Spatiotemporal dynamics of biogeochemical reactions in an intertidal beach aquifer: a field, laboratory, and numerical modeling study

Date
2019
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University of Delaware
Abstract
The intertidal zone of coastal aquifers hosts biogeochemical reactions that alter the form and concentration of land-derived nutrients prior to their coastal discharge. The location and rate of reactions within the beach aquifer are dependent on groundwater flow patterns for the delivery and transport of reactants, which are highly dynamic due to transient hydrologic conditions. This dissertation focuses on spatially characterizing the relationship between groundwater flow and biogeochemical activity within the intertidal beach aquifer over various timescales. ☐ A field, laboratory, and numerical modeling study investigated the spatial relationship between flow paths and chemical reactions over one sampling event. Aerobic respiration was primary controlled by the infiltration and flow of oxic seawater, with high oxygen consumption rates near the seawater infiltration point and along the landward freshwater-saline water flowpath. Denitrification, on the other hand, progressively increased along the flowpath and to the discharge zone. ☐ A two-year field and laboratory study of porewater revealed the transient and spatially-variable distribution of marine particulate organic carbon in the beach aquifer. Differential mobility of particulate and dissolved components led to a variable distribution of particulate carbon across the aquifer that diverged from salinity-indicated flow paths. This created reaction dynamics that deviated from salinity patterns, showing that reaction characteristics within the beach aquifer are not completely predictable based on groundwater advection alone. ☐ A variable-density groundwater flow and a reactive solute transport model were linked to quantify the effects of retarded particulate carbon transport on the biogeochemical reactions of the aquifer. The pool of marine particulate carbon resulting from its retarded mobility intermittently supported denitrification when transient hydrologic conditions allowed for anoxic groundwater to move over its location. The study demonstrated that sediment-entrapped particulate organic carbon contributes to beach reactivity patterns and increases the denitrification potential of the aquifer beyond that previously considered. ☐ These investigations collectively show that biogeochemical processes within beach intertidal zones are highly dynamic in time and space, influenced by, but often independent from physical hydrologic changes. These findings have noteworthy implications for quantification of chemical reactivity within beach systems over various timescales, and ultimately for the management of coastal chemical fluxes and budgets.
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