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Biogeochemical processes in land-ocean interfaces, surface estuaries, subterranean estuaries and sediment-water interface
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Spatiotemporal variability of biogeochemical reactions in an intertidal beach aquifer
Monday 7th @ 1030-1050, Conference Room 1
Kyra H. Kim* , University of Delaware, Department of Geological Sciences
Holly Michael, University of Delaware, Department of Geological Sciences
William Ullman, University of Delaware, School of Marine Science and Policy
Presenter Email: kyrakim@udel.edu |
| Intertidal areas of sandy beaches host dynamic mixing zones between fresh groundwater and saline seawater. Fresh groundwater discharges seaward with an in-land hydraulic gradient, and mixes with saline seawater as it comes up the beachface during waves and tides. The mixing of nitrate-rich, hypoxic to anoxic freshwater with oxygen- and carbon-rich seawater promotes active biogeochemical reactions along the mixing interface, altering the concentrations of terrestrial nutrients prior to their coastal discharge. As hydrologic conditions and flowpaths change (tidal amplitude, freshwater flux), the delivery of solutes and gases to a given location within the aquifer is also altered, subsequently changing the spatial patterns of reactions. Over two years, we investigated the spatial relationship between chemical reaction zones within the intertidal circulation cell to its physical setting. Porewater samples were collected from multi-level wells along a transect perpendicular to the shoreline at Cape Shores, Lewes, Delaware. Samples were analyzed for particulate carbon and reactive solutes, and incubated to obtain rates of oxic respiration and denitrification. Porewater incubation results showed high oxic respiration in the landward mixing zone between freshwater and seawater within the circulation cell where oxygen availability was high. Denitrification was more active towards the seaward mixing zone closer to the discharge point, accumulating nitrogen gas within the intertidal circulation cell. High respiration rates did not correlate with particulate carbon concentrations in porewater, suggesting that dissolved organic carbon or particulate carbon trapped within the sediment can contribute to and alter bulk reactivity. A better understanding of beach aquifer reactivity and its potential impacts to the coastal environment in times of sea-level rise and aquifer salinization will improve our ability to predict nutrient fluxes to estuaries and oceans. |
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