Tides and Heterogeneity Drive Trapping and Chaotic Mixing in Coastal Aquifers

The combined effect of tidal forcing and aquifer heterogeneity leads to intricate transport patterns in coastal aquifers that impact both solute residence times and mixing dynamics. We study these patterns through detailed numerical simulations of density‐dependent flow and transport in a three‐dimensional, heterogeneous, confined coastal aquifer under tidal forcing. Advective particle tracking from both the freshwater and seawater domains reveals the formation of chaotic and periodic orbits in the freshwater‐saltwater transition zone, which traps contaminants. Furthermore, we found that increasing heterogeneity results in enhanced trapping and increased mixing entropy, suggesting that the chaotic orbits enhance mixing between contaminants from the freshwater and seawater domains. The results highlight the long‐term contamination risks of coastal aquifers due to trapping and the creation of hotspots for chemical and biological reactions through chaotic mixing in the transition zone. Our findings reveal novel mechanisms of contaminant transport in coastal aquifers, emphasizing the need to consider particle entrapment in environmental conservation and management strategies and to further explore these processes through laboratory and field investigations.