A screening of the capacity of Louisiana freshwater wetlands to process nitrate in diverted Mississippi River water

Abstract

Mississippi River water is currently being diverted into Louisiana coastal wetlands for slowing or reversing marsh deterioration attributed to the rapid subsidence and accompanying saltwater intrusion. In this study, nitrate processing and removal was quantified in a 3700 ha ponded freshwater wetland through which the diverted Mississippi River water enters Louisiana Barataria Basin estuary. Nitrate removal rates using a mass balance approach (measuring changes in nitrate content between the inlet and the outlets) showed that the fresh water wetland removed practically all the nitrate in the diverted river water (≈35 m 3 s −1) during a discharge event in April 2003. Denitrification was a major process in removing nitrate in diverted river water. The capacity of the ponded wetland to remove all nitrate in diverted river water was strongly influenced by discharge rate. The ponded wetland did not remove all the NO 3 in river water at high discharge rate (>100 m 3 s −1) in December 2003. Results demonstrate that freshwater diversion through Louisiana coastal wetland can effectively process nitrogen in diverted Mississippi River water. However, discharge or pulsing rate should be regulated where possible to maximize nitrogen removal for limiting amount of nitrogen reaching lower reaches of Louisiana's Barataria Basin estuary.