Long-term assimilation wetlands in coastal Louisiana: Review of monitoring data and management

The term 'assimilation wetland' has been applied to natural wetlands in Louisiana into which disinfected, secondarily treated municipal effluent is discharged with the dual purpose of improving regional water quality and enhancing vegetation productivity and soil accretion. Some municipalities began discharging treated effluent into wetlands prior to state regulations, which began in 1992. Here we review data and observations from five assimilation wetlands in the Mississippi River Delta receiving discharge of treated effluent for 26-70 years. In addition, we examine two adjacent forested wetlands, one that receives periodic Mississippi River input and one that does not. Information from these sites provides insight into how long-term nutrient input impacts coastal wetlands. Analysis of tree-ring, leaf litter, accretion, and water quality data shows that input of freshwater containing nutrients and sediments leads to enhanced wetland productivity and soil accretion via increased organic matter burial. In addition, long-term data indicate that assimilation wetlands continue to be nutrient sinks even after decades of effluent discharge, with both nitrogen and phosphorus reduced to background levels. Collectively, these data demonstrate that wetlands benefit from long-term discharge of treated municipal effluent. Properly managed wetland assimilation systems can function for long periods and lead to enhancement of degrading wetland communities in coastal Louisiana.