ABSTRACT
During periods of low fl ow, the San Marcos/Cazones River that goes by the city of Poza Rico, Mexico, appears an intense, beautiful green color. But this appearance masks a major problem because what makes the river so green is the rich growth of algae and plants in it. This plant life thrives so well in the tropical climate because of nutrient-rich wastes that enter the river from untreated sewage. It illustrates a major water pollution problem called eutrophication discussed later in this chapter.
