Density-dependent flow, solute transport and mixing in the Fraser River delta aquifer.

Groundwater flow in coastal aquifers is influenced by density differences between freshwater and saline water, forming a complex density-dependent flow system. The relatively narrow zone where freshwater and saline water mix is the location of strong geochemical gradients, which can drive reactions. Understanding the dynamics of this mixing zone is crucial to understand the biogeochemical reactions that occur during saline intrusion. In this study, we characterized flow and dispersive mixing in a confined sand aquifer subjected to saline intrusion, using both field observations and numerical modeling. Observations from an extensive monitoring network revealed a saline wedge extending approximately 500 m inland. We simulated density-dependent flow with MIN3P and used PEST++ to estimate dispersivity and diffusion from field observations. In effect, we use salinity data to measure the dispersivity through inverse modeling. The longitudinal and transverse dispersivity were calibrated at 0.1 m and 0.001 m, respectively, while the molecular diffusion coefficient was 1.2 × 10-9 m2/s. Sensitivity analyses show that transverse dispersivity affects mixing-zone characteristics more than longitudinal dispersivity. Our results highlight the importance of accurately estimating dispersivity values to model the width of the mixing zone, which strongly controls geochemical reactions at the site. These findings provide insights into the processes driving freshwater-saline water mixing in coastal aquifers and offer guidance for future modeling studies on geochemical dynamics during saline intrusion.