Seasonal Patterns of Nitrogen Removal in Restored and Natural Wetlands Influenced by Wastewater Along the Texas Coast

Wastewater treatment plant (WWTP) can release high levels of nitrogen (N) species into the environment. Excess N can lead to hypoxia which is a large problem in the Gulf of Mexico and its surrounding estuaries. Wetlands can naturally remove excessive N through denitrification. However, there has been degradation of natural wetlands over time due to increased anthropogenic impacts. Investments are being made to restore these wetlands ecosystems along the Texas coast. This project aims to evaluate economic value of N mitigation service provided by the natural and reconstructed wetlands along Texas coast. The study sites include three WWTP and the adjacent wetlands impacted by the WWTP effluents. Seasonal N loading (nitrate, nitrite, and ammonium) in WWTP effluents are measured in monthly-collected water samples. Stable isotope ratios in different N species are determined to assess potential N sources in the wetlands. Potential denitrification rates are measured using the acetylene blocking method to investigate the effects of seasonal temperature changes on the rates of denitrification. The nitrate concentrations in water samples collected monthly from October 2018 through July 2019 at the Portland, Oso, and Bayside WWTP range from 3 to 1363 μM, 17 to 314 μM, and 0 to 60 μM, respectively. Nitrite concentrations are generally below 10 μM except occasional high levels detected in Portland WWTP (up to 71.5 μM). Ammonium is usually the second most abundant N species, but its concentration was three times higher in Bayside WWTP than nitrate. The two natural wetland sites adjacent to the Oso Bay WWTP have potential denitrification rates ranging from 11 to 31 and 9 to 50 ppm N₂O/ hour, respectively. Denitrification rates for the Nueces Bay Restoration site adjacent to the Portland WWTP range from 1 to 10 ppm N₂O/ hour, and the rates for the Egery Flats Restoration site adjacent to the Bayside WWTP range from 0.3 to 3 ppm N₂O/ hour. These denitrification rates will be used to monetize the N mitigation service provided by wetlands in this study. Understanding the denitrification process in natural and restored wetlands can aid in ecosystem health assessment, help to inform stakeholders and policy makers of the importance of these ecosystems, and assist with justification of conservation and restoration in Texas Coastal Bend wetlands.