Hydrogeologic controls of surface water-groundwater nitrogen dynamics within a tidal freshwater zone

Microbial processing of reactive nitrogen in stream sediments and connected aquifers can remove and transform nitrogen prior to its discharge into coastal waters, decreasing the likelihood of harmful algal blooms and low oxygen levels in estuaries. Canonical wisdom points to the decreased capacity of rivers to retain nitrogen as they flow towards the coast. However, how tidal freshwater zones, which often extend hundreds of kilometers inland, process and remove nitrogen remains unknown. Using geochemical measurements and numerical models, we show that tidal pumping results in the rapid cycling of nitrogen within distinct zones throughout the riparian aquifer. Near the fluctuating water table nitrification dominates, with high nitrate concentrations (>10 mg N L-1) and consistent isotopic composition. Beneath this zone, isotopes reveal that nitrate is both denitrified and added over the tidal cycle, maintaining nitrate concentrations >3-4 mg N L-1. In most of the riparian aquifer and streambed, nitrate concentrations are <0.5 mg N L-1, suggesting denitrification dominates. Model results reveal that oxygen delivery to groundwater from the overlying unsaturated soil fuels mineralization and nitrification, with subsequent denitrification in low oxygen, high organic matter regions. Depending on flow paths, tidal freshwater zones could be sources of nitrate in regions with permeable sediment and low organic matter content.

溪流沉积物与连通含水层中的活性氮（reactive nitrogen）微生物转化过程，可在氮素排入近海水体前将其移除并转化，进而降低河口有害藻华暴发与低氧现象的发生概率。学界主流共识认为，河流向海岸径流时，其氮素截留能力会逐步降低。然而，通常可向内陆延伸数百公里的潮汐淡水区（tidal freshwater zones）如何处理并移除氮素，目前仍不明确。本研究通过地球化学测量与数值模拟手段，证实潮汐泵效应可驱动河岸含水层（riparian aquifer）不同分区内的氮素发生快速循环。在波动水位带附近，硝化作用（nitrification）占据主导地位，此时硝酸盐（nitrate）浓度高于10 mg N L⁻¹，且同位素组成稳定均一。该带下方的同位素分析显示，在潮汐周期内，硝酸盐既发生反硝化消耗，又得到持续补充，维持硝酸盐浓度处于3~4 mg N L⁻¹以上。在多数河岸含水层与河床区域，硝酸盐浓度低于0.5 mg N L⁻¹，表明反硝化作用占据主导地位。模型结果显示，上层非饱和土壤向地下水输送的氧气可驱动矿化作用与硝化作用，随后在低氧、高有机质的区域进行反硝化过程。根据径流路径的差异，在沉积物渗透性较强且有机质含量较低的区域，潮汐淡水区可能成为硝酸盐的释放源。