Which activity most directly introduces nitrates and phosphates into waterways?

• A. Fertilizers, detergents, animal waste, sewage, and agricultural runoff
• B. Planting trees along riverbanks
• C. Desalination of seawater
• D. Filtering through sand

Answer: (D)

The key idea is that nitrates and phosphates enter waterways mainly from human activities that add nutrients to the land and water. Fertilizers are designed to supply nitrogen and phosphorus to crops, so theyContain high levels of nitrates and phosphates. Detergents historically contained phosphates, and waste from animals and sewage adds both nutrients. When rain or irrigation washes over soils or waste, these nutrients are carried into rivers, lakes, and streams as agricultural runoff, animal waste runoff, or treated/untreated sewage discharge. Once in water, nitrates and phosphates fuel algal blooms, which can deplete oxygen and harm aquatic life.

Planting trees along riverbanks helps by trapping and absorbing nutrients before they reach the water, so it reduces nutrient input rather than introducing it. Desalination removes salt from seawater and doesn’t add nutrients. Filtering through sand is a treatment step that can remove nutrients, not introduce them. Therefore, the activity that most directly introduces nitrates and phosphates into waterways is the use and disposal of fertilizers, detergents, animal waste, sewage, and agricultural runoff.