Groundwater flow and moisture dynamics in the swash zone: effects of heterogeneous hydraulic conductivity and capillarity

A density-dependent, variably saturated groundwater flow and solute transport model was used to investigate the influence of swash motions on subsurface flow and moisture dynamics in beach aquifers with heterogeneous distributions of hydraulic conductivity (K) and capillarity. The numerical simulations were performed within a Monte Carlo framework using field measurements conducted in the swash zone of a sandy beach. Our results show that heterogeneous capillarity causes spatially variable capillary rise above the groundwater table. In response to swash motions, heterogeneity creates capillary barriers that result in pockets of elevated moisture content beneath the swash zone. These moisture hotspots persist within the unsaturated zone even at ebb tide when the swash motions recede seaward. Heterogeneous capillarity also results in highly tortuous preferential flow paths and alters the flow rates from the sand surface to the water table. Heterogeneous K greatly enhances the seawater infiltration into the swash zone and modulates its spatial distribution along the beach surface. Due to heterogeneous K and capillarity, complex mixing patterns emerge. Both strain-dominated and vorticity-dominated flow regions develop and dissipate as tides and waves move across the beach surface. Complex mixing patterns of seawater percolating from the swash zone surface to the water table, with localized areas of high and low mixing intensities, are further demonstrated by analysis of dilution index. Our findings reveal the influence of geologic heterogeneity on swash zone moisture and flow dynamics, which may have important implications for sediment transport and chemical processing in beach aquifers.本研究采用密度依赖型变饱和地下水流动与溶质运移模型，探究了激浪运动（swash motions）对导水率（hydraulic conductivity，简称K）与毛细作用（capillarity）呈非均质性分布的海滩含水层地下水流及水分动力学的影响。数值模拟以某砂质海滩冲激带（swash zone）的野外实测数据为基础，在蒙特卡洛框架下开展。研究结果显示，非均质性毛细作用会导致地下水位（groundwater table）以上出现空间分布不均的毛细上升现象。在激浪运动作用下，非均质性会形成毛细屏障，进而在冲激带下方形成高含水率区域。即便在退潮、激浪运动向海退去时，这类水分热点仍会持续留存于非饱和带（unsaturated zone）中。非均质性毛细作用还会形成高度曲折的优先流路径，并改变砂质表面至地下水位的水流速率。非均质性导水率会显著增强海水向冲激带的入渗过程，并调控其沿海滩表面的空间分布格局。受导水率与毛细作用的非均质性影响，系统中会形成复杂的混合模式。随着潮汐与波浪掠过海滩表面，应变主导型与涡度主导型流场区域会先后形成并消散。通过对稀释指数（dilution index）的分析，进一步证实了海水从冲激带表面向地下水位渗流时存在复杂的混合模式，且伴随局部高、低混合强度区域。本研究揭示了地质非均质性对冲激带水分与流场动力学的影响，该结论对海滩含水层中的泥沙输运与化学过程具有重要的参考价值。