Mahmood Muhammad Nasir, Schmidt Christian, Fleckenstein Jan H, Trauth Nico
Helmholtz Centre for Environmental Research - UFZ, Permoserstraße 15, 04318 Leipzig, Germany.
Ground Water. 2019 Jan;57(1):140-152. doi: 10.1111/gwat.12664. Epub 2018 Apr 17.
The biogeochemical composition of stream water and the surrounding riparian water is mainly defined by the exchange of water and solutes between the stream and the riparian zone. Short-term fluctuations in near stream hydraulic head gradients (e.g., during stream flow events) can significantly influence the extent and rate of exchange processes. In this study, we simulate exchanges between streams and their riparian zone driven by stream stage fluctuations during single stream discharge events of varying peak height and duration. Simulated results show that strong stream flow events can trigger solute mobilization in riparian soils and subsequent export to the stream. The timing and amount of solute export is linked to the shape of the discharge event. Higher peaks and increased durations significantly enhance solute export, however, peak height is found to be the dominant control for overall mass export. Mobilized solutes are transported to the stream in two stages (1) by return flow of stream water that was stored in the riparian zone during the event and (2) by vertical movement to the groundwater under gravity drainage from the unsaturated parts of the riparian zone, which lasts for significantly longer time (> 400 days) resulting in long tailing of bank outflows and solute mass outfluxes. We conclude that strong stream discharge events can mobilize and transport solutes from near stream riparian soils into the stream. The impact of short-term stream discharge variations on solute exchange may last for long times after the flow event.
溪流水和周边河岸水的生物地球化学组成主要由溪流与河岸带之间的水和溶质交换所决定。近溪流液压头梯度的短期波动（例如，在溪流流量事件期间）会显著影响交换过程的程度和速率。在本研究中，我们模拟了在不同峰值高度和持续时间的单次溪流排放事件中，由溪流阶段波动驱动的溪流与其河岸带之间的交换。模拟结果表明，强烈的溪流流量事件会引发河岸土壤中的溶质迁移，随后向溪流输出。溶质输出的时间和数量与排放事件的形状有关。更高的峰值和更长的持续时间会显著增强溶质输出，然而，峰值高度被发现是总体质量输出的主要控制因素。迁移的溶质分两个阶段输送到溪流中：（1）通过事件期间储存在河岸带的溪流水的回流；（2）通过重力排水从河岸带非饱和部分垂直移动到地下水，这一过程持续时间长得多（>400天），导致河岸流出和溶质质量流出出现长拖尾现象。我们得出结论，强烈的溪流排放事件能够使近溪流河岸土壤中的溶质迁移并输送到溪流中。短期溪流排放变化对溶质交换的影响在流量事件之后可能会持续很长时间。
