Assess nonpoint source nitrogen contribution to the Texas Coastal Zone from septic systems

The goal of this project was to quantify and compare the amount of nitrogen (NH4 +, NO3 - , and NO2 - ) released into water bodies near Corpus Christi, Texas from wastewater treatment plants and septic systems in the surrounding communities. Sample collection began in November 2019 and carried on until September 2021, although it was interrupted during the height of the pandemic in spring of 2020. The sampling occurred at five different wastewater treatment plants ([WWTPs], Oso, Kingsville, Rockport, Portland, and Whitecap) as well as seven different septic systems in Corpus Christi (Oso), Kingsville, Portland, and Rockport near the WWTPs so that the nitrogen loading of these two different sources of treated effluent could be meaningfully compared. Effluents from WWTPs and on-site septic systems are sources of the nutrients responsible for aquatic nutrient pollution, releasing nitrate (NO3 - ), nitrite (NO2 - ), and ammonium (NH4 +) into local bodies of water, which could lead to eutrophication and hypoxia. Two different sources of effluent were sampled during this study: wastewater treatment plants (WWTP) and septic systems. Both WWTPs and septic systems separate solid waste from liquid wastewater, treat the wastewater, then discharge this cleaned effluent back into the natural environment; however, WWTPs serve a much larger area than septic systems, which collect and treat wastewater from a single home. Each WWTP and septic system may have different methods of treating wastewater and removing excess nutrients, if they are required to. This study detected significant levels of NO3 - , NO2 - , and NH4 + in bodies of water that served as reservoirs for effluents released by the septic systems and WWTPs. This suggests that further N removal processes would be needed if lower nitrogen concentrations in treated effluents were desired. Rockport WWTP is the only WWTP studied in this project that has a dedicated N removal process, which incorporates an anoxic tank and discharged the less N per day than other WWTPs studied in this project. The N released by Rockport WWTP was about 50% of the estimated N loading from combined onsite septic systems located in the same county. This suggests that N removal technique employed by WWTP is effective in nitrogen reduction, and N removal techniques should be employed by both WWTPs and septic system to reduce anthropogenic N contribution to local bodies of water.