Fertilizer runoff occurs when excess fertilizers used in agriculture wash away from fields into nearby water bodies, such as rivers and lakes. This typically happens during heavy rainfall or irrigation, where the nutrients, primarily nitrogen and phosphorus, are carried away from the soil. The runoff can lead to water pollution, causing problems like algal blooms, which deplete oxygen in the water and harm aquatic life. This process can disrupt ecosystems and affect drinking water quality, making it a significant environmental concern that requires careful management of fertilizer application practices.