Some agricultural pesticides used in the Midwest can also be carried to the wetland through surface runoff. Nitrates are lost from upland sites primarily through subsurface drainage. Nitrate-N is efficiently removed from wetland surface waters by aquatic plants. Ammonium-N enters wetlands primarily through surface runoff. In the wetland, ammonia is absorbed by plants or converted to nitrogen gas through volatilization.
Nitrification can also occur, changing ammonia into nitrites and nitrates.
The nitrate form of nitrogen is more readily removed from surface water by wetland plants than the ammonium form. By holding water, a wetland allows sediment and large particles to settle on the wetland bottom. The root systems of wetland plants then absorb nutrients from the sediment. Much like phosphorus, some pesticides also bind to sediment materials. Surface runoff carries the sediment materials to the wetlands and deposits them on the wetland bottom.