How can fertilizer and pesticide runoff pollute rivers and groundwater?

Fertilizer and pesticide runoff significantly degrade water quality through two primary mechanisms: nutrient loading and chemical contamination.

Nutrient Loading and Eutrophication

Agricultural fertilizers containing nitrogen and phosphorus often wash into rivers and lakes via runoff and soil erosion. This process, known as cultural eutrophication, accelerates the growth of algae and aquatic plants. These blooms block sunlight and, upon decomposing, consume dissolved oxygen, leading to hypoxia—a condition that can cause massive fish kills and create "dead zones" where aquatic life cannot survive. Furthermore, high concentrations of nitrates and nitrites in groundwater pose direct toxicity risks to both humans and livestock.123

Chemical Contamination

Pesticides—including insecticides, herbicides, and fungicides—can enter drainage and irrigation systems. These chemicals can kill fish in farm ponds and contaminate crops or livestock through soil residues and water uptake. Because these substances can persist in the environment, their management is strictly regulated to prevent broader ecological and agricultural damage.4