A model analysis of the tidal engine that drives nitrogen cycling in coastal riparian aquifers

In coastal rivers, tides facilitate surface water-groundwater exchange and strongly coupled nitrification-denitrification near the fluctuating water table. We used numerical fluid flow and reactive transport models to explore hydrogeologic and biogeochemical controls on nitrogen transport along an idealized tidal freshwater zone based on field observations from White Clay Creek, Delaware, USA. The capacity of the riparian aquifer to remove nitrate depends largely on nitrate transport rates, which initially increase with increasing tidal range but then decline as sediments become muddier and permeability decreases. Over the entire model reach, local nitrification provides a similar amount of nitrate as surface and groundwater contributions combined. More than half (~66%) of nitrate removed via denitrification is produced in-situ, while the vast majority of remaining nitrate removed comes from groundwater sources. In contrast, average nitrate removal from surface water due to tidal pumping amounts to only ~1% of the average daily in-channel riverine nitrate load or 1.77 kg of nitrate along the reach each day. As a result, tidal bank storage zones may not be major sinks for nitrate in coastal rivers but can act as effective sinks for groundwater nitrate. By extension, tidal bank storage zones provide a critical ecosystem service, reducing contributions of groundwater nitrate, which is often derived from septic tanks and fertilizers, to coastal rivers.

在海滨河流中，潮汐作用促进了地表水与地下水的交换，并在波动的水位附近强烈耦合硝化-反硝化过程。本研究基于美国特拉华州怀特克莱河的实地观测数据，利用数值流体流动和反应性传输模型，探讨了氮素在理想化潮汐淡水区沿线的迁移过程中，水文地质学和生物地球化学因素的控制作用。河岸含水层去除硝酸盐的能力主要取决于硝酸盐的迁移速率，该速率随着潮汐幅度的增加而最初上升，但随着沉积物变得更为混浊以及渗透性降低而随后下降。在整个模型范围内，局部硝化提供的硝酸盐量与地表水和地下水贡献的总和相当。通过反硝化去除的硝酸盐中，超过一半（约66%）是在原位生成的，而剩余大部分去除的硝酸盐则源自地下水。相比之下，由于潮汐泵浦作用从地表水中去除的平均硝酸盐量仅占平均每日河中硝酸盐负荷的约1%，或沿河段每日1.77公斤硝酸盐。因此，潮汐岸线储存区可能不是海滨河流中硝酸盐的主要汇，但可以作为地下水硝酸盐的有效汇。由此延伸，潮汐岸线储存区提供了关键的生态系统服务，减少了源自化粪池和化肥的地下水硝酸盐对海滨河流的贡献。