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  1. Free, publicly-accessible full text available October 1, 2024
  2. Abstract

    Groundwater mixing dynamics play a crucial role in the biogeochemical cycling of shallow wetlands. In this paper, we conducted groundwater simulations to investigate the combined effects of evaporation and local heterogeneity on mixing dynamics in shallow wetland sediments. The results show that evaporation causes groundwater and solutes to upwell from deep sediments to the surface. As the solute reaches the surface, evaporation enhances the accumulation of the solute near the surface, resulting in a higher solute concentration than in deep sediments. Mapping of flow topology reveals that local heterogeneity generates spatially varied mixing patterns mainly along preferential flow pathways. The upwelling of groundwater induced by surface evaporation through heterogeneous sediments is likely to create distinct mixing hotspots that differ spatially from those generated by lateral preferential flows driven by large‐scale hydraulic gradients, which enhances the overall mixing in the subsurface. These findings have strong implications for biogeochemical processing in wetlands.

     
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  3. Abstract

    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. HeterogeneousKgreatly enhances the seawater infiltration into the swash zone and modulates its spatial distribution along the beach surface. Due to heterogeneousKand 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.

     
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