Determining source apportionment of Din and Don in a Gulf of Mexico watershed and airshed

• Main Author: Qiu, Yixi
• Other Authors: Felix, Joseph, Murgulet, Dorina, Abdulla, Hussain, Wetz, Michael
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2023
• Subjects: Baffin Bay; Baffin
• Online Access: https://hdl.handle.net/1969.6/97223

A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Coastal and Marine Systems Science. Dissolved organic nitrogen (DON) profiles and contributing sources are poorly characterized in estuarine systems despite studies showing it constitutes a significant portion of the total nitrogen pool and inputs. The characterization of dissolved inorganic nitrogen (DIN) and DON processing along the pathways of surface/subsurface flow and wet deposition is also not well constrained. This dissertation work used stable isotope techniques to complete a comprehensive investigation of DIN and DON profiles, transformations, and sources across the hydrosphere (ground, surface, pore and rainwater) of a semi-arid estuary system (Baffin Bay, TX). Rainwater directly deposited 5.2 kg N/(ha*yr) to the watershed and had relatively low concentrations and varied nitrogen isotopic compositions of NH4+ (~27 µM; -10-7‰), NO3- (~17 µM; -7-10‰), and DON (~11 µM; -7-18‰). Isotope mixing model results implied agriculture and vehicle emissions as the two primary competing sources to ambient NH3 in the atmosphere. Vehicles were the dominant contributors to NOx emissions, however intermittent sources (e.g., lightning and biomass burning) rivaled vehicle emission during different seasons. Vehicle, fertilizer, and marine emissions were primary contributors to atmospheric DON and higher secondary DON formation contributions were observed when agriculture and/or biomass burning emissions were more prevalent. Tributaries in the watershed had varied NH4+ (0-300 µM), NO3- (0-272 µM), and DON (1-302 µM) concentrations that can reach the bay through riverine discharge. Several DIN hotspots were found at downstream WWTP outfalls, indicating wastewater contamination, which was consistent with the DON and NO3- dual isotope mixing model results. Groundwater had high NO3- (~1064 µM) and DON (~65 µM) concentrations, which could reach the bay through subsurface discharge. The varied ?15N-NO3- ...