This study explores how tidal forcing and aquifer heterogeneity influence contaminant transport in coastal aquifers. Numerical simulations of density-dependent flow and solute transport were conducted in a three-dimensional, heterogeneous coastal aquifer subjected to tidal forcing. Advective transport was then analysed using particle tracking, tracing contaminants from both freshwater and seawater sources. Poincaré sections reveal that the interaction between tidal dynamics and aquifer heterogeneity induces chaotic and periodic orbits in the coastal salinity transition zone, leading to persistent contaminant trapping. The stochastic analysis further shows that greater heterogeneity enhances both trapping and mixing entropy while reducing segregation intensity, indicating that chaotic orbits promote mixing between contaminants from freshwater and seawater domains. These findings highlight two key processes in the transition zone: (1) contaminant trapping, which prolongs residence times and potentially increases long-term contamination risks, and (2) chaotic mixing, which promotes chemical and biological interactions.