Subterranean estuaries (STE) are hotspots of biogeochemical reactions. Here, dissolved constituents in waters of terrestrial and marine origin are transformed before they discharge to the coastal oceans. The involved biogeochemical reactions are complex and non-linear, calling for the application of numerical reactive transport modeling (RTM) to improve the process understanding. The aim of this study was to assess the roles of organic matter degradation and coupled secondary mineral reactions for the fate of dissolved species in STEs of sandy beaches. A comprehensive RTM approach was applied for this purpose, accounting for the effects of ion activities, pH, pe, redox reactions, mineral equilibria (calcite, goethite, siderite, iron sulfide, hydroxyapatite and vivianite) as well as surface complexation. Results show that the STE biogeochemistry and associated species fluxes are very sensitive to the assumed reaction network. For example, inorganic carbon and pH were largely controlled by calcite and siderite dynamics, and dissolved Fe and HS were precipitated as goethite, siderite and/or iron sulfides. Moreover, PO concentrations were affected by both the formation of vivianite or hydroxyapatite as well as surface complexation. This work helped to establish the relative importance of some of the major biogeochemical processes in the STE. However, further field studies are needed to understand which processes play a role in real-world STEs, including an exploration of the deep subsurface of STEs. Such field-based observations will improve our conceptual process understanding, which is key to developing well-constrained RTMs.

中文翻译：

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矿物反应和表面络合对地下河口溶解物质输送的影响：综合反应输送建模方法的应用


地下河口（STE）是生物地球化学反应的热点。在这里，陆地和海洋来源的水中溶解的成分在排放到沿海海洋之前会发生转化。所涉及的生物地球化学反应复杂且非线性，需要应用数值反应输运模型（RTM）来提高对过程的理解。本研究的目的是评估有机物降解和耦合次生矿物反应对沙滩 STE 中溶解物种命运的作用。为此应用了综合 RTM 方法，考虑了离子活性、pH、pe、氧化还原反应、矿物平衡（方解石、针铁矿、菱铁矿、硫化铁、羟基磷灰石和六铁矿）以及表面络合的影响。结果表明，STE 生物地球化学和相关物种通量对假设的反应网络非常敏感。例如，无机碳和pH值主要受方解石和菱铁矿动力学控制，溶解的Fe和HS以针铁矿、菱铁矿和/或硫化铁的形式沉淀。此外，PO 浓度还受到紫铁矿或羟基磷灰石的形成以及表面络合的影响。这项工作有助于确定 STE 中一些主要生物地球化学过程的相对重要性。然而，需要进一步的实地研究来了解哪些过程在现实世界的 STE 中发挥作用，包括对 STE 深层地下的探索。这种基于现场的观察将提高我们对概念过程的理解，这是开发约束良好的 RTM 的关键。