Nutrient runoff occurs when excess nutrients, primarily nitrogen and phosphorus, wash away from agricultural fields into nearby water bodies. This often happens after heavy rainfall or irrigation, where fertilizers used to promote plant growth can be carried away by water, leading to pollution in rivers, lakes, and oceans. The influx of these nutrients can cause harmful algal blooms, which deplete oxygen in the water and harm aquatic life. This process can disrupt ecosystems and affect water quality, making it a significant environmental concern for communities relying on freshwater sources for drinking and recreation.