What is the primary source of severe nitrogen loading as indicated on certain maps?

The primary source of severe nitrogen loading in various water systems is agricultural fertilization. This occurs when fertilizers, which are rich in nitrogen, are applied to crops to enhance growth and yields. Rainfall and irrigation can lead to the runoff of excess fertilizers into nearby waterways, significantly increasing nitrogen levels in those bodies of water. This process can result in a variety of environmental issues, including eutrophication, where nutrient overloads cause excessive algae growth that depletes oxygen in the water and harms aquatic life.

While urban runoff, industrial waste, and sewage discharge can also contribute to nitrogen loading, their impact is often less significant in comparison to the extensive use of fertilizers in agriculture. Agricultural practices are widespread and often involve large amounts of nitrogen application, making them a major contributor to nutrient pollution in surface and groundwater.