Seawater transport through quartz pores of coastal aquifers: A molecular dynamics study

Abstract

Classical molecular dynamics (MD) simulations were carried out to analyze the diffusion of saltwater in aquifers at a salinity of 3.5 wt%, and temperatures between 5 and 50 °C. Porous systems were built from the quartz stable alpha phase to represent typical compositions and structures reported for aquifer surfaces. The SPC/E model was used to describe the interactions between water molecules in the system, while the quartz intramolecular bonds were characterized through a Buckingham–type potential. It was demonstrated from MD trajectories that the diffusivity in small pores of both, water molecules and NaCl ions, depends strongly on the crystallographic orientation of quartz surfaces exposed to the quartz/saltwater interface. The three most stable quartz surfaces, favor the diffusion of Na+ over Cl– in confinement, principally due to strong nonbonded interactions between chloride and silicon atoms with partial positive charges that remain exposed at surface terminations. The exposed quartz surfaces also determined the structure of confined water and the distribution of ions along the normal direction of the pore surfaces. The results reported in this study provide a molecular-scale description of the diffusive transport of seawater through quartz pores, which represents a potential scenario for seawater intrusion in coastal aquifers.