Mechanisms of Dinitrogen Production in Coastal Permeable Sediments

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ABSTRACTOCE-0524994In coastal watersheds with soils of high hydraulic conductivity and permeable marine sediments, a major source of coastal pollution is groundwater-transported nitrogen (N), which is added to the aquifer primarily from wastewater, fertilizers, and atmospheric deposition. This nitrogen-bearing groundwater enters coastal waters from a seepage face near the intertidal zone or flows directly into the sea as fresh, brackish or saline submarine groundwater discharge. Depending on the biogeochemical setting, removal of fixed N due to N2 gas producing processes in the near shore aquifer may significantly attenuate land-derived N loads. In addition to the environment being poorly understood, our understanding of microbial nitrogen transformations for removal of fixed nitrogen is in the process of transformation due to recent discovery of the new pathway of anaerobic ammonium oxidation (anammox), a microbial process that may be so significant as to require recalculation of existing global N budgets. Anammox provides a mechanism for short-circuiting the N cycle by eliminating the need for nitrification prior to N2 gas production. Based on compelling preliminary evidence that anammox may be an important pathway for dinitrogen gas formation in advection dominated, coarse, sandy sediments, three young scientists from the Woods Hole Oceanographic Institution will investigate the consequences of advection through permeable sediments on N transport and transformations. They will employ a series of experiments designed to simulate water transport through permeable marine sediments and utilize a new approach that combines of isotope pairing and membrane inlet mass spectrometry techniques to quantify and identify all pathways of N2 production. The new approach is required as currently available procedures for measuring N2 production are not adequate to distinguish among the reaction pathways that may be occurring in these systems. The proposed project will benefit society by helping to develop a better understanding of behavior of groundwater-transported N loads and of internal N cycling within the large proportion of coastal waters and continental shelf areas characterized by permeable sediments. With 75% of the world's population expected to live within 35 miles of a coastline by 2020, it is vitally important that we understand how anthropogenic contamination is transformed in the environment as it is transported from land to the ocean. The proposed project will also foster education by involving high school students and undergraduate interns in both laboratory and field aspects of the research.