Coastal aquifers are commonly layered, and thus, a clear understanding of groundwater flow and salt transport in layered coastal aquifers is important for managing fresh groundwater. However, the influence of leakage between adjacent aquifers on flow and transport processes remains largely unknown where the influence of tides is considered. This study used laboratory experiments and numerical simulation to examine the processes of flow and transport within a tidal aquifer-aquitard system (i.e., an unconfined aquifer underlain by a semi-confined aquifer, with an intervening thin aquitard). The laboratory-scale observations of the current study are the first observations of offshore fresh groundwater within a semi-confined coastal aquifer. The numerical and laboratory results are in close agreement, revealing that upward leakage from the semi-confined aquifer into the saltwater wedge of the overlying unconfined aquifer caused buoyant instabilities to form. The development of freshwater fingers created complex saltwater-freshwater mixing, leading to mixed saltwater influx-efflux patterns across the sloping aquifer-ocean interface. Compared with non-tidal conditions, tidal forces reduced the net upward leakage from the semi-confined aquifer to the overlying unconfined aquifer. This increased the horizontal flow toward the sea, which in turn reduced the extent of the saltwater wedge in the semi-confined aquifer. The higher rates of both fresh and saline submarine groundwater discharge (SGD), caused by tides, led to lower groundwater ages in the semi-confined aquifer. These findings have important implications for unveiling the complex characteristics of seawater intrusion, SGD and geochemical hotspots within layered coastal aquifers.

中文翻译：

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沿海含水层-弱水层系统中的流动和输送：实验和数值分析

沿海含水层通常是分层的，因此，清楚地了解分层沿海含水层中的地下水流和盐输送对于管理淡水地下水非常重要。然而，在考虑潮汐影响的情况下，相邻含水层之间的渗漏对水流和运输过程的影响仍然很大程度上未知。本研究使用实验室实验和数值模拟来研究潮汐含水层-弱透水系统（即半承压含水层下方的无承压含水层，中间有薄弱透水层）内的流动和输送过程。本研究的实验室规模观测是对半封闭沿海含水层内近海淡水地下水的首次观测。数值结果和实验室结果非常一致，表明从半承压含水层向上渗漏到上覆非承压含水层的咸水楔中导致了浮力不稳定性的形成。淡水指的发展造成了复杂的咸水-淡水混合，导致在倾斜的含水层-海洋界面上出现混合的咸水流入-流出模式。与非潮汐条件相比，潮汐力减少了从半承压含水层到上覆非承压含水层的净向上渗漏。这增加了流向海洋的水平流，从而减少了半承压含水层中咸水楔的范围。潮汐引起的淡水和咸水海底地下水排放率（SGD）较高，导致半承压含水层地下水年龄较低。这些发现对于揭示沿海层状含水层内海水入侵、SGD 和地球化学热点的复杂特征具有重要意义。