Preferential flow between rivers and aquifers in alluvial floodplains

Preferential flow between rivers and aquifers in alluvial floodplains may be a core component of shallow groundwater transport and, consequently, its understand- ing is key to modelling and managing groundwater resources. At a clay wedge separating present-day streamflow and bank storage from an adjacent shallow aquifer, we image a suspected sand-dominated structure. This structure cuts through the clay wedge and possesses temporally dynamic electrical resistivity as seen in time-lapse electrical resistivity tomographic (ERT) images collected over a 61-day study period. During days 11–12, following heavy rainstorms, the cross section of the electrically resistive sand fades into the background resistivity structure, reappearing the following day. This research shows that preferential flow can be imaged in time-lapse ERT in buried sand-dominated structures between a floodplain and the adjacent river. Our analysis demon- strates that sand conduits can transport infiltrated rainwater from the floodplain into the river as a bank spring and, hypothetically, at high-stage streamflow, from the river into the adjacent shallow aquifer. In both directions, these conduits for preferential flow exert an important control on the regulation and distribution of water, sediments and contaminants. This phenomenon will help hydrological models to incorporate more real-world phenomena and ultimately better prepare groundwater managers to sustainably steward shallow groundwater resources.冲积洪泛平原内河流与含水层之间的优先流（preferential flow），或许是浅层地下水运移的核心组成部分，因此对其的深入认知是地下水资源建模与管理的关键所在。在分隔当前河道水流与岸带储水、毗邻浅层含水层的黏土楔体中，我们发现一处疑似以砂质为主的构造。该构造贯穿该黏土楔体，且在为期61天的研究周期内采集的时移电阻率层析成像（ERT）图像中，呈现出随时间动态变化的电阻率特征。在强暴雨过后的第11至12天，该高电阻率砂质构造的断面逐渐融入背景电阻率结构，并于次日重新显现。本研究证实，通过时移ERT成像，可在洪泛平原与毗邻河流间的埋藏砂质构造中观测到优先流信号。我们的分析表明，砂质通道可作为岸泉，将洪泛平原下渗的雨水输送至河流；而在高水位河道水流条件下，理论上也可将河水反向输送至毗邻的浅层含水层。无论水流方向如何，这类优先流通道均对水、沉积物与污染物的运移调控与空间分布起到关键控制作用。这一发现将助力水文模型纳入更多真实水文过程，最终帮助地下水管理者更高效地实现浅层地下水资源的可持续管理与守护。