New insights on porewater geochemistry in organic-rich coastal sediments

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University of Alabama Libraries

Previous studies of groundwater nutrient dynamics in coastal Baldwin County, AL indicate that groundwater is contaminated with NO3-. However, recently, nutrient fluxes' mass-balance indicates positive fluxes of reduced nitrogen species, NH4+ and dissolved organic nitrogen (DON), while NO3- fluxes were negative. Further, it was suggested that geochemical transformations within an organic-rich layer comprising the subterranean estuary (STE) in Mobile Bay could be responsible for the observed changes in the groundwater-derived nitrogen fluxes. This study aims to examine the nitrogen geochemical transformations occurring in organic-rich shallow coastal sediments and to identify the quantity and quality of carbon exported by groundwater. In a laboratory-based study, sediment cores with identified organic-rich layers collected from a hypoxia-impacted shoreline of Mobile Bay were incubated with 400µM and 800µM NO3- solutions, natural groundwater (GW), and ultra-pure carbon-free water (UPCFW) to evaluate how the sediment reacts to different NO3- loading. Findings from the incubation studies show that the organic-rich sediments are indeed responsible for the NO3- loss in groundwater as all treatments with NO3- amendments experienced NO3- removal. Also, that the organic-rich sediments are a source of reduced N, in the form of DON and NH4+, evidenced by net-N production observed in three of four treatments and the N produced correlates with DON, and that higher NO3- loading promotes increased DON production. Additionally both DNRA and denitrification occurred, but the dominant pathway of NO3- removal was denitrification. Average NO3- removal rates increased with increases in NO3- loading, but complete NO3- removal was not observed. The STE sediment has the potential to denitrify significant amounts of anthropogenic NO3-, however, in the process producing NH4+, DON, DOC, and DIC.

Electronic Thesis or Dissertation
Geochemistry, Environmental geology