Impacts of Evaporation‐Induced Groundwater Upwelling on Mixing Dynamics in Shallow Wetlands

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. Plain Language Summary In shallow wetlands, groundwater mixing and exchange have been identified as critical factors affecting biogeochemical cycling and transformation in sediments. Our results for the first time demonstrate evaporation causes a significant upwelling of groundwater and solutes 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, including the Okubo‐Weiss parameter and dilution index, reveals that evaporation and local heterogeneity generates dynamic mixing patterns along preferential flow pathways. Such mixing mechanisms would strongly affect biogeochemical conditions in near‐surface sediments of shallow wetlands, which have strong implications for wetland ecosystems. Key Points Evaporation causes groundwater and solutes to upwell from deep sediments to the surface, resulting in near‐surface solute accumulation Heterogeneity causes strain‐dominated and vorticity‐dominated flow regions to coexist at small spatial scales along preferential flow paths Upwelling groundwater creates distinct mixing hotspots that differ spatially from those generated by large‐scale hydraulic gradients