Dense Contaminants Mixing Into the Saltwater Wedge in Coastal Aquifers: Laboratory and Numerical Investigations

The saltwater-freshwater mixing zones in coastal aquifers can host complex physical exchange and biogeochemical transformations. The land-sourced dense contaminant plumes could be transferred into the mixing zone of the saltwater wedge due to the density effect prior to discharge to the sea. However, the mixing process between dense contaminants and the saltwater wedge has not received much attention, largely due to the lack of physical evidence. This study used laboratory experiments and numerical simulations to investigate the transport and discharge behaviors of variable-density contaminant plumes in tidally influenced unconfined coastal aquifers. Results demonstrate that the highly dense contaminants mix with the underlying saltwater and finally merge with the saltwater wedge. This process significantly extends the contaminant discharge durations, thereby reducing the peak value of contaminant efflux. The dense contaminants are elongated along the landward margin of the saltwater wedge, leading to a larger spreading area (M_s) than that of constant-density contaminants. The sensitivity analysis indicates that the high density of contaminants acts as a trigger to induce the mixing of them and wedges. The higher hydraulic conductivity, lower dispersivities and reduced inland freshwater flux significantly increase the residence times (R_t) and discharge duration (D_t) by enhancing the mixing of dense contaminants with seawater. In contrast, both R_t and D_t values are not only non-monotonic functions of tidal amplitudes but also less sensitive to tidal effects. Compared with the non-tidal condition, however, the addition of tides significantly increases both R_t and M_s values of dense contaminant plumes. The results presented herein provide valuable insights into the mechanisms of dense contaminants mixing into saltwater wedges, which could guide practitioners in designing effective strategies to protect coastal environments from land-sourced contaminants.