Coastal aquifers are dynamic hotspots of complex hydro-geochemical interactions between fresh groundwater and saline seawater. Understanding the reactive transport of major solute species (eg. Ca2+, CO32-) is critical to advance our understanding of the carbonate system in saline aquifers which fundamentally impacts carbon cycling. This study employs a commercially available FEFLOW package to simulate the reactive transport of dissolved Ca2+ and CO32- species in a saline aquifer, incorporating both density-driven hydrodynamic mixing and geochemical reactions. Originally developed to investigate CO2 injection in saline aquifers as a climate change mitigation technique, this modeling framework provides insights into how the CO2 influx in dissolved form affects the geochemical system, especially carbonate equilibrium. A steady-state conservative mixing model was first developed using hydraulic head and solute concentration (TDS, Ca2+, CO32-) boundary and initial conditions. Transient reactive transport simulations were then performed to evaluate the geochemical evolution of Ca2+ and CO32- species, focusing on their enrichment or depletion in the model domain. The model dynamically solves the reaction-transport equation, coupling advective-diffusive transport with precipitation-dissolution kinetics based on carbonate saturation state. Preliminary results indicate that mixing-induced geochemical reactions significantly alter the spatial distribution of dissolved Ca2+ and CO32- concentrations with time (in years) within the model domain. The findings from the present study highlight the importance of considering reactive transport processes in saline aquifer models to better assess the impact of CO2 injections on water-rock interactions. This study will potentially provide deeper insights into the evolution of water chemistry in coastal aquifers with implications for carbon sequestration and long-term CO2 storage in coastal regions.