: nutrient runoff -> algal bloom -> decomposition -> oxygen drop -> aquatic life stress. If a graph shows nutrients rising, algae rising, and dissolved oxygen falling, the best explanation is usually eutrophication.