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.

海滩含水层的潮间带（intertidal zone）承载着由海水入渗带来的活性有机碳所介导的陆源营养盐生物地球化学转化过程。既往研究多认为溶解有机碳是唯一的活性有机碳组分，但经平流输送与截留留存的颗粒有机碳（particulate organic carbon, POC）同样可支撑相关化学反应。相较于地下水流，颗粒有机碳的平流过程发生阻滞，进而在海滩沉积物中形成活性碳库；当溶解溶质因瞬态地下水流穿过该沉积物时，该碳库可为生物地球化学反应提供支撑。本研究借助地下水流模型（SEAWAT）与反应性迁移模型（PHT3D），模拟了颗粒有机碳对海滩反应过程的贡献，并识别出调控其相对贡献的关键参数。结果显示，由于水文因子变化导致盐水循环单元的空间范围随时间动态改变，颗粒有机碳对反硝化作用的贡献呈现瞬态特征。在颗粒有机碳沉积阶段，颗粒有机碳阻滞作用减弱与潮汐振幅升高会促使其分布范围扩张，进而提升其对海滩反应活性的相对贡献。沉积后阶段，导水率降低与潮汐振幅升高会使含氧盐水完全包裹颗粒有机碳库，从而减少其用于反硝化作用的碳消耗量。研究结果凸显，颗粒有机碳是一类间歇利用的碳源，其与地下水流条件及整体海滩生物地球化学过程之间存在复杂的空间关联。本研究证实，在贫碳海滩含水层中，颗粒有机碳或许是一类周期性发挥重要作用但长期被忽视的生物地球化学反应贡献者。本数据集包含基准模型的输入与输出文件。