The Use of Coastal Wetlands for Wastewater Assimilation and Restoration in the Gulf of Mexico

A hypoxic zone has developed off the shore of Louisiana in the Gulf of Mexico where hypolimnetic waters with dissolved oxygen less than 2 mg/L oxygen now extend over an area of 13,000 to 20,000 square km. Nitrogen, particularly nitrate nitrogen, is the most probable cause: 80% of the nitrogen input is from 3-million square km Mississippi River basin, which in turn comprises 41% of the conterminous U. S. The control of this hypoxia is important in the Gulf because the continental shelf fishery in the Gulf is approximately 25% of the U.S. total, and 50% of the Mexico total. A number of approaches are being considered for controlling nitrogen flow into the Gulf. Many of them involve large-scale modifications of land use practice (e.g., reduced fertilizer use or alternate cropping techniques), tertiary treatment (biological, chemical, physical) of point sources, landscape restoration (e.g., riparian buffers and wetland creation) to control nonpoint source pollution from farmland, stream and delta restoration, and atmospheric controls of NOx. The approach that appears to have highest probability of success with minimum impact on farming in the Midwest U.S. is landscape restoration. It has been suggested that 2-million-ha of restored and created wetland and about 77,000 square km of restored riparian buffers would be necessary to provide enough denitrification to substantially reduce the N entering the Gulf of Mexico. Restoring the Gulf requires restoring 3% of the Mississippi River basin. Over the past several thousand years, inputs from the Mississippi River formed the Mississippi delta, an area of about 25,000 square km. Over the past century, there has been a high loss of coastal wetlands of about 4,800 square km. The main causes of this loss are the near complete isolation of the river from the delta, mostly due to the construction of flood control levees, and pervasive hydrological disruption of the delta plain. Future coastal restoration efforts should also focus on less energy-intensive, ecologically engineered management techniques that use the energies of nature as much as possible. Diversion may be as important for controlling salinity as for providing sediments and nutrients for restoring coastal wetlands.