Numerical studies of vertical Cl-, δ2H and δ18O profiles in the aquifer-aquitard system in the Pearl River Delta, China

Abstract

Sedimentation may have a significant effect on the transport of solutes and environmental isotopes in sediment. The depth profiles of the Cl, δ2H, and δ18O in a borehole in the aquifer-aquitard system in the Pearl River Delta (PRD), China were obtained by centrifuging the core sediment samples. A one-dimensional model based on the sedimentation and sea level changes of the PRD during the Holocene was built to investigate numerically the transport mechanisms of Cl, δ2H, and δ18O. The sedimentation process was modeled as a moving boundary problem with the moving rate equal to the sedimentation rate. The model was calibrated and the parameters were obtained by comparing simulated and measured data. Very good agreement between all the three observed profiles and the simulated ones demonstrates the reliability of the model and the parameters. Simulation results show that the shapes of the curves are controlled by the combination of sedimentation and upper boundary conditions. Diffusion solely is adequate to reconstruct the observed profiles, which indicates that diffusion is the dominant vertical transport mechanism. The effective diffusion coefficient of the aquitard and the aquifer equals to 5.0 × 10-11 m2/s and 2.0 × 10-10 m2/s, respectively. The results of this study will help understand the transport mechanisms of solutes and environmental tracers in deltas with geology and hydrogeology similar to the PRD.