Modeling hydrologic controls on particulate organic carbon contributions to beach aquifer biogeochemical reactivity

The intertidal zone of beach aquifers hosts biogeochemical transformations of terrestrially-derived nutrients that are mediated by reactive organic carbon from seawater infiltration. While dissolved organic carbon is often assumed the sole reactive organic carbon component, advected and entrapped particulate organic carbon (POC) is also capable of supporting chemical reactions. Retarded advection of POC relative to groundwater flow forms pools of reactive carbon within beach sediments that support biogeochemical reactions as dissolved solutes move across them due to transient groundwater flow. In this work, we simulate the contribution of POC to beach reactions and identify parameters that control its relative contribution using a groundwater flow model (SEAWAT) and reactive transport model (PHT3D). Results show transient contributions of POC to denitrification, as the spatial extent of the saline circulation cell varies over time due to changing hydrologic factors. A decrease in POC retardation and an increase in tidal amplitude during POC deposition resulted in POC expansion, which increased the relative contributions of POC to beach reactivity. Decreased hydraulic conductivity and increased tidal amplitude post-deposition decreased the utilization of POC for denitrification by allowing the oxic, saline water to completely encompass the pool of POC. Results highlight that POC is an intermittently-utilized source of carbon that displays complex spatial relationships with groundwater flow conditions and overall beach biogeochemistry. This work demonstrates that POC may be a periodically important, but overlooked contributor to biogeochemical reactions in carbon-poor beach aquifers. This resource contains the input and output files for the base case model.