The importance of submarine groundwater discharge on nutrient fluxes and dissolved organic matter molecular composition in a semi-arid, highly disturbed estuary

This dissertation advances our understanding of the temporal and spatial variability of submarine groundwater discharge (SGD) rates, SGD-derived nutrient fluxes and its role in the nitrogen budget, and the dissolved organic matter (DOM) molecular composition of surface water and porewater in semi-arid, hydrological disturbed estuaries. Nueces Bay was selected as the study area because it is highly disturbed both anthropogenically (e.g., dredging, oil/gas development, and reduced freshwater inflows) and naturally (e.g., drought, flooding) and the nitrogen budget remains unbalanced after multiple attempts. This study collected surface water and porewater samples quarterly for 2 years and employed radon and radium mass balances and Darcy estimates to assess SGD rates, principal components analysis and a partial hierarchical two-way ANOVA to evaluate the water quality and relative importance of SGD-derived nutrient fluxes to the overall nutrient budget, and PPL-SPE and UPLC-Orbitrap Fusion Tribrid mass spectrometry to assess DOM molecular composition. This study found significant spatial and temporal variability in SGD rates that vary over 1-2 orders of magnitude depending on method (i.e., radon or radium mass balance, Darcy estimate), location, and groundwater endmember. The high SGD rates, compared to literature, are driven by steeper gradients nearshore, shortcircuiting of confining layers due to substantial oil/gas development and dredging, sediment heterogeneity, and reduced confining layer integrity favoring vertical advective flux. The observed spatiotemporal variability in SGD was related to nutrient fluxes and nitrogen budgets, suggesting that SGD brings 2-4 orders of magnitude more nitrogen and other nutrients into the system than surface runoff under all hydroclimatic conditions. Thus, the average SGD flux supplies more nitrogen to the system than previously accounted for creating an excess of vi 370.6x109 g N⸱yr-1 . Finally, a molecular characterization of the DOM indicates that surface water DOM composition is significantly different from flooding and baseflow conditions (6- and 9- months post flooding) during flood recession (3-months post flooding) with more heteroatom compounds detected. Surface water and porewater are most similar during flooding recession, indicating greater SGD-derived DOM. While other studies have suggested that semi-arid systems receive significant SGD, this dissertation further suggests that SGD in highly anthropogenically disturbed systems may be derived from both shorter (shallower) and longer (deeper) flowpaths and lag the climatic conditions by weeks (surficial aquifers) and months or longer (deeper aquifers). As anthropogenic disturbances continue to increase along with a changing climate, the groundwater-surface water interactions will be impacted and the long-term effects of these changes on nutrient and DOM composition might be significant, though potential consequences remain largely unknown.