How do fertilizer use and soil erosion contribute to nonpoint pollution?

• A. Fertilizer use reduces nutrient runoff.
• B. Erosion reduces pollutants.
• C. Diffuse pollution from land runoff.
• D. Excess fertilizers increase nutrient runoff; erosion transports attached pollutants; both raise nutrient and sediment loads to water bodies.

Answer: (D)

Nonpoint pollution from agricultural lands happens when runoff from many acres moves contaminants into streams and lakes, rather than a single discharge. When fertilizer is applied, nutrients such as nitrogen and phosphorus can remain in the soil. If rainfall or irrigation is intense, these nutrients are washed away with runoff or can seep into groundwater, increasing the nutrient load entering water bodies. At the same time, soil erosion loosens and transports soil particles. Those particles often carry nutrients and pesticides attached to them, so erosion moves both sediment and bound pollutants into waterways. Put together, excess fertilizer boosts nutrient runoff and erosion moves pollutants attached to soil, both raising nutrient and sediment loads to surface waters. That combination is what makes this the best explanation for how fertilizer use and soil erosion contribute to nonpoint pollution. The other statements don’t fit because fertilizer usage doesn’t reduce runoff, erosion doesn’t reduce pollutants, and a vague note about diffuse pollution doesn’t describe the mechanisms at play.