Nitrogen biogeochemistry of groundwater fluxes to the estuaries in the northern Gulf of Mexico

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The Gulf of Mexico hosts some of the most diverse ecosystems in the world; however, its ecological diversity is currently threatened by poor water quality and water resource scarcity. Currently, the largest hypoxic zone in U.S. coastal waters is found in its northern part and it is correlated with freshwater nitrogen (N) flux, which originates from the Mississippi River Delta. Nevertheless, recent studies indicate that river water is a minor contributor to the local hypoxia problem; instead, they point to potential contribution of nutrient-rich groundwater discharge into the coastal water. In this study, I aim to (1) quantify groundwater-derived N fluxes, (2) determine freshwater N sources, and (3) assess N transformation in the organic-rich coastal sediment found in the northern Gulf of Mexico. Specifically, I propose to focus on several smaller estuaries along coastlines, in which groundwater discharge has not been studied due to their proximate locations to the Mississippi River Delta, the largest delta in the continental U.S. I hypothesize that groundwater-derived N fluxes are partially responsible for the hypoxia problem in the northern Gulf of Mexico, and the organic-rich coastal sediment acts as a sink of anthropogenic N through a combination of biogeochemical processes. To test my hypotheses, I propose to do a combination of field and laboratory works, including measurements of radon, radium, dissolved organic and inorganic N concentration, and microbial sequencing. The results of this study can be used to assess the contribution of groundwater to total regional N budget, determine the role of subterranean estuary in transforming or/and attenuating anthropogenic N transport, and further contribute to better fundamental understanding of the drivers and remediation of water quality in the Gulf of Mexico. On a broader scale, the knowledge obtained by this project will improve our predictive ability of the contribution of groundwater to the global nutrient marine budget.