Fertilizer contains different types of phosphates and nitrates. When they are used to fertilize large fields, it can be beneficial in the matter of food production. But when it rains those nitrates, phosphates, etc can run off into a river,lake,stream and cause eutrophication .
Eutrophication causes an abundance of plant growth such as algae blooms. Algae blooms can suffocate fish and can have an impact on human health.